biology

RENDEZVOUS

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ALL HUMANKIND

HUMAN GENOME PROJECT has reached completion, hailed by a proud humanity. We

might pardonably wonder whose genome has been sequenced. Has an il- lustrious dignitary been singled out for the honour, or is it a random no- body pulled off the street, or even an anonymous clone of cells from a tissue culture lab? It makes a difference becanse we vary. I have brown eyes while you, perhaps, have blue. I can't curl my tongue into a tube, whereas it's 50/50 that you can. Which version of the tongue-curling gene makes it into the published human genome? What is the canonical eye colour?

I raise the question only to draw a parallel. This book traces 'our' an- cestors back through time, but whose ancestors are we talking about: yours or mine, a Bambuti Pygmy's or a Torres Strait Islander's? I shall come to the question presently. But first, having raised the analogous question about the Human Genome Project, I can't just leave it dangling. Whose genome is chosen for analysis? In the case of the 'official' Human Genome Project the answer is that, for the low percentage of DNA letters that vary, the canonical genome is the majority (vote' an1ong a couple of hundred people chosen to give a good spread of racial diversity. In the case of the rival project initiated by Dr Craig Venter, the genome ana- lysed was mostly that of ... Dr Craig Venter. This was announced by the man himself,* to the mild consternation of the ethics committee which

* v\lhen his team went on to decipher the dog genome, it ,vas no surprise to discover that the individual honoured was Dr Venler's own poodle, Shadow.

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ALL HUMANKIND 39

had recommended, for all sorts of warm and worthy reasons, that the donors should be anonymous and drawn from a spread of different

races. There are,other projects for the study of human genetic diversity

itself, which, bizarrely, come under recurrent political attack as though it were somehow improper to admit that humans vary. Thank goodness we do, if not very much.

But now, to our backwards pilgrimage. Whose ancestors are we going to trace? If we go sufficiently far hack, everyhody's ancestors are shared. All your ancestors are n1ine, whoever you are, and all 1nine are yours. Not just approximately but literally. This is one of those truths that turns out, on reflection, to need no new evidence. We prove it by pure reason, using the mathematician's trick of reductio ad absurdum. Take our imaginary time machine ahsurdly far back, say 100 million years, to an age when our ancestors resembled shrews or opossums. Somewhere in the world at that ancient date, at least one of my personal ancestors must have been living, or I wouldn't be here. Let us call this particular little mammal Henry (it happens to be a family name). We seek to prove that if Henry is my ancestor he must be yours too. Imagine, for a moment, the contrary: 1 am descended from Henry and you are not. For this to be so, your lin- eage and mine would have to have marched, side by side yet never touch- ing, through 100 million years of evolution to the present, never inter- breeding yet ending up at tbe same evolutionary destination - so alike that your relatives are still capable of interbreeding with mine. This re- ductia is clearly absurd. If Henry is my ancestor he has to be yours too. If not mine, he cannot be yours.

Without specifying how ancient is 'sufficiently', we have just proved that a sufficiently ancient individual with any human descendants at all must be an ancestor of the entire hmnan race. Long-distance ancestry, of a particular group of descendants such as the human species, is an all-or- nothing affair. Moreover, it is perfectly possible that Henry is my ances- tor (and necessarily yours, given that you are human enough to be read-

Opposite: Humankind. A stylised impressio of the human family tree. It is not intended as an accurate depiction - the real tree would be unmanageably dense. Moving down the page means going back in time, with the geological timescale (see page 16) given by the bar on the right. White lines illustrate patterns of interbreeding, with lots of it within continents and occasional migration between them. The numbered circle marks Concestor 0, the most recent common ancestor of all living humans. Verify this by following routes upwards from Concestor 0: you can reach any of the modern-day-human end points.

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40 RENDEZVOUS 0

ing this book) while his hrother Eric is the ancestor of, say, all the surviv- ing aardvarks. Not only is it possible. 1t is a remarkable fact that there must be a m01nent ln history when there were two animals in the same species, one of whom became the ancestor of all humans and no aard- varks, while the other became the ancestor of all aardvarks and no hu- mans. They may well have met, and may even have been brothers. You can cross out aardvark and substitute any other modern species you like, and the statement must still be true. Think it through, and you will find that it follows from the fact that all species are cousins of one another. Bear in mind when you do so that the 'ancestor of all aardvarks' will also be the ancestor of lots of very different things besides aardvarks (in this case, the entire major group called Afrotheria which we shall meet at Rendezvous 13, and which includes elephants and dugongs, hyraxes and Madagascan tenrecs).

My reasoning was constructed as a reductio ad absurdum. It assumed that 'Henry' Jived long enough ago for it to be obvious that he begat ei- ther all living humans, or none. How long is long enough? That'sa harder question. A hundred million years is more than enough to assure the conclusion we seek. If we go back only a hnndred years, no individual can claim the entire human race as direct descendants. Between the obvi- ous cases of 100 years and 100 million, what can we say about unobvious intermediates such as 10,000, 100,000 or 1 million years? The precise cal- culations were beyond me when I explained this reductio in River Out of Eden but, happily, a Yale University statistician called Joseph T. Chang has now made a start on them. His conclusions and their implications form the Tasmanian's Tale, a tale of particular relevance to this rendez- vous because Concestor 0 is the 1nost recent common ancestor of all liv- ing humans. It is more elaborate versions of calculations like Chang's that we need to do in order to date Rendezvous 0.

Rendezvous O is the time when, on our backwards pilgrimage, we first meet a common human ancestor. But according to our reductio there is a point further in the past when every individual that we encounter with our time machine is either a con1mon ancestor or no ancestor at all. And although no one ancestor can be singled out for attention at this more distant milestone, it is worth a nod as we go by, because it marks the point where we can stop worrying about whether it is your ancestors we trace or mine: from that milestone on, all my readers march, shoulder to shoulder, in a phalanx of pilgrims towards the past.

TBE TASMANIA N'S TALE 41

THE TASMANIAN'S TALE

·written with Yan Wong

R AC I N G ANCESTORS is a beguiling pastime. As with history itself,

there are two methods. You can go backwards, listing your two par- ents, four grandparents, eight great-grandparents, and so on. Or you can pick a distant ancestor and go forwards, listing his children, grandchil- dren, great-grandchildren, until you end up with yourself. Amateur ge- nealogists do both, going back aud forth between generations un\il they have filled in the tree as far as parish registers and family Bibles allow. This tale, like the book as a whole, uses the backwards method.

Pick any two people and go backwards and, sooner or later, we hit a most recent com1non ancestor ~- MRCA. You and 1ne, the plumber and the queen, auy set of us must converge on a single concestor (or couple). But unless we pick dose relatives, finding the concestor requires a vast family tree, and most of it will be unknown. This applies a fortiori to the concestor of all humans alive today. Dating Concestor 0, the most recent common ancestor of all living hmnans, is not a task that can be under- taken by a practising genealogist. It is a task in estimation: a task for a mathematician.

An applied mathematician tries to understand the real world by set- ting up a simplified version of it - a 'model'. The model eases thought, while not losing all power to illuminate reality. Sometimes a model gives us a baseline, departures from which elucidate the real world.

ln framing a 1nathe1natical model to date the common ancestors of all surviving humans, a good simplifying assumption - a sort of toy world ··- is a breeding population of fixed and constant size, living on an island with uo immigration or emigration. Let it be an idealised popula- tion of Tas1nanian aboriginals, in happier tin1es before they were exter- minated as agricultural vennin by nineteenth-century settlers. The last pure-bred Tasmanian, Truganinni, died in 1876, soon after her friend 'King Billy' whose scrotum was made into a tobacco pouch (shades of Nazi lamps). The Tasmanian aboriginals were isolated some 13,000 years ago when land bridges to Australia were flooded by rising sea levels, and they then saw no outsiders until they saw them with a vengeance in their nineteenth-century holocaust. For ouf n1odelling purposes, we consider Tasmania to be perfectly isolated from the rest of the world for 13,000 years until 1800. Our notional 'present', for modelling purposes, will be defined as 1800 AD.

The next step is to model the mating pattern. In the real world people

42 RENDEZVOUS O

fall in love, or into arranged marriages, but here we are modellers, ruth- lessly replacing human detail by tractable mathematics. There's more than one mating model we could imagine. The random diffusion mode] has men and women behaving as particles diffusing outwards from their birthplace, more likely to bump into near than distant neighbours. An even simpler and less realistic model is the random mating model. Here, we forget about distance altogether and simply assume that, strictly within the island, mating between any male and any female is equally likely.

Of course neither model is remotely plausible. Random diffusion as- sumes that people walk in any direction from their starting point. In re- ality there are paths or roads which guide their feet: narrow gene con- duits through the island's forests and grasslands. The random mating model is even more unrealistic. Never n1ind. We set up 1nodels to see what happens under ideally simplified conditions. It can be surprising. Then we have to consider whether the real world is more surprising or less, and in which directions.

Joseph Chang, following a long tradition of mathematical geneticists, opted for random mating. His model ignored population size by assum- ing it constant. He did not deal with Tasmania in particular but we shall assume, again as a calculated oversimplification, that our toy population remained constant at 5,000, which is one estimate for Tasmania's aborig- inal population in 1800 before the massacres began. l must repeat that such simplifications are of the essence in mathematical modelling: not a weakness of the method hut, for certain purposes, a strength. Chang of course doesn't believe people mate at random, any more than Euclid be- lieved lines have no breadth. We follow abstract assumptions to see where they lead, and then decide whether the detailed differences from the real world matter.

So, how many generations would you have to go back, in order to be reasonably sure of finding an individual who was ancestor to everybody alive in the present? The calculated answer from the abstract model is the logarithm (base 2) of the population size. The base 2 logarithm of a number is the number of times you have to multiply 2 by itself to get that number. To get 5,000, you need to m1tltiply 2 by itself about 12.3 tin1es so, for our 'Thsmanian example, theory tells us to go back 12.3 gen- erations to find the concestor. Assuming four generations per century, this is less than four centuries. It's even less if people reproduce younger than 25.

I give the na1ne 'Chang One' to the date of the most recent common ancestor of some specified population. Continuing backwards from

rHE TASMANIAN's TALE 43

Chang One, it doesn't take long before we hit the point"-· I shall call it 'Chang Two' - at which everybody is either a common ancestor or has no surviving descendants. Only during the brief interregnum between Chang One and Chang Two does there exist an intermediate category of people who have son1e surviving descendants but are not common an- cestors of everybody. A surprising deduction, whose rationale I won't spell out, is that at Chang Two a large number of people are universal an- cestors: about 80 per cent of individuals in any generation will in theory be ancestors of everybody alive in the distant future.

As for the timing, well, the mathematics yield the result that Chang Two is approximately I. 77 times older than Chang One. I. 77 times 12.3 gives just under 22 generations, between five and six centuries. As we ride our time machine backwards in Tasmania, therefore, around the time of Geoffrey Chaucer in England we enter 'all or nothing' territory. From there on backwards, to the time when Tasn1ania was joined to Aus- tralia and all bets are off, everyone our time machine encounters will have either the entire population as descendants or no descendants at all.

] don't know about you, but 1 find these calculated dates astonishingly recent. vVhat's more, the conclusions don't change much if you assume a larger population. Taking a model population the size of Britain's today, 60 million, we still need to go back only 23 generations to reach Chang One and our youngest universal ancestor. If the model applied to Britain, Chang Two, when everybody is either the ancestor of all modern British people or of none, is only about 40 generations ago, or about 1000 AD. lf the assumptions of the model are true (of course they aren't) King Alfred the Great is the ancestor of either all today's British or none.*

I must repeat the cautions with which I began. There are all sorts of differences between 'model' and 'real' populations, in Britain or Tasma- nia or anywhere else. Britain's population has climbed steeply in histori- cal time to reach its present size, and that completely changes the calcula- tions. In any real population, people don't mate at random. They favour their own tribe, language group or local area, and of course they all have individual preferences. Britain's history adds the complication that, al- though a geographical island, its popnlation is far from isolated: Waves of external immigrants have swept in from Europe over the centuries: Romans, Saxons, Danes, and Normans among them.

If Tasmania and Britain are islands, the world is a larger 'island' since it has no immigration or emigration (give or take alien abductions in fly- ing saucers). But it is imperfectly subdivided into continents and smaller

*Seep. 642.

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islands, with not just seas but n1ountain ranges, rivers and deserts im- peding the movement of people to varying degrees. Complicated depar- tures from random mating confound our calculations, not just slightly but grossly. The present population of the world is 6 billion, but it would be absurd to look up the logarithm of 6 billion and swallow the resulting mediaeval date for Rendezvous 0! The real date is older, if only because pockets of humanity have been separated far longer than the orders of magnitude we are now calculating. If an island has been isolated for 13,000 years, as Tasmania was, it is impossible for the human race as a whole to have a universal ancestor younger than 13,000 years. Even par- tial isolation of sub-populations plays havoc with our all-too-tidy calcu- lations, as does any kind of non-random mating.

The date when the most isolated island population in the world be- came isolated sets a lower bound on the date of Rendezvous 0. But to take this lower bound seriously, isolation must be absolute. This follows from the calculated figure of 80 per cent that we met earlier. A single mi- grant to Tasmania, once he has been sufficiently accepted into society to reproduce normally, has an 80 per cent chance of eventually becoming a comn1011 ancestor to all Tasmanians. So even tiny amounts of mjgration are enough to graft the family tree of an otherwise isolated population to that of the mainland. The timing of Rendezvous 0 is likely to depend on the date at whicb tbe most isolated pocket of humanity became com- pletely isolated from its neighbour, plus the date at which its neighbour then became completely isolated from its neighbour, and so on. A few is- land hops may be needed before we can join all the family trees together, but it is then an insignificant number of centuries back until we tumble upon Conces tor 0. That would put Rendezvous 0 some few tens of thou- sands of years ago, conceivably somewhere in the high tens of thousands, no more.

As to where Rendezvous O took place, this is almost as surprising. You 1night be inclined to think of Africa, as was my initial reaction. Africa houses the deepest genetic divides within humankind, so it seems a logi- cal place to look for a common ancestor of all living humans. It has been well said that if you wiped out sub-Saharan Africa you would lose the great majority of human genetic diversity, whereas you could wipe out everywhere except Africa and nothing 1nuch would change. Nevertheless Concestor 0 may well have lived outside Africa. Concestor 0 is the most recent common ancestor that unites the most geographically isolated population -- Tasmania for the sake of argument -- with the rest of the world. If we assume that populations throughout the rest of the world, including Africa, indulged in at least some interbreeding during a long

THE T A S M A N I A N S TALE 45

period when Tasmania was totally isolated, tbe logic of Chang's calcula- tions could lead us to suspect that Concestor 0 lived outside Africa, near the take-off point for tbe migrants whose offspring became Tasmanian immigrants. Yet African groups still retain most of humanity's genetic diversity. This seeming paradox is resolved in the next tale, when we ex- plore family trees of genes rather than of people.

Our surprising conclusion is that Concestor 0 probably lived tens of thousands of years ago, and very possibly not even in Africa.' Other spe- cies too may generally have quite recent common ancestors. But this is not the only part of the Tasmanian's 1ale that forces us to examine bio- logical ideas in a new light. To professional Darwinian specialists, it seems a paradox that 80 per cent of a population will become universal ancestors. Let me explain. We are used to thinking of individual organ- isms as striving to maximise a quantity called 'fitness'. Exactly what fitness means is disputed. One favoured approxi1nation is 'total nu1nber of children'. Another is 'total number of grandchildren', but there is no obvious reason to stop at grandchildren, and many authorities prefer to say something like 'total number of descendants alive at some distant date in the future'. But we seem to have a problem if, in our theoretically idealised population in the absence of natural selection, 80 per cent of the population can expect to have the maximum possible 'fitness': tbat is, they can expect to claim the entire population as their descendants! This matters for Darwinians because tbey widely presnme that 'fitness' is what all animals constantly struggle to maximise.

I have long argued that the only reason an organism behaves as a quasi- purposeful entity at all an entity capable of maximising any- thing -- is that it is built by genes that bave survived through past gener- ations. There is a temptation to personify and ·impute intention: to turn 'gene survival in the past' into something like 'intention to reproduce in the future'. Or 'individual intention to bave lots of descendants in the fu- ture'. Such personification can also apply to genes: we are tempted to see genes as influencing individual bodies to behave in such a way as to in- crease the number of future copies of those same genes.

Scientists who use such language, whether at the level of the i11divid- ual or the gene, know very well tbat it is only a figure of speech. Genes are just DNA molecules. You'd have to be barking mad to think that 'selfish' genes really have deliberate intentions to survive! We can always translate hack into respectable language: the world becomes full of those genes that have survived in the past. Because the world has a certain sta-

*Seep. 642.

46 RENDEZVOUS O rHE TASMANIAN's TALE 47 "'- ,,. --

bility and doesn't change capriciously, the genes that have survived in the past tend to be the ones that are going to be good at surviving in the fu. ture. That means good at programming bodies to survive and make chil- dren, grandchildren and long-distance descendants. So, we have arrived back at our individual-based definition of fitness looking into the future. But we now recognise that individuals matter only as vehicles of gene survival. Individuals having grandchildren and distant descendants is only a means to the end of gene survival. And this brings us again to our paradox. 80 per cent of reproducing individuals seem to be crammed up against the ceiling saturated out at maximum fitness!

To resolve the paradox, we return to the theoretical bedrock: the genes. We neutralise one paradox by erecting another, almost as if two wrongs could make a right. Think on this: an individual organism can be a universal ancestor of the entire population at some distant time in the future, and yet not a single one of his genes survives into that future! How can this be?

Every time an individual has a child, exactly half his genes go into that child. Every time he has a grandchild, a quarter of his genes on average go into that child. Unlike the first generation offspring where the percentage contribution is exact, the figure for each grandchild is statistical. It could be more than a quarter, it could be less. Half your genes come from your father, half from your mother. When you make a child, you put half of your genes into her. But which half of your genes do you give to the child? On average they will be drawn equally from the ones you origi- nally got from the child's grandfather and the ones you originally got from the child's grandmother. But, by chance, you could happen to give all your mother's genes to your child, and none of your father's. In tbis case, your father would have given no genes to his grandchild. Of course such a scenario is highly unlikely, but as we go down to more distant de- scendants, total non-contribution of genes becomes more possible. On average yon can expect one-eighth of your genes to end up in each great- grandchild, one-sixteenth in each great-great-grandchild, but it could be more or it could be less. And so on until the likelihood of a literally zero contribution to a given descendant becomes significant.

In our hypothetical Tasmanian population, the Chang Two date is 22 generations back. So when we say that 80 per cent of the population can expect to be ancestors of all surviving individuals, we are talking about their 22-greats-grandchildren. The fraction of an ancestor's genome , which, on average, we can expect to find in a particular one of his 22··greats-grandchildren is one four-millionth part. Since the human ge-

nome has only tens of thousands of genes, it would appear that one four- millionth part is going to be fairly thinly spread! It won't be quite like that, of course, because the population of our hypothetical Tasmania is only 5,000. Any individual may be descended from a particular ancestor through many different routes. But still, it could easily happen by chance that some universal ancestors happen to encl up contributing none of their genes to distant posterity.

Perhaps I am biased, but I see this as yet another reason to return to the gene as the focus of natural selection: to think backwards about the genes that have survived up to the present, rather than forwards about individuals, or indeed genes, trying to survive into the future. The 'for- ward intentional' style of thought can be helpfnl if used carefnlly and not misunderstood, but it is not really necessary. 'Backwards gene' language is just as vivid when you get used to it, is closer to the truth, and is less likely to yield the wrong answer.

In the Tasmanian's Talc we have talked about genealogical ancestors: historical individuals who are ancestors of modern ones in the conventional genealogist's sense: 'people ancestors'. But what you can do for people you can do for genes. Genes too have parent genes, grandparent genes, grandchild genes. Genes too have pedigrees, family trees, 'Most Recent Common Ancestors' (MRCAs). Genes too have their own Ren- dezvous O and here we really can say that, for the majority of genes, their own Rendezvous O was in Africa. This apparent contradiction will take some explaining, and this is the purpose of Eve's Tale.

Before proceeding, I must clear up a possible confusion over the meaning of the word gene. It can mean lots of things to different people, but the particular confusion that threatens here is the following. Some biologists, especially molecular geneticists, strictly reserve the word gene for a location on a chromosome ('locus'), and they use the word 'allele' for each of the alternative versions of the gene that might sit at that locus. To take an oversimplified example, the gene for eye colour comes in dif- ferent versions or alleles, including a blue allele and a brown allele. Other biologists, especially the kind to which I belong, who are sometimes called sociobiologists, behavioural ecologists or ethologists, tend lo use the word gene to mean the same as allele. When we want a word for the slot in the chromosome which could be filled by any of a set of alleles, we tend to say 'locus'. People like me are apt to say 'Imagine a gene for blue eyes, and a rival gene for brown eyes'. Not all molecular geneticists like that, but it is a well- established habit with my kind of biologist and J shall occasionally follow them.

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RENDEZVOUS 0 48

EVES TALE 49

EVE'S TALE

Written with Win Wong H ER E 'S A TELLING difference between 'gene trees' and 'people

trees'. Unlike a person who is descended from two parents, a gene has one parent only. Each one of your genes must have come from either your mother or your father, from one and only one of your four grand- parents, from one and only one of your eight great-grandparents, and

so on. But when whole people trace their ancestors in the conven- tional way, they descend eqnally from two parents, four grandparents, eight great-grandparents and so on. This means that a 'people genealogy' is much more mixed up than a 'gene genealogy'. In a sense, a gene takes a single path chosen from the maze of crisscrossing routes mapped by the (people) family tree. Surnames behave like genes, not like people. Your surname picks out a thin line through your full family tree. It high- lights your male to male to male ancestry. DNA, with two notable excep- tions whichI shall come to later, is not so sexist as a surname: genes trace their ancestry through males and females with equal likelihood.

Some of the best-recorded human pedigrees are of European royal families. In the family tree of the house of Saxe-Coburg (opposite), look at the princes Alexis, Waldemar, Heinrich, and Rupert. The 'gene tree' of

one of their genes is easy to trace because, unfortunately for them but fortunately for us, the gene concerned was defective. It gave the four princes, and many others of their ill-favoured family, the easily recognised blood disease haemophilia: their blood wouldn't clot properly. Haemophilia is inherited in a special manner: it is carried on the X chromosome. Males have only one X chromosome which they inherit from their mother. Fe- males have two X chromosomes, one inherited from each parent. They suffer from the disease only if they have inherited the defective version of the gene from both their mother and their father (i.e. haemophilia is 're- cessive'). Males suffer from the disease if their single 'unguarded'X chro- mosome bears the defective gene. Extremely few females suffer from haemophilia, therefore, but lots of females are 'carriers'. They have one copy of the faulty gene, and a 50 per cent chance of passing it on to each child.

Carrier females who are pregnant always hope for a daughter, but they still have a substantial risk of haemophiliac grandsons. If a haemo- philiac male lives long enough to have children, he cannot pass the gene on to a son (males never receive their X cl1romoson1e from their father), but he must pass it on to a daughter (females always receive their father's only X chromosome). Knowing these rules, and knowing which royal males

Bloodlines in the ill-fated House of Saxe-Coburg

had haemophilia, we can trace the faulty gene. Here is the backwards fam- ily tree, with the path the haemophilia gene must have taken in bold.

It seems that Queen Victoria herself was the mutant. It wasn't Albert, because his son, Prince Leopold, was haemophiliac, and sons don't get their X chromosome from their father. None of Victoria's collateral rela- tives suffered from haemophilia. She was the first royal individual to carry the gene. The miscopying must have occurred either in an egg of her mother, Victoria of Saxe-Coburg, or, which is more likely for reasons explained by my colleague Steve Jones in The Language of the Genes, 'in the august testicles of her father, Edward Duke of Kent'.

Although neither of Victoria's parents carried or suffered from haemophilia, one of them did have a gene (strictly an allele) which was the pre-mutated 'parent' of the royal haemophilia gene. We can think about (though we cannot detect) the ancestry of Victoria's haemophilia gene, back before it mutated to become a haemophilia gene. For our pur- poses it is irrelevant, except as a matter of diagnostic convenience, that

50 RENDEZVOUS 0

Victoria's copy of the gene was diseased while its predecessors were not. As we trace back the family tree of the gene we ignore its effects, except insofar as they render it visible. The gene's lineage must go back hefore Victoria, but the visible trail goes cold when it wasn't a haemophilia gene. The lesson is that every gene has one parent gene even if, through mutation, it is not identical to that parent gene. Similarly it has only one grandparent gene, only one great-grandparent gene, and so on. This may seem an odd way to think, but remember that we are on an ancestor- hunting pilgrimage. The present exercise is to see what an ancestor-hunt- ing pilgrimage would look like from a gene's point of view, instead of an individual's.

In the Tasmanian's Tale we encountered the acronym MRCA (Most Recent Common Ancestor) as an alternative to 'concestor'. I want to re- serve 'concestor' for the most recent com1non ancestor in an entire (peo- ple or organism) genealogy. So when talking about genes I shall use 'MRCA'. Two or more alleles in different individuals (or even, as we shall see, in the same individual) certainly do have an MRCA. It is the ances- tral gene of which they are each a (possibly mutated) copy. The MRCA of the haemophilia genes of Princes Waldemar and Heinrich of Prussia sat on one of the two X chromosomes of their mother, Irene von Hesse und bei Rhein. When she was still a foetus, two copies of the one haemophilia gene she carried were peeled off and passed successively into two of her egg cells, the progenitors of her luckless sons. These genes in turn share an MR.CA with the haemophilia gene of Tsarevitch Alexis of Russia (1904-1918), in the form of a gene carried by their grandmother, Princess Alice of Hesse. Finally, the MR.CA of the haemophilia gene in all four of our chosen princes is the very one that flagged itself up for attention in the first place, the mutant gene of Victoria herself.

Geneticists have a word for this sort of backwards tracing of a gene: it is called the coalescent. Looking backwards in time, two gene lineages can be said to coalesce into one at the point where, looking forwards again, a parent runs off two copies of the gene for two successive children. The point of coalescence is the MRCA. Any gene tree has many coalescence points. The haemophilia genes of Waldemar and Heinrich coalesce into the MRCA gene carried by their mother, Irene. That then coalesces with the lineage heading backwards from Tsarevitch Alexis. And, as we've seen, the grand coalescence of all the royal haemophilia genes occurs in Queen Victoria. Her genome holds the MRCA haemo- philia gene for the whole dynasty.

In my example, the coalescence of the haemophilia genes of all four princes occurs in the very individual (Victoria) who happens also to

EVE'S TALE 51

be their most recent common genealogical ('people') ancestor, their con- cestor. But that is just coincidence. If we we e to choose another gene (say for eye colour), then the path it took through the family tree would be quite diffetent, and the genes would coalesce in a more distant ances- tor than Victoria. If we picked a gene for brown eyes in Prince Rupert and one for blue eyes in Prince Heinrich, then the coalescence must be at least as far off as the separation of an ancestral eye-colour gene into two forms, brown and blue, an event buried in prehistory. Each piece of DNA has a genealogy which may be traced in a way that is separate but parallel to the sort of genealogy where we follow surnames through records of Births, Marriages and Deaths.

We can even do this for two identical genes in the same person. Prince Charles has blue eyes, which means, since blue is recessive, that he has two blue-eyed alleles. Those two alleles must coalesce somewhere in the past, but we can't tell when or where. It could be centuries or millennia ago, but in the special case of Prince Charles it is possible that the two blue-eyed alleles coalesce in as recent an individual as Queen Victoria. This is because, as it happens, Prince Charles is descended from Victoria twice: once via King Edward VII and once via Princess Alice of Hesse. On this hypothesis, a single blue-eyed gene of Victoria made two copies of itself at different times. These two copies of the same gene came down to the present Queen (Edward VII's great-granddaughter) and to her hus- band, Prince Philip (Princess Alice's great-grandson) respectively. Two copies of one Victorian gene could therefore have met again, on two dif- ferent chromosomes, in Prince Charles. In fact, that almost certainly has happened for some of his genes, whether for blue eyes or not. And re- gardless of whether his two blue-eyed genes coalesce in Queen Victoria or in somebody farther back, those two genes must have had an MRCA at some specific point in the past. It doesn't matter whether we are talk- ing about two genes in one person (Charles) or in two people (Rupert and Heinrich): the logic is the same. Any two alleles, in different people or in the same person, are fair game for the question: When, and in whom, do these genes coalesce as we look back? And, by extension, we can ask the same question of any three genes, or any number of genes in the population, at the same genetic location ('locus').

Looking much further back still, we can ask the same question for pairs of genes at different loci) because genes give rise to genes at differ- ent loci by the process of 'gene duplication'. We shall meet this phenome- non again in the Howler Monkey's Tale, and in the Lamprey's Tale.

Individual people who are closely related share a large number of gene trees. We share the majority of our gene trees with our dose kin. But

52 RENDEZVOUS 0

some gene trees deliver a '111inority vote', placing us closer to our other- wise more distant relatives. We can think of closeness of kinship among people as a kind of majority vote among genes. Some of your genes vote for, say, the Queen, as a close cousin. Others argue that you are closer to seemingly much 1nore distant individuals (as we shall see, even mem- bers of other species). When quizzed, each piece of DNA has a different view of what history is all about, because each has blazed a different path through the generations. We can hope to gain a comprehensive view only by questioning a large number of genes. But at this point we must be suspicious of genes situated close to each other on a chromo- some. To see why this is, we need to know something about the phenom- enon of recombination, which happens every time a sperm or an egg is made.

In recombination, randomly chosen sections of matching DNA are swapped between chromosomes. On average, only one or two swaps are seen per hunrnn chromosome (fewer when 111aking sperm, more when making eggs: it is not known why). But over nnmerous generations, many different parts of the chromosome will eventually be swapped around. So, generally speaking, the nearer two pieces of DNA are on a chromosome, the lower is the chance of a swap occurring between them, and the more likely they are to be inherited together.

When taking 'votes' from genes, therefore, we have to remember that the nearer a pair of genes are to each other on a chromosome, the more likely they are to experience the same history. And this motivates genes which are close colleagues to back up each other's vote. At the extreme are sections of DNA so tightly bonnd together that the entire chunk has travelled through history as a single unit. Such fellow-travelling chunks are known as 'haplotypes', a word that we shall meet again. Among such caucuses within the genetic parliament, two stand out, not because their view of history is more valid, but because they have been extensively used to settle biological debates. Both hold sexist views, because one has come down entirely through female bodies, and the other has never been out- side a male body. These are the two major exceptions to unbiased gene inheritance that I previously mentioned.

Like a surname, the (non-recombining portion of the) Y chromo- some always passes through the male line only. Together with a few other genes, the Y chromosome contains the genetic material that actually switches an embryo into the male pattern of development rather than the female one. Mitochondrial DNA, on the other hand, passes exclu- sively down the female line (although in this case it is not responsible for making the embryo develop as a female: males have mitochondria, it is

EvE's TALE 53

just that they don't pass them on). As we shall see in the Great Historic Rendezvous, mitochondria are tiny bodies inside cells, relics of once-free bacteria who, probably about 2 billion years ago, took up exclusive resi- dence inside cells where they have been reproducing, nonsexually by simple division, ever since. They have lost many of their bacterial quali- ties and most of their DNA, but they retain enough to be useful to genet- icists. Mitochondria constitute an independent line of genetic reproduc- tion inside our bodies, unconnected with the 111ajn nuclear line which we think of as our 'own' genes.

Because of their mutation rate, Y chromosomes are most useful for studies of recent populations. One neat study took samples of Y-chro- mosomal DNA in a straight line across modern Britain. The results showed that Anglo-Saxon Y chromosomes moved west across England from Europe, stopping rather abruptly at the Welsh border. It is not hard to imagine reasons why this male-carried DNA is unrepresentative of the rest of the genome. To take a more obvious example, Viking ships carried cargoes of Y chromosomes (and other genes) and spread them among widely scattered populations. The distribution of Viking Y-chromosome genes today presumably shows them to be slightly more 'travelled' than other Viking genes, which were statistically more likely to favour home- acre over Widow-maker:

What is a ivoman that you forsake her, And the hearth-fire and the home-acre, To go with the old grey Widmv-maker?

RUDYARD KIPLING

"Harp Song of the Dane Women"

Mitochondrial DNA too can be revealing, particularly for very ancient patterns. If we compare your mitochondrial DNA with mine, we can tell how long ago they shared an ancestral mitochondrion. And, since we all get our mitochondria from our mothers, and hence maternal grand- mothers, nrnternal great-grandmothers, etc., mitochondrial cornparison can tell us when our most recent female-line ancestor lived. The san1e can be done for Y chromosomes, to tell us when our 111ost recent male- line ancestor lived but, for technical reasons, it is not so easy. The beauty of Y- chromosomal and mitochondrial DNA is that neither of them is contaminated by sexual mixing. This makes tracing these particular classes of ancestor easy.

The mitochondrial MRCA of all humanity, which pinpoints the 'peo-

RENDEZVOUS 0 54

pie' common ancestor in the all-female line, is sometimes called Mito- chondrial Eve - she whose tale this is. And of course the equivalent in the all-male line might as well be called Y-chromosome Adam. All hu- man males have Adam's Y chromoso1ne (creationists please refrain frmn deliberate misquotation). If surnames bad always been strictly inherited by modern Western mies we'd all have Adam's surname too, which would rather lose the point of having a surname.

Eve isa great temptress to error and it is good to be forearmed. The errors_ are quite instructive. First, it is important to understand that Eve and Adam are only two out of a multitude of MRCAs that we could reach if we traced our way back through different lines. They are the spe- cial- case common ancestors that we reach if we travel up the family tree from mother to mother to mother, or father to father to father respec- tively. But there are many, many other ways of going up the family tree: mother to father to father to mother, mother to mother to father to fa- ther, and so forth. Each of these possible pathways will havea different MRCA.

Second, Eve and Adam were not a couple. It would bea major coinci- dence if they ever met, and they could well have been separated by tens

of thousands of years. As a subsidiary point, there are independent rea- sons to believe that Eve preceded Adam. Males are more variable in re- productive success than females: where some females have five times as many children as other fe.inales, the most successful males could have hundreds of times as many children as unsuccessful males.A male witha large harem finds it easy to become a universal ancestor.A female, since she is less likely to have a large family, needs a larger number of genera- tions to achieve the same feat. And indeed, today's best 'molecular clock' estimates for their respective dates are about 140,000 years ago for Eve

and only about 60,000 for Adam. Third, Adam and Eve are shifting honorific titles, not names of par-

ticular individuals. If, tomorrow, the last member of some outlying tribe were to die, the baton of Adam, or of Eve, could abruptly be thrown for- ward several thousand years. The same is true of all the other MRCAs de- fined by different gene trees. To see why this is so, suppose Eve had two daughters, one of whom eventually gave rise to the Tasmanian aborigi- nes and the other of whom spawned the rest of humanity. And suppose, entirely plausibly, that the female-line MRCA uniting 'the rest of hu- manity' lived 10,000 years later, all other collateral lines descending from Eve having gone extinct apart from the Tasmanians..Vlhen Trnganinni, the last Tasmanian, died, the title of Eve would instantly have jumped forward 10,000 years.

EVE'S TALE 55

Fourth, there was nothing to single out either Adam or Eve for partic- ular notice in their own times. Despite their legendary namesakes, Mito- chondrial Eve and Y-chromosome Adam were not particularly lonely. Both woukl have had plenty of companions, and each may well have had many sexual partners, with whom they may also have surviving descen- dants. The only thing that singles them out is that Adam eventually turned out to be hugely endowed with descendants down the male line, and Eve with descendants down the female line. Others among their contemporaries may have left as many descendants all told.

While I was writing this, somebody sent me a videotape of a BBC television documentary called Motherland, hyped as 'an incredibly poi- gnant film', and as 'truly beautiful, a really memorable piece'. The heroes of the film were three 'black'* people whose families had immigrated to Britain from Jamaica. Their DNA was matched up against worldwide da- tabases, in an attempt to trace the part of Africa from which their ances- tors were taken as slaves. The production company then staged lachry- mose 'reunions' between our heroes and their long-lost African families. They used Y-chromosomal and mitochondrial DNA because, for the rea- sons we have seen, they are more traceable than genes in general. But un- fortunately, the producers never really came clean about the limitations this imposed. In particular, no doubt for sound televisual reasons, they came close to actively deceiving these individuals, and also their long- lost African 'relatives', into becoming far more emotional about the re- unions than they had any right to be.

Let me explain. When Mark, later given the tribal name Kaigama, vis- ited the Kanuri tribe in Niger, he believed he was 'returning' to the land of 'his people'. Beaula was welcomed as a long-lost daughter by eight women of the Bubi tribe on an island off the coast of Guinea, whose mi- tochondria matched hers. Beaula said,

It was like blood touching blood ... It was like family .....I was just cry- ing, my eyes were just filled with tears, my heart was pounding. All I just kept thinking was: Tm going to my motherland.'

Sentimental rnbbish, and she should never have been deceived into thinking this. All that she, or Mark, were really visiting - at least as far as there was any evidence to suppose - were individuals who shared their mitochondria. As a matter of fact, Mark had already been told that his Y chromosome came from Europe (which upset him and he was later pal-

* For explanation of the inverted commas around 'black', see the Grasshopper's Tale.

56 RENDEZVOUS 0

pably relieved to discover respectable African roots for his mitochon· .dria!). Beaula, of course, has no Y chromosome, and apparently they didn't bother to look at her father's although that would have been inter· esting, for she was quite light-skinned. But it was explained to neither Beaula nor Mark, nor the television audience, that genes outside their mitochondria almost certainly came from a huge variety of 'homelands', nowhere near those identified for purposes of the documentary. If their other genes had been traced, they could have had equally emotional 're- unions' in hundreds of different sites, all over Africa, Europe and very probably Asia too. That would have spoiled the dramatic impact, of course.

As I have been continually reiterating, reliance on a single gene can be misleading. But the combined evidence from many genes gives us a pow· erful tool for reaching back into history. The gene trees of a population, and the coalescence points which define them, reflect the events of the past. Not only can we identify these coalescence points, we can also guess at their dates because of the molecular clock. And herein lies the key, be- cause the pattern of branchings through time tells a story. Random mat· ing, the assumption made in the Tasmanian's Tale, generates a very dif- ferent pattern of coalescence from various kinds of non-random 1nating - each of which, in turn, imprints its own shape on the coalescence tree. Fluctuations in population size, too, leave their own characteristic signa- ture. So we can work backwards from today's patterns of gene distribu- tion and make inferences about population sizes, and about the timings of migrations. For example, when a population is small, coalescence events will occur more frequently. An expanding population is signified by trees with long end branches, so coalescence points will be concen· trated near the base of the tree, back when the population was small. With the aid of the molecular clock, this effect can be used to work out when the population expanded, and when it contracted in 'bottlenecks'. (Although unfortunately, by wiping out genetic lineages, severe bottle- necks tend to erase the traces of what happened before them.)

Coalescent gene trees have helped resolve a long-standing debate over human origins. The 'Out of Africa' theory holds that all surviving peo· pies outside Africa are descended from a single exodus around a hundred thousand years ago, 1nore or less. At the other extreme are the 'Separate Origins' theorists or 'Multiregionalists', who believe that the races still living in, say, Asia, Australia aud Europe are anciently divided, separately descended from regional populations of the earlier species, Homo erec· tus. Both names are misleading. 'Out of Africa' is unfortunate because everybody agrees that our ancestors are from Africa if you go back far

EVE'S TALE 57

enough. 'Separate Origins' is also not an ideal name because, again if you go back far enough, the separation must disappear on any theory. The disagreement concerns the date when we came out of Africa. It might be better to call the two theories 'Young Out of Africa' (YOOA) and 'Old Out of Africa' (OOOA). This bas the added advantage of emphasising the continuum between them.

If today's non-Africans all stein fro_m a single recent e1nigration fron1 that continent, we would expect n1odern gene distributions to de1non-- strate a recent, Africa-centred, small-population 'bottleneck'. Coales- cence points would be concentrated around the tl1ne of the exodus. If we are separately descended from regional H. erectus, however, then genes should instead show evidence of anciently separated genetic lineages in each region. At the time when YOOA supporters claim an exodus, we would instead see a dearth of coalescence points. Which is it?

By expecting a single answer to this question we have fallen into the same trap as the Motherland television documentary. Different genes tell different stories. It is perfectly possible for some of our genes to have re- cently come out of Africa, while others have been passed to us from sepa- rate H. erectus populations. Or to put it another way, we can be both de- scendants of a recent African exodus, and simultaneously descendants of regional 1-f. erectus, because at any given ti1ne in the past we have a huge number of genealogical ancestors. Some could have recently left Africa. Some could have been resident in, say, Java for thousands of years. And we could have inherited African genes frmn some and Javan genes from others. A single chunk of DNA, such as from a mitochondrion or Y chromosome, gives as impoverished a view of the past as a single sen- tence from a history book. Yet the YOOA position is often supported on the basis of the placement of Mitochondrial Eve. Vv'hat happens if we quiz the other members of the parliament of genes?

This is, in effect, what the evolutionary biologist Alan Templeton did, and he came up with his engagingly titled theory 'Out of Africa Again and Again'. Templeton used a type of coalesceuce theory, similar to that in our hae1nophilia discussion, but he did it for lots of separate genes in~ stead of just one. This enabled him to reconstruct the history and geog- raphy of genes over the whole world and over hundreds of thousands of years. At the moment, I favour Templeton's 'Out of Africa Again and Again' theory, because he seems to 1ne to use all the available informa- tion in a way that rnaxi1nises its power to generate inferences; and be- cause he bent over backwards, at every step of his work, to guard against overreaching the evidence.

Here is what Templeton did. Ile looked through the genetic litera-

58 RENDEZVOUS 0

ture, using strict criteria to skim the cream: he wanted only large studies of.human genetics, where samples had been taken from different parts of the world, including Europe, Asia and Africa. The genes examined be- longed to long-lived 'haplotypes'. A haplotype, as we have seen, is a chunk of genome which is either impervious to being broken up by sex- ual recombination (as with Y-.chromosomal and mitochondrial DNA),

EVES TALE 59

Africa S. Europe N. Europe S. Asia N. Asia Pacific Americas

Recurrent gene flow with isolation by

or (as with certain smaller parts of the genome) can be recognised in- tact through enough generations to cover the timescale of interest. A haplotype is a long-lived, recognisable chunk of genome. You don't go too far wrong if you think of it as a large 'gene'.

Templeton zeroed in on 13 haplotypes. For each of them, he calcu- lated their 'gene tree', and dated the various coalescence points using the molecular clock which is ultimately calibrated with fossils. From these dates, and from the geographical distribution of the samples, he was able to pull out inferences about the genetic history of our species over the past couple of million years. He summarised his conclusions in a helpful diagram, reproduced on page 59.

Templeton)s main conclusion is that there were not two major migra- tions out of Africa but three. In addition to the OOOA (Homo erectus) exodus around 1.7 million years ago (which everyone accepts and for which the evidence is mostly from fossils) and the recent migration as promoted by the YOOA theory, there was another Great Trek from Af- rica to Asia between 840,000 and 420,000 years ago. This middle emigra- tion - shall we call it MOOA? - is supported by extant 'signals' from three of the 13 haplotypes. The YOOA emigration is supported by mito- chondrial and Y- chromosomal evidence. Other genetic 'signals' betray a

distance shown by mtDNA, Y-DNA, X-li11ked DNA and

autosomal DNA

0.08 to0.15 Mya

0.42 to0.84 Mya

1.7Mya

Africa S. Europe S.Asia

Range extensions shown by mtDNA, MX1, M5205, MClR and EON

Out of Asia expansion shown by Y-DNA and the haemoglobin fi locus

Out of Africa expansion shown by mtDNA and Y·DNA

Recurrent gene flow with isolation by distance hown by Xq13.3, haemoglobin /3, ECP, EON and PDHA1

Out of Afrlca expansion shown hy haemoglobin fi, MS205 and MClR

Rernrrent gene flow with i,olation by distance shown by Xq13.3, haemoglobin fl. ECP, EDN and PDHA1?

Rernrrent ger1e flow with isolation by distance shown by MXl?

Out of Africa expansior1 of Homo erectu.1 shown by fo sil data

major back-migration from Asia to Africa about 50,000 years ago. A lit- tle later, mitochondrial DNA and various smaller genes disclose other migrations: fron1 ·southern to northern Europe, from southern Asia to northern Asia, across the Pacific and to Australia. Finally, as shown by mitochondrial DNA and archaeological evidence, North America was colonised across what was then the Bering land bridge from north-east Asia, around 14,000 years ago. Colonisation of South America through the Isthmus of Panama rapidly followed. The suggestion, by the way, that either Christopher Columbus or Leif Ericsson 'discovered' America is nothing short of racist. Equally distasteful, in my view, is relativist 're- spect' for Native American oral histories which ignorantly deny that their ancestors ever lived outside A1nerica.

Between Templeton's three major migrations out of Africa, other ge- netic signals reveal continual eddies of gene flow back and forth between

Africa, southern Europe and southern Asia. His evidence suggests that

Out of Africa again and again. Templeton's summary of major human migrations, based on the study of 13 haplotypes. Vertical lines represent genetic descent; diagonal lines represent gene flow, The major human migrations indicated by genetic data are shown by the thick arrows. Adapted from Templeton [284] (square brackets refer to sources in the Bibliography).

major and minor immigrations have usually been followed by some in- terbreeding with indigenous populations, rather than - as might just as well have happened- complete extermination of one side or the other. Clearly this has large implications for our evolutionary ancestry.

This tale, and Templeton's study, focused on humans and their genes. But of course all species have family trees. All species inherit genetic ma- terial. All species with two sexes have an Adam and an Eve. Genes and gene trees are a ubiquitous feature of life on Earth. The techniques that we apply to recent human history can also be applied to the rest of life.

60 RENDEZVOUS 0

Cheetah DNA reveals a 12,000-year-old population bottleneck impor- tant to feline conservationists. Maize DNA has stamped upon it the un- mistakable signature of its 9,000-year Mexican domestication. The coa- lescence patterns of HIV strains can be used by epidemiologists and medical doctors to understand and contain tbe virus. Genes and gene trees reveal the history of the flora and fauna of Europe: the vast migra- tions driven by ice ages whose waxing pushed temperate species into southern-European refuges, and whose waning stranded Arctic species on isolated mountain ranges. All these events and more can be traced in the distribution of DNA around the globe, a historical reference book which we are only just learning to read.

We have seen how different genes have different stories to tell, which can be pieced together to reveal something of our history, both modern and ancient. How ancient? Amazingly, our oldest MR.CA genes can even date back before we were human at all. This is especially so when natural selection favours variety in the population for its own sake. Here's how it works.

Suppose there are two blood types called A and B, which confer im- munity to different diseases. Each blood type is susceptible to the disease against which the other type has im1nunity. Diseases flourish when the blood type that they can attack is abundant, because an epidemic can get going. So if B people, say, happen to be common in the population, the disease that hurts them will enjoy an epidemic. Consequently, B people will die until they cease to he common, and the A people increase - and vice versa. v\Thenever we have two types, the rarer of which is favoured because it is rare, it is a recipe for polymorphism: the positive 1nainte- nance of variety for variety's sake. The ABO blood group system is a fa- mous polymorphism which has probably been maintained for this kind of reason.

Some polymorphisms can be quite stable - so stable that they span the change from an ancestral to a descendant species. Astonishingly, our ABO polymorphism is present in chimpanzees. It could be that we and chimps have independently 'invented' the polymorphism, and for the same reason. But it is more plausible that we have both inherited it from om shared ancestor, and independently kept it going during our six mil- lion years of separate descent, because the relevant diseases have been continuously at large throughout that time. This is called trans-specific polymorphism, and it may apply to far more distant cousins than chim- panzees are to us.

A stunning conclusion is that, for particular genes, you are more closely related to some chin1panzees than to son1e humans. And I am

EVE' S TALE 61

closer to some chimpanzees than to you (or to 'your' chimpanzees). Hu- mans as a species, as well as humans as individuals, are temporary vessels containing a mix of genes from different sources. Individuals are tempo- rary meeting points on the crisscrossing routes that genes take through history. This is a tree-based way to express the central message of The Selfish Gene, my first book. As I put it there, 'When we have served our purpose we are cast aside. But genes are denizens of geological time: genes are forever.' At the concluding banquet to a conference in America, I recited the same message in verse:

An itinerant selfish gene Said 'Bodies a-plenty I've seen. You think you're so clever But I'll live for ever. You're just a survival machine.'

And, as the body's immediate reply to the gene, I parodied the very same Ha,p Song of the Dane Women quoted previously:

What is a body that first you take her, Grow her up, and then forsake her, To go with the old blind watchmaker?

We estimated the date of Rendezvous Oas probably tens of thousands of years ago, and at most hundreds of thousands. We have not travelled far on our backward pilgrimage. The next rendezvous, our meeting with the chimpanzee pilgrims at Rendezvous I, is millions of years away, and most of our rendezvous are hundreds of millions beyond that. To stand a chance of completing our pilgrimage, we shall need to speed up, and be- gin the move into 'deep time'. We must accelerate past the rest of the 30 or so ice ages that punctuate the last three million years, past such drastic events as the drying and refilling of the Mediterranean that occurred be- tween 4.5 and 6 million years ago. To ease this initial acceleration, I shall take the otherwise unusual liberty of stopping at a few intermediate milestones en route, and allowing dead fossils to tell tales. The fossilised 'shadow' pilgrims we shall meet, and the tales they tell, will help satisfy our natural preoccupation with our direct ancestors.

0

'·

11

ARCHAIC HOMO SAPIENS 63

ARCFlAIC HOMO SAPIENS

UR FIRST MILESTONE on theway back to Ren- dezvous 1 is in the depths of the ice age before

last, about 160,000 years ago. I have chosen this way station to look at fossil finds from Herto in the Afar depression of Ethiopia.* The Herta hunrnns are intriguing because, in the words of their discoverers, Tim White and his colleagues, they are from a 'population that is on the verge of anatomical modernity but not yet fully modern'. The distinguished palaeoanthropologist Christopher Stringer regards 'the Herto material as the oldest definite record of what we currently think of as modern H. sa-

Archaic forms persisted alongside Modern forms until at least 100,000 years ago (longer still if we include the Neanderthals, of whom more in a,moment). Archaic fossils are fonnd all around the world, dat- ing fromvarious times during the last few hundred thousand years: examples are the German 'Heidelberg man', 'Rhodesian man' from Zam- bia (which used to be called Northern Rhodesia), and the Chinese 'Dali man'. Archaics had big brains like us, averaging 1,200 to 1,300 cubic centimetres. This is a little smaller than our average of 1,400 cubic centi- metres but the range comfortably overlaps with ours. Their bodies were more robust than ours, their skulls were thicker, and they had more pronounced brow ridges and less pronounced chins. They looked more like Erects than we do, and hindsight justly sees them as intermediate. Some taxonomists recognise them as a subspecies of Homo sapiens called Homo sapiens heidelbergensis (where we would be Homo sapiens sapiens). Others do not recognise the Archaics as Homo sapiens at all, but call them Homo heidelbergensis. Yet others divide the Archaics into more than one species, for instance Homo heidelbergensis, Homo rhodesiensis, and Homo antecessor. If you think about it, we should be worried if there was not disagreement over the divisions. On the evolutionary view of life, a continuous range of intermediates is to be expected.

Modern Homo sapiens sapiens are not the only offshoot of the Archaics. Another species of advanced humans, the so-called Neanderthals, were our contemporaries for mnch of our prehistory. They resembled the Archa cs more than we do in some respects, and they seem to have

piens'a, record previously held by younger Middle Eastern fossils dating emerged from an Archaic root between about one and two hnndred

I ii 1.:

11 I,

lj:

I.I from about 100,000 years ago. Regardless of hair-splitting distinctions between 'modern' and 'nearly modern', it is clear that the Herto people are on the cusp between modern humans and those predecessors that we know by the catch-all name of 'Archaic Homo sapiens'. Certain authorities use this name back to about 900,000 years ago where it grades into an earlier species, Homo erectus. As we shall see, others prefer to give various Latin names to the bridging archaic forms. I shall sidestep the disputes by using anglicisms in the style of my colleague Jonathan Kingdon: 'Mod- erns', 'Archaics', 'Erects', and others that I'll mention as we come to them. We should not expect to draw a neat line between early Archaics and the Erects from whom they evolved, or between Archaics and the earliest Moderns who evolved from them. Don't be confused, incidentally, by the fact that the Erects were even more archaic (with a small a) than the Archaics (with a large A), and that all three types were erect witha small el

* The same A.far' after which the much older Australopitherns afarensis, or Lucy, is named.

t h o u s a n d y e a r s a g o - i n t h i s c a s e n o t i n A f r i c a b u t

in Enrope and the Middle East. Fossils from these regions show a gradual transition from Archaics to Neanderthals with the first unequivocal Neanderthal fossils found just before the beginning of the last Ice Age, about 130,000 years ago. They then persisted in Europe for most of this cold period, vanish- ing about 28,000 years ago. In other words, for their entire existence Neanderthals were contemporaries of Enropean Modern emigres from Africa. Some people believe that Moderns were responsible for their ex- tinction, either by killing them directly or by competing with them.

Neanderthal* anatomy was sufficiently different from ours that some

* Pedants' Corner: Thal or, in modern German, Tal, means valley. The Neander Valley is where the first fossil of this type was discovered. When German spelling was reformed at the end of the nineteenth century, the valley changed from a Thal to a Till, but the Latin name, Homo nea11derthalcnsis, was left high and dry, trapped by the laws of zoo- logical terminology. 'fo conform with tradition and with the Latin, I prefer to leave the English spelling in its original form and stick with the h.

A

64 ARCHAIC HOMO SAPIENS

people prefer to give them a separate species name, Homo neander- thalensis. They retained some features of Archaics such as large brow ridges which Moderns did not (which is why some authorities classify them as just another type of Archaic). Adaptations to their cold environ- ment include stockiness, short limbs and enormous noses, and they surely must have been warmly clothed, presumably in animal furs. Their brains were as big as ours or even bigger. Much is made of slight indica- tions that they ceremonially buried their dead. Nobody knows whether they could speak, and opinions differ on this important question. Ar- chaeology hints that technological ideas may have passed both ways be- tween Neanderthals and Moderns, but this could have been by imitation rather than by language.

The rules for the pilgrimage stated that only modern animals setting off from the present were entitled to tell tales. We are making an excep- tion for the dodo and the elephant bird, because they lived in recent his- torical times. And the fossils Homo erectus and Homo habilis qualify as 'shadow pilgrims' because a plausible case could be made that they are our direct ancestors. Do the Neanderthals, too, qualify under this rubric? Are we descended from them? Well, as it happens, that very question is the topic of the tale that the Neanderthals want to tell. Think of the Neanderthal's Tale as a plea to be allowed to tell it.

THE NEANDERTHAL'S TALE 1Vritten with Yan h'ong

RE WE DESCENDED from Neanderthals? If so, they would have to have interbred with Homo sapiens sapiens. But did they? They over-

lapped for a long time in Europe, and there was surely contact between them. But did it go beyond contact? Do modern Europeans inherit any

Neanderthal genes? This is a hotly debated issue, recently reignited by a remarkable extraction of DNA from late Neanderthal bones. So far,

we have extracted only the maternally inherited mitochondrial DNA, but this is enough for a tentative verdict. Neanderthal mitochondria are

quite distinct from those of all surviving humans, suggesting that Neanderthals are no closer to Europeans than to any other modern peo- ples. In other words, the female-line common ancestor of Neanderthals

and all surviving humans long pre-dates Mitochondrial Eve: about 500,000 years as opposed to 140,000. This genetic evidence suggests that

successful interbreeding between Neanderthals and Moderns was rare.

THE NEANDERTHAL'S TALE 65

And so it is often said that they died out without leaving any descen- dants. But don't let's forget that '80 per cent' argument which so surprised

us in the Tasfuanian's Tale. A single immigrant who managed to break into the Tasmanian breeding population had an 80 per cent chance of joining the set of universal ancestors: the set of individuals who could call themselves ancestors of all surviving Tasmanians in the distant fu- ture. By the same token, if only one Neanderthal male, say, bred into a sa- piens population, that gave him a reasonab]e chance of being a com1non ancestor to all Europeans alive today. This can be true even if Europeans contain no Neanderthal genes at all. A striking thought.

So although few, if any, of our genes come from Neanderthals, it is possible that some people have many Neanderthal ancestors. This was the distinction we met in Eve's Tale between gene trees and people trees. Evolution is governed by the flow of genes, and the moral of the Neanderthal's Tale, if we allow him to tell it, is that we cannot, should not, look at evolution in terms of pedigrees of individuals. Of course in- dividuals are important in all sorts of other ways, but if we are talking pedigrees it is gene trees that count. The words 'evolutionary descent' re- fer to gene ancestors, not genealogical ancestors.

Fossil changes too are a reflection of gene pedigrees) not (or only inci- dentally) genealogical pedigrees. Fossils indicate that Modern anatomy passed to the rest of the world via young out-of-Africa migrations. But Alan Templeton's work (described in Eve's Tale) suggests that we are also partly 'descended from' non-African Archaics, possibly even non-African Homo erectus. The description is both simpler and more powerful if we switch from people talk to gene talk. The genes that determine our Mod- ern anatomy were carried ont of Africa by the YOOA migrants, leaving fossils in their wake. At the same time, Templeton's evidence suggests that other genes we now possess were flowing around the world by dif- ferent rontes, but left little anatomical evidence to show for it. Most of our genes probably took the young out-of-Africa route, while just a few came to ns through other routes. What could be a more powerful way to express it?

So, have the Neanderthals established their right to tell a tale? Maybe a tale of genealogy if not a tale of genes.

M

ERGASTS 67

! "{ .I

-; it

Ill:

ERGASTS

OV IN G DEEPER IN TIME, we touch down again at one million years in search of ances-

tors, The only likely candidates of this age are of the type usually called Homo erectus, although some would call the African ones Homo· ergaster and I shall follow them. In seeking an anglicised form for these creatures, I shall call them Ergasts rather than Erects, partly because I believe the majority of our genes trace back to the African form, and partly because, as I've already remarked, they were no more erect than their predecessors (Homo habilis) or their successors (us). Whatever name we prefer, the Ergast type persisted from about L 8 million until about a quarter of a million years ago. They are widely accepted as the immediate predeces- sors, and partial conte1nporaries, of the Archaics who are in turn the predecessors of us Moderns,

The Ergasts were noticeably different from modern Homo sapiens, and, unlike the Archaic sapiens people, they differed from us in some respects that show no overlap, Fossil finds show they lived in the Middle East and Far East including Java, and represent an ancient migration out of Africa, You may have heard them referred to by their old names of Java Man and Peking Man, In Latin, before they were admitted into the Homo fold, they had the generic names Pithecanthropus and Sinanthropus. They walked on two legs like us, but had smaller brains (900 cc in early specimens to 1,100 cc in late ones), housed in lower, less domed, more 'swept-back' skulls than ours, and they had receding chins. Their jutting brow ridges made a pronounced horizontal ledge above the eyes, set in wide faces, with a pinching in of the skull behind the eyes.

Hair doesn't fossilise, so there is no natural place in our history to dis- cuss the obvious fact that at some point in our evolution we lost most of our body hair, with the luxuriant exception of the tops of our heads, Very likely the Erg,:ists were hairier than us, but we can't rule out the possibil- ity that Ergasts had already lost their body hair by a million years ago. They could have been as hairless as we are, Equally, nobody should com- plain of an imaginative reconstruction as hairy as a chimpanzee, or any intermediate level of shagginess. Modern people, males at least, remain quite variable in how hairy they are, Hairiness is one of those character- istics that can increase or decrease in evolution again and again. Vestigial hairs, with their associated cellular support structures, lurk in even the barest-seeming skin, ready to evolve into a full coat of thick hair at short notice (or shrink again) should natural selection at any time call them out of retirement Look at the woolly mammoths and woolly rhinocer- oses that rapidly evolved in response to the recent ice ages in Eurasia. We shall return to the evolutionary loss of human hair in - strangely enough - the Peacock's Tale,

Subtle evidence of repeatedly used hearths suggests that at least some groups of Ergasts discovered the use of fire -- with hindsight a momen- tous event in our history The evidence is less conclusive than we might hope. B]ackening from soot and charcoal does not survive immense timespans, but fires leave other traces that last longer. Modern experi- menters have systematically constructed fires of various kinds and then examined them afterwards for their trace effects, It emerges that deliber- ately built campfires magnetise the soil in a way that distinguishes them from bush/ires and from burnt-out tree stumps - I don't know why. But such signs provide evidence that Ergasts, both in Africa and Asia, had camp fires nearly one and a half million years ago. This doesn't have to mean that they knew how to light fires. They could have begun by cap- turing and tending naturally occurring fire, feeding it and keeping it alive as one might look after a Tamagochi pet Maybe, before they began to cook food, they used fires to scare away dangerous animals and provide light, heat, and a social focus.

The Ergasts also shaped and used stone tools, and presumably wooden and bone ones too. Nobody knows whether they could speak, and evidence is hard to come by, You might think that 'hard to come by' is an understatement, but we have now reached a point in our backward journey when fossil evidence starts to tell. Just as campfires leave traces in the soil, so the needs of speech call forth tiny changes in the skeleton: nothing so dramatic as the hollow bony box in the throat with which the howler monkeys of the South American forests amplify their stentorian

68 ERGASTS

voices, but still telltale signs such as one might hope to detect in a few fossils. Unfortunately, the signs that have been unearthed are not telltale enough to settle the 111atter, and it re1nains controversial.

There are two parts of the modern human brain which seem to go with speech. vVhen in our history did these parts - Broca's Area and Wernicke's Area - enlarge? The nearest approach we have to fossil brains is endocasts, to be described in the Ergast's Tale. Unfortunately the lines dividing different regions of the brain do not fossilise very clearly, but some experts think they can say that the speech areas of the brain were already enlarged before two million years ago. Those who want to believe that Ergasts possessed the power of speech are encouraged by this evi- dence.

They are discouraged, however, when they move down the skeleton. The most complete Homo ergaster we know is the Turkana Boy, who died near Lake Turkana, in Kenya, about 1.5 million years ago. His ribs, and the small size of the portholes in the vertebrae through which the nerves pass, suggest that he lacked the fine control over breathing that seems to he associated with speech. Other scientists, studying the base of the skull, have concluded that even Neanderthals, as recently as 60,000 years ago, were speechless. The evidence is that their throat shape would not have allowed the full range of vowels that we deploy. On the other hand, as the linguist and evolutionary psychologist Steven Pinker has remarked, 'e lengeege weth e smell nember ef vewels cen remeen quete expresseve'. If written Hebrew can be intelligible without vowels, I don't see why spo- ken Neander or even Ergaster couldn't too. The veteran South African anthropologist Philip Tobias suspects that language may pre-date even Homo ergaster, and he may just possibly be right. As we have seen, there are a few who go to the opposite extreme and date the origin oflanguage to the Great Leap Forward, just a few tens of thousands of years ago.

This may be one of those disagreements that can never be resolved. All considerations of the origin of language begin by citing the Linguistic Society of Paris which, in 1866, banned discussion of the question be- cause it was deemed unanswerable and futile. It may be difficult to an- swer, but it is not in principle unanswerable like some philosophical questions. Where scientific ingenuity is concerned, I am an optimist. Just as continental drift is now sewn up beyond all doubt, with multiple threads of convincing evidence, and just as DNA fingerprinting can es- tablish the exact source of a bloodstain with a confidence that forensic experts could once only dream of, I guardedly expect that scientists will one day discover some ingenious new method of establishing when our ancestors started to speak.

ERGASTS 69

Even I, however, have no hope that we shall ever !mow what they said to each other, or the language in which they said it. Did it begin with pure words and no grammar: the equivalent of an infant babbling nounspeak? Or did grammar come early and - which is not impossible and not even silly- suddenly? Perhaps the capacity for grammar was al- ready deep in the brain, being used for something else like mental plan- ning. Is it even possible that gram1nar, as applied to communication at least, was the sudden invention of a genius? I doubt it, but in this field I wouldn't rule anything out with confidence.

As a small step towards finding out the date at which language arose, some promising genetic evidence has appeared. A family code-named KE suffers from a strange hereditary defect. Out of approximately 30 family members spread over three generations, about half are normal, but fifteen show a curious linguistic disorder, which seems to affect both speech and understanding. It has been called verbal dyspraxia, and it first shows itself as an inability to articulate clearly in childhood. Other au- thorities think the trouble stems from 'feature blindness', meaning an in- ability to grasp certain grammatical features such as gender, tense and number. What is clear is that the abnormality is genetic. Individuals ei- ther have it or they don't) and it is associated with a mutation of an im- portant gene called FOXP2, which the rest of us have in unmutated form. Like most of our genes, a version of FOXP2 is present in mice and other species, and it probably does various things in the brain and elsewhere.* The evidence of the KE family suggests that in humans FOXP2 is impor- tant for the development of some part of the brain that is involved in language.

So, we naturally want to compare human FOXP2 with the same gene in animals that lack language. You can compare genes either by looking at the DNA sequences themselves, or by looking at the amino acid se- quences in the proteins that they encode. There are ti1nes when it 1nakes a difference, and this is one of them. FOXP2 codes for a protein chain 715 amino acids long. The mouse and chimpanzee versions of the gene differ in only one amino acid. The human version differs from botb these animals in an additional two amino acids. You see what this might mean? Although humans and chimpanzees share the great majority of their evolution and their genes, the FOXP2 gene is one place where humans seem to have evolved rapidly in the short time since we split from them. And one of the most important respects in which we differ from chimpanzees is that we have language and they don't. A gene that

11 lv1any genes have more than one _effect: a phenomenon known as plciotropism.

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70 ERGASTS

changed somewhere along the line towards us, but after the separation from chimpanzees, is exactly the sort of gene we should be looking for if we are trying to understand the evolution of language. And it is the very same gene that has mutated in the unfortunate KE family (and also, in a different way, in a wholly unrelated individual with the same kind oflan- guage defect). Perhaps it was changes in FOXP2 that made humans, as opposed to chimpanzees, capable of language. Did the Ergasts have the mutated FOXP2 gene?

Wouldn't it be wonderful if we could use this genetic hypothesis to date the origin of language in our ancestors? Wbile we can't do it with certainty, we can do something quite suggestive, along these lines. The obvious approach would be to triangulate backwards from variants among modern humans, and try to calculate the antiquity of the FOXP2 gene. But with the exception of rare unfortunates like the members of the KE family, there is no variation among humans in any of the FOXP2 amino acids. So there isn't enough variation there to triangulate from. Luckily, however, there are other parts of the gene which are never trans- lated into protein and which are therefore free to mutate without natural selection 'noticing': they are 'silent' code letters, in those parts of the gene that are never transcribed and are called intrans (as opposed to 'exons' which are 'expressed' and therefore 'seen' by natural selection). The silent letters, unlike the expressed ones, are quite variable among individual hunians, and between hmnans and chi1npanzees. We can get some un- derstanding of the evolution of the gene if we look at the patterns of variation in the silent areas. Even though the silent letters are not subject to natural selection themselves, they can he swept along by selection of neighbouring exons. Even better, the mathematically analysed pattern of variation in the silent intrans gives a good indication of when the sweeps of natural selection occurred. And the answer for FOXP2 is less than 200,000 years ago. A naturally selected change to the human version of FOXP2 seems roughly to coincide with the change from archaic Homo sapiens to anatomically modern Homo sapiens. Could this be when lan- guage was born? The margin of error in this sort of calculation is wide, but this ingenious genetic evidence counts as a vote against the theory that Homo ergaster could talk. More importantly for me, the unexpected new method boosts my optimism that one day science will find a way to confound the pessimists of the Linguistic Society of Paris.

Homo ergaster is the first fossil ancestor we have met on our pilgrim- age who is unequivocally of a different species from ourselves. We are about to embark on a portion of the pilgrimage in which fossils provide the n1ost important evidence, and they will continue to bulk large -

THE ERGAST'S TALE 71

though they will never overwhelm molecular evidence until we reach extremely ancient times and relevant fossils start to peter out. lt is a good moment to lookin more detail at fossils, and how they are formed. The Ergast will tell the tale.

THE ERGAST'S TALE

ICH A RD LEAKEY movingly describes the discovery, by his col- league Kimoya Kimeu on 22 August 1984, of the Turkana Boy (Homo

ergaster), at 1.5 million years the oldest near-complete hominid skeleton ever found. Equally moving is Donald Johanson's description of the older, and unsurprisingly less complete, australopithecine familiarly known as Lucy. The discovery of 'Little Foot', yet to he fully described, is just as remarkable (see page 88). Whatever freak conditions blessed Lucy,

'Little Foot', and the Turkana Boy with their version of immortality, would we not wish it for ourselves when our time comes? What hurdles must we cross to achieve this ambition? How does any fossil come to he formed? This is the subject of the Ergast's Tale. To begin, we need a small digression into geology.

Rocks are built of crystals, though these are often too small for the unaided eye to see. A crystal is a single giant molecule, its atoms arranged in an orderly lattice with a regular spacing pattern repeated billions of times until, eventually, the edge of the crystal is reached. Crystals grow when atoms come out of the liquid state and build up on the expanding edge of an existing crystal. The liquid is usually water. On other occa- sions, it is not a solvent at all but the molten mineral itself. The shape of the crystal, and the angles at which its plane facets meet, is a direct rendi- tion, in the large, of the atomic lattice. The lattice shape is sometimes projected very large indeed, as in a diamond or amethyst whose facets betray to the naked eye the three-dimensional geometry of the self- assembled atomic arrays. Usually, however, the crystalline units of which rocks are made are too small for the eye to detect them, which is one reason why most rocks are not transparent. Among important and com- mon rock crystals are quartz (silicon dioxide), feldspars (mostly silicon dioxide again, but some of the silicon atoms are replaced hy aluminium atoms), and calcite (calcium carbonate). Granite is a densely packed mixture of quartz, feldspar and mica, crystallised out of molten magma. Limestone is mostly calcite, sandstone mostly quartz, in both cases ground small and then compacted from sediments of sand or mud.

Igneous rocks begin as cooled lava (which in turn is molten rock). Of-

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72 ERGASTS

ten, as with granite, they are crystalline. Sometimes their shape may be visibly that of a glass-like solidified liquid and, with great good fortune, molten lava may sometimes be cast in a natural mould, such as a dino- saur's footprint or an empty skull. But the main usefulness of igneous rock to historians of life is in dating. As we shall see in the Redwood's Tale, the best dating methods are available for igneous rocks alone. Fos- sils usually cannot be precisely dated themselves, but we can look for ig- neous rocks in the vicinity. We then either assume that the fossil is con- temporaneous, or we seek two datable igneous samples that sandwich our fossil and fix upper and lower bounds to its date. This sandwich dat- ing is open to the slight risk that a corpse has heen carried by floodwater, or by hyenas or their dinosaur equivalents, to an anachronistic site. With luck this will usually be obvious; otherwise we have to fall back on con- sistency with a general statistical pattern.

Sedimentary rocks such as sandstone and limestone are formed from tiny fragments, ground by wind or water from earlier rocks or other hard materials such as shells. They are carried in suspension, as sand, silt or dust, and deposited somewhere else, where they settle and compact themselves over time into new layers of rock. Most fossils lie in sedimen- tary beds.

It is iri the nature of sedimentary rock that its materials are continu- ally being recycled. Old mountains such as the Scottish Highlands have been slowly ground down by wind and water, yielding materials which later settle into sediments and may ultimately push up again somewhere else as new mountains like the Alps, and the cycle resumes. In a world of such recycling, we have to curb our importunate demands for a continu- ous fossil record to bridge every gap in evolution. It isn't jnst bad luck that fossils are often missing, but an inherent consequence of the way sedimentary rocks are made. It would be positively worrying if there were no gaps in the fossil record. Old rocks, with their fossils, are actively being destroyed by the very process that goes to make new ones.

Often fossils are formed when mineral~charged water penetrates the fabric of a buried creature. In life, bone is porous and spongy, for good engineering and econon1ic reasons. When water seeps through the inter- stices of a dead bone, minerals are slowly deposited as the ages pass. I say slowly almost as a ritual, but it isn't always slow. Think how fast a kettle furs up. On an Australian beach I once found a bottletop embedded in stone. But the process usually is slow. Whatever the speed, the stone of a fossil eventually takes on the shape of the original bone, and that shape is

11 revealed to us millions of years later, even if - which doesn't always hap·

I pen - every atom of the original bone has disappeared. The petrified

THE RRGAST S TALE 73

forest in the Painted Desert of Arizona consists of trees whose tissues were slowly replaced by silica and other minerals leached out of ground water. 1\vo lrnndred million years dead, the trees are now stone through and through, but many of their microscopic cellular details can still be clearly seen in petrified form.

I've already mentioned that sometimes the original organism, or a part of it, forms a natural mould or imprint from which it is subse- quently removed, or dissolved. I fondly recall two happy days in Texas in l 987 spent wading through the Paluxy River examining, and even put- ting my feet in, the dinosaur footprints preserved in its s1nooth lirne- stone bed. A bizarre local legend grew up that some of these are giant manprints contemporary with undoubted dinosaur prints, and in conse- quence the nearby town of Glen Rose became home to a thriving cottage industry, artlessly faking giant manprints in blocks of cement (for sale to gullible creationists who know, all too well, that 'There were giants in the earth in those days': Genesis 6:4). The story of the real footprints has been carefully worked out, and is fascinating. The obviously dinosaurian ones are three-toed. The ones that look faintly like a human foot have no toes, and were made by dinosaurs walking on the back of the foot rather than running on their toes. Also, the viscous mud would have tended to ooze back in at the sides of the footprint, obscuring the side toes of the dinosaurs.

More poignant for us, at Laetoli in Tanzania are the companionable footprints of three real hominids, probably Australopithecus afarensis, walking together 3.6 n1illion years ago in what was then fresh' volcanic ash. Who does not wonder what these individuals were to each other, whether they held hands or even talked, and what forgotten errand they shared in a Pliocene dawn?

Sometimes, as I mentioned when discussing lava, the 1nould n1ay become filled with a different material, which subsequently hardens to form a cast of the original animal or organ. I am writing this on a table in the garden whose top is a six-inch thick, seven--foot square slab of Purbeck sedi111entary lirnestone, of Jurassic age, perhaps 150 1nillion years old.' Along with lots of fossil mollusc shells, there is an alleged (by the distinguished and eccentric sculptor who procured it for me) dino- saur footprint on the underside of the table, but it is a footprint in relief, standing out from the surface. The original footprint (if indeed it is gen- * !\ jonrnalist interviewed me at this two-tonne megalith for over an hour and then de- scribed it in his newspaper as a 'white wrought--iron table': my favourite example of the

of eye1vitness evidence.

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74 ERGASTS

uine, for it looks pretty nondescript to me) must have served as a mould, into which the sediment later settled. The mould then disappeared. Much of what we know about ancient brains comes to us in the form of such casts: 'endocasts' of the insides of skulls, often imprinted with sur- prisingly full details of the brain surface itself.

Less frequently than shells, bones or teeth, soft parts of animals some- times fossilise. The most famous sites are the Burgess Shale of the Cana- dian Rockies, and the slightly older Chengjiang in South China which we shall meet again in the Velvet Worm's Tale. At both these sites, fossils of worms and other soft, boneless and toothless creatures (as well as the usual hard ones) wonderfully record the Cambrian Period, more than half a billion years ago. We are outstandingly lucky to have Chengjiang and the Burgess Shale. Indeed, as I have already remarked, we are pretty lucky to have fossils at all, anywhere. It has been estimated that 90 per cent of all species will never be known to us as fossils. If that is the fig- ure for whole species, just think how few individuals can ever hope to achieve the ambition with which the tale began, and end up as fossils. One estimate puts the odds at one in a million among vertebrates. That sounds high to me, and the true figure must be far less among animals with no hard parts.

HABILINES

ACK ANOTHER mi1lion years frmn Homo ergaster, 2 million years ago there is no longer any doubt in

which continent our genetic · roots lie. Everyone agrees, in1ulti- regionalists' included, that Africa is the place. The most compelling fossil bones at this age are normally classified as Homo habilis. Some authori- ties recognise a second, very similar contemporary type, which they call Homo rudolfensis. Others equate it with Kenyapithecus, described by the Leakey team in 2001. Yet others ca ,1tiously refrain from giving these fos- sils a species name at all, and just call them all 'Early Homo'. As usual I shan't take a stand on names. What matters is the real flesh and bone creatures themselves, and I shall use 'Habilines' as an anglicism for all of them. Habiline fossils, being older, are understandably less plentiful than Ergasts. The best-preserved skull bears the reference number KNM-ER 1470 and is widely known as Fourteen Seventy. It lived about J.9 million years ago.

The Habilines were about as different from Ergasts as Ergasts from us, and, as we should expect, there were intermediates which are hard to classify. Habiline skulls are less robust than Ergast skulls, and lack the pronounced brow ridges. In this respect, Habilines were more like us. This should cause no surprise. Robustness and brow ridges are peculiari- ties that, possibly like hair, hominids seem able to acquire and lose again at the drop of an evolutionary hat.

Habilines mark the place in our history where the brain, that most dramatic of hmnan pecu]iarities, starts to expand. Or more accurately, starts to expand beyond the normal size of the already large brains of other apes. This distinction, indeed, is the rationale for placing the

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76 HABILINES

Habilines in the genus Homo at.all. For many palaeontologists, the large brain is the distingnishing feature of our genus. Habilines, with their brains pushing the 750 cc barrier, have crossed the rubicon and are hu- man.

As readers may soon become tired of hearing, I am not a lover of rubicons, barriers and gaps. ln par.ticular, there is no reason to expect an early Habiline to be separated from its predecessor by a bigger gap than from its successor. It might seem tempting because the predecessor has a different generic name (Australopithecus) whereas the successor (Homo ergaster) is 'entirely' another Homo. It is true that when we look at living

THE HANDYMAN'S TALE 77 ~··~~ •

given its body size. This is a topic worthy of a tale in its own right, and Homo habilis, handyman, from his uneasy vantage point straddling the brain-size 'rubicon), will tell it.

THE HANDYMAN'S TALE

E WANT TO KNOW whether the brain of a particular creature snch as Homo habilis is larger or smaller than it 'ought' to be,

given that animal's body size. We accept (slightly unwillingly in my case

species we expect members of djfferent genera to be less alike than but I'll let it pass) that large animals just have to have large brains and

members of different species within the sa1ne genus. But it can't work like that for fossils, if we have a continuous historical lineage in evolu- tion. At the borderline between any fossil species and its immediate pred- ecessor, there must be some individuals about whom it is absurd to ar- gue, since the reductio of such an argument must he that parents of one species gave birth to a child of the other. It is even more absurd to suggest that a baby of the genus Homo was born to parents of a completely dif- ferent genus, Australopithecus. These are evolutionary regions into which our zoological naining conventions were never designed to go.*

Setting names to one side frees us for a n1ore constructive discussion about why the brain suddenly started to enlarge. How would we measure the enlargement of the hominid brain and plot a graph of average brain size against geological ti1ne? There is no problem about the units in which ·we measure time: 1nillions of years. Brain size is harder. Fossil skulls and endocasts allow us to estimate brain size in cubic centimetres, and it is easy enough to convert this to grams. But absolute brain size is not necessarily the measure you want. An elephant has a bigger brain than a person) and it isdt just vanity that makes us think we are brainier than elephants. Tyrannosaurus's brain was not much smaller than ours, but all dinosaurs are regarded as small-brained, slow-witted creatures. What makes us cleverer is that we have bigger brains for our size than di- nosaurs. But what, more precisely) does 'for our size' mean?

There are mathe1natical methods of correcting for absolute size, and expressing an animal's brain size as a function of how big it 'ought to be'

* The 750 cc rubicon for the definition of Flomo was originally chosen by Sir Arthur Keith. As Richard Leakey tells us in The Origin of Humnnkind, when Louis Leakey first described Homo lrnbilis his specimen had a brain capacity of 650 cc, and Leakey actually moved the rubicon to accommodate it. Later specimens of Homo lrnhilis retrosrectively vindicated him by turning in figures closer to 800 cc. All grist to my anti-rubicon mill.

small animals small brains. Making allowance for this, we still want to know whether some species are 'brainier' than others. So, how do we make allowance for body size? We need a reasonable basis for calculating the expected brain size of an animal from its body size, so that we can de- cide whether the actual brain of a particular animal is larger or smaller than expected.

In our pilgrimage to the past, we happen to have met the problem in connection with brains, but similar questions can arise with respect to any part of the body. Do some animals have larger (or smaller) hearts, or kidneys, or shoulder-blades than they 'ought' to have for their size? If so, this might suggest that their way of life makes special demands on the heart (kidney or shoulder-blade). How do we know what size any bit of an animal 'ought' to be, given that we know its total body size? Note that 'ought to be' doesn't mean 'needs to have for functional reasons'. lt means 'would be expected to have, knowing what comparable animals have'. Since this is the Handyman's Tale, and since the Handyman's most surprising feature is his brain, we'll go on using brains for the sake of dis- cussion. The lessons we learn will be more general.

We begin by making a scatter plot of brain mass against body mass for a large number of species. Each symbol in the graph on the following page (from my colleague the distinguished anthropologist Robert Mar- tin) represents one species of living mammal - 309 of them, ranging from the smallest to the largest. In case you are interested, Homo sapiens is the point with the arrow, and the one irnmediately next to us is a dol- phin. The heavy black line drawn through the middle of the points is the straight line that, according to statistical calculation, gives the best fit to all the points.*

"" It is the line that minimises the sum of the squares of the distances of the points from it.

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78 HABILINES THE HANDYMAN'S TALE 79

kind of intuition from simple aritbmetic scales, which is useful for differ- ent purposes.

There are at least three good reasons for using a logarithmic scale. First, it makes it possible to get a pygmy shrew, a horse and a blue whale on the same graph without needing a hundred yards of paper. Second, it makes it easy to read off multiplicative factors, which is sometimes what

we want to do. We don't just want to know that we have a bigger brain than we should have for our body size. We want to know that our brain is, say, six times as big as it 'should' be. Such multiplicative judgements can be read directly off a logarithmic graph: that is what logarithmic means. The third reason for preferring logarithmic scales takes a little, longer to explain. One way of putting it is that it makes our scatterpoints fall along straight lines instead of curves, but there is more to it than that. Let me try to explain to my fellow dysnumerics.

Suppose you take an object like a sphere or a cube, or indeed a brain, and you inflate it evenly so it is still the same shape but ten times the size.

log body mass (g)

Log-log plot of brain mass against body mass for different species of placental mammal. Filled triangles represent primates. Adapted from Martin [ I 85].

A slight complication, which will make sense in a moment, is that things work better if we make the scales of both axes logarithmic, and that is how this graph was made. We plot the logarithm of an animal's brain mass against the logarithm of its body mass. Logarithmic means that equal steps along the bottom of the graph (or equal steps up the side) represent multiplications by some fixed number, say ten, rather than additions of a number, as in an ordinary graph. The reason ten is conve- nient is that we can then think of a logarithm as a count of the number of noughts. If you have to multiply a mouse's mass by a million to get an elephant's, this means you have to add six noughts to the mouse's mass: you have to add six to the logarithm of the one, to get the loga- rithm of the other. Half way between them on the logarithmic scale - three noughts - lies an animal that weighs a thousand times as much as a mouse, or a thousandth of an elephant: a person, perhaps. Using round numbers like a thousand and a million is just to make the explanation easy. 'Three and a half noughts' means somewhere between a thousand and ten thousand. Note that 'half way between' when we are counting noughts is a very different matter from half way between when we are counting grams. This is all taken care of automatically by looking up the logarithms of the numbers. Logarithmic scales call on a different

In the case of the sphere, this means ten times the diameter. In the case of the cube, or the brain, it means ten times the width (and height and depth). In all these cases of proportionate scaling up, what will happen to the volume? It will not be ten times as great - it will be a thousand times as great! You can prove it for cubes if you imagine stacking sugar lumps. The same applies to uniformly inflating any shape you like. Multiply length by ten and, provided the shape doesn't change, you automati- cally multiply volume by a thousand. In the special case of a tenfold infla- tion, th;s is equivalent to adding three noughts. More generally, volume is proportional to the third power of length, and the logarithm is multi- plied by three.

We can do the same sort of calculation for area. But area increases in proportion to the second power of length rather than the third power. Not for nothing is raising to the second power called squaring while rais- ing to the third power is called cubing. The volume of a sugar lump de- termines how much sugar there is, and what it costs. But how fast it dis- solves will be determined by its surface area (not a simple calculation because, as it dissolves, the remaining surface area will shrink 1nore slowly than the volume of sugar remaining). \Nhen you uniformly inflate an object by doubling its length (width, etc.), you multiply tbe surface area by 2 X 2 = 4, Multiply its length by ten, and you multiply the sur- face area by IO X 10 = 100 or add two noughts to the number. The loga- rithm of area increases as double the logarithm of length, while the loga- rithm of volume increases as treble the logarithm of length. A two- centimetre sugar lump will contain eight times as 1n11eh sugar as a one-

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80 HABILINES

centimetre lump, but it will release that sugar into the tea only four times as fast (at least initially), because it is the surface of the lump that is ex- posed to the tea.

Now imagine that we make a scatter plot of sugar lumps of a wide range of sizes, with mass of lump (proportional to volume) along the bottom axis, and (initial) rate of dissolving up the side of the graph (as- sumed proportional to area). In a non-logarithmic graph, the points will fall along a curved line, which will be quite hard to interpret and not very helpful. But if we plot the logarithm of mass against the logarithm of ini- tial dissolving rate, we shall see something much more informative. For every threefold increment of log mass, we shall see a doubling of log snr- face. On the log-log scale, the points will not fall along a curve, they will fall along a straight line. What is more, the slope of the straight line will mean something very precise. It will be a slope of two-thirds: for every two steps along the area axis, the line takes three steps along the volume axis. For every doubling of the logarithm of area, the logarithm of vol- ume is tripled. Two-thirds is not the only informative slope of line we might see in a log-log plot. Plots of this kind are informative because the slope of the line gives us an intuitive feel for what is going on vis-a-vis such things as volumes and areas. And volumes and areas and the com- plicated relationships between them are extremely important in under- standing living bodies and their parts.

I am not particularly mathematical - that's putting it mildly- but even I can see the fascination of this. And it gets better, because the same principle works for all shapes, not just tidy ones like cubes and spheres, but complicated shapes like animals and bits of animals such as kidneys and brains. All that is required is that size change should come about by simple inflation or deflation without a change of shape. This gives us a sort of null-expectation, against which to compare real measurements. If one species of animal is 10 times the length of another, its mass will be 1,000 times as great, but only if the shapes are the same. In fact, shape is very likely to have evolved to be systematically different as you go from small animals to large, and we can now see why.

Big animals need to be a different shape from small animals, if only because of the area/volume scaling rules we have just seen. lf you turned a shrew into an elephant just by inflating it, retaining the same shape, it wouldn't survive. Because it is now about a million times heavier, a whole lot of new problems arise. Some of the problems an animal faces depend upon volume (mass). Others depend on area. Still others depend on some con1plicated function o.f the two, or on some different consider· ation altogether. Like a sugar lump's rate of dissolving, an animal's rate of

THE HANDYMAN S TALE 81

losing heat, or of losing water through the skin, will be proportional to the area that it presents to the outside world. But its rate of generating heat is probably more related to the number of cells in the body, which is a functioh of volume.

A shrew scaled up to elephant size would have spindly legs that would break under the strain, and its slender muscles would be too weak to work. The strength of a muscle is proportional not to its volume but to its cross-sectional area. This is because muscular movement is the summed movement of millions of molecular fibres, sliding past each other in parallel. The number of fibres you can pack into a muscle de- pends upon the area of its cross-section (second power of linear size). But the task that the muscle has to perform - supporting an elephant, say- is proportional to the mass of the elephant (third power of linear size). So, the elephant needs proportionately more mnscle fibres than a shrew, in order to support its mass. Therefore the cross-sectional area of elephant muscles needs to be larger than you'd expect from simple scal- ing up, and the volume of n1uscle in an elephant must be n10re than you'd expect from simple scaling up. For different particular reasons, the conclusion is similar for bones. This is why large animals like elephants have massive tree-trunk shaped legs.

Suppose an elephant-sized animal is 100 times as long as a shrew- sized animal. With no change of shape, the area of its outer skin would be 10,000 times as great as the shrew's and its volume and mass a million times as great. If touch-sensitive cells are equally spaced through the skin, the elephant will need 10,000 times as many of them, and the part of the brain that services them will perhaps need to be scaled in propor- tion. The total number of cells in the elephant's body will be a million times as great as in the shrew, and they'll all have to be serviced by capil- lary blood vessels. What does this do to the number of miles of blood vessel that we expect in a large animal, as distinct from a small one? That's a complicated calculation, and one that we'll return to in a later tale. For the moment, it is enongh for us to understand that when we cal- culate it we cannot ignore these scaling rules for volumes and areas. And the logarithmic plot is a good method for getting intuitive clues to snch things. The main conclusion is that, as animals get larger or smaller in evolution, we positively expect their shape to change in predictable di- rections.

We got into this through thinking about brain size. We can't just com- pare our brains with those of Homo habilis, Australopithecus or any other species withont making allowance for body size. We need some index of brain size which 1nakes allowance for body size. We can't divide brain

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82 HABILINES

size by body size, though that would be better than just comparing abso- lute brain sizes. A better way is to make use of the logarithmic plots we have just been discussing. Plot the logarithm of brain mass against the

logarithm of body mass for lots of species of different sizes. The points will probably fall around a straight line, as indeed they do in the graph

on page 78. If the slope of the line is 1/ 1 (brain size exactly proportional to body size) it will suggest that each brain cell is capable of servicing some

fixed number of body cells. A slope of 2/, would suggest that brains are like bones and muscles: a given volume of body (or number of body

cells) demands a certain surface area of brain. Some other slope would need yet a different interpretation. So, what is the actual slope of the line?

It is neither 1/ 1 nor 2/, but something in between. To be exact, it is a re- markably good fit to¾. Why¾? Well, that is a tale in itself, which will be told, as you will no doubt have guessed, by the cauliflower (well, a brain does look a bit like a cauliflower). Without pre-empting the Cauliflower's

Tale, I will just say that the¾ slope is not special to brains, but crops up all over the place in all sorts of living creatures, including plants like

cauliflowers. Applied to brain size, and with the intuitive rationale that must wait for the Cauliflower's Tale, this observed line, with its ¾ slope, is the meaning we are going to attach to the word 'expect' as it was used

in the opening paragraphs of this tale. Although the points cluster about the 'expected' straight line of slope

¾, not all the points fall exactly on the line. A 'brainy' species is one whose point on the graph falls above the line. Its brain is larger than 'ex- pected' for its body size. A species whose brain is smaller than 'expected' falls below the line. The distance above, or below, the line, is our measure of how much bigger than 'expected', or smaller, it is. A point that falls ex- actly on the line represents a species whose brain is exactly the size ex- pected for its body size.

Expected on what assumption? On the assumption that it is typical of the set of species whose data contributed to calculating the line. So, if tlie line was calculated from a representative range of land vertebrates, from geckos to elephants, the fact that all mammals fall above the line (and all reptiles below) means that mammals have bigger brains than you would 'expect' of a typical vertebrate. If we calculate a separate line from a rep- resentative range of mammals, it will be parallel to the vertebrate line, still with a slope of¾, but its absolute height will be higher. A separate line calculated from a representative range of primates (monkeys and apes) will be higher again, but still parallel with a slope of¾. And Homo sapiens is higher than any of them.

The human brain is 'too' big, even by the standards of primates, and

THE HANDYMAN'S TALE 83

the average prin1ate brain is too big by the standards of ma1n1nals gener- ally. For that matter, the average mammal brain is too big by the stan- dards of vertebrates. Another way to say all this is that the scatter of points in tJie vertebrate graph is wider than the scatter of points on the mammal graph, which is in turn wider than the primate scatter which it includes. The xenarthran scatter of points on the graph (xenarthrans are an order of South American mam1nals, including sloths, anteaters and armadillos) sits below the average of mammals, of which the xenarthran scatter forms a part.

Harry Jerison, the father of fossil brain size studies, proposed an in- dex, the Encephalisation Quotient or EQ, as a measure of how much big- ger, or smaller, the brain of a particular species is than it 'should' be for its size, given that it is a member of some larger grouping, such as the vertebrates or the mammals. Notice that the EQ requires us to specify the larger group which is being used as the baseline for comparison. The EQ of a species is its distance above, or below, the average line for the specified larger grouping. Jerison thought the slope of the line was 2/,, whereas modern studies agree that it is ¾, so Jerison's own estimates of EQ have to be amended accordingly, as was pointed out by Robert Mar- tin. When this is done, it turns out that the modern human brain is about six times as big as it should be, for a mammal of equivalent size

( the EQ would be larger, if calculated against the standard of the verte- brates as a whole, rather than the mammals as a whole. And it would be smaller if calculated against the standard of primates as a whole).* A modern chimpanzee's brain is about twice the size it should be for a typi- cal mammal, and so are the brains of australopithecines. Homo habilis and Homo erectus, the species that are probably intermediate in evolu- tion between Australopithecus and ourselves, are also intermediate in brain size. Both have an EQ of about 4, meaning that their brains were about four times as big as they should have been for a mammal of equiv- alent size.

The graph on the following page shows an estimate of EQ, the 'brain- iness index', for various fossil primates and ape-men, as a function of the time at which they lived. With considerable pinches of salt you could

*Much the same applies to IQ. It is not an absolute measure of intelligence. Rather, your IQ reflects how much more (or less) intelligent you are than the average for a par- ticular population, that average being standardised at 100. My IQ if standardised against the background population of Oxford University ·would be lower than if stand- ardi sed against the background population of England. Hence the joke about the politi- cian lamenting the fact that half the population has an IQ less than 100.

84 HABlLINES THE HANDYMAN S TALE 85

raised, remember, to a power in the EQ formula. So, the scatter of points about the line largely reflects erratic estimation of body mass. On the other hand, the trend over time, as represented by the line, is probably real. The methc,ds explained in this tale, in particular the estimates of EQ

in the graph at the end, bear out our subjective impression that one of the most important things that has happened during the last 3 million years of our evolution was the ballooning of our already large primate brain. The next obvious question is why. What Darwinian selection pres- sure drove the enlargement of the brain during the past three million years?

Because it happened after we rose up on our hind legs, some people have suggested that brain inflation was driven by tbe freeing of the hands and the opportunity this offered for precision-controlled manual dexter-

Plot of EQ or 'braininess index' against time, for various fossil species. Time, in mil- lions of years, is plotted on a log scale. The results have been corrected for a slope of¾ for the reference baseline (see text).

read it as a rough graph of decreasing braininess as we go backwards in evolutionary time. At the top of the graph is modern Homo sapiens with an EQ of 6, meaning our brain is six times as heavy as it 'should' be for a typical mammal of our size. At the bottom of the graph are fossils who might possibly represent something like Concestor 5, our common an- cestor with the Old World monkeys. Their estimated EQ was about 1, meaning they had a brain which would be 'about right' for a typical mammal of their size today. Intermediate on the graph are various spe- cies of Australopithecus and Homo who might be close to our ancestral line at the time they lived. The drawn line is, once again, the straight line which best fits the points on the graph.

I advised pinches of salt, and let me raise that to ladles of salt. The EQ 'braininess index' is calculated from two measured quantities, the brain mass and the body mass. In the case of fossils, both these quantities have to be estimated from the fragments that have come down to us, and there is a huge margin of error, especially in the estimation of body mass. The point on the graph for Homo lwbilis shows it as 'brainier' than Homo erectus. I don't believe this. The absolute brain size of H. erectus is unde- niably larger. The inflation of the H. habilis EQ comes from the much lower estimated body mass. But to get an idea of the margin of error, think of the enormous range of body mass in modern humans. EQ as a measure is extremely sensitive to error in measuring body 1nass, which is

ity. In a general way I find this a plausible idea, though no more than sev- eral others that have been offered. But the enlargement of the human brain looks, as evolutionary trends go, explosive. I think inflationary evolution demands a special kind of inflationary explanation. In Un- weaving the Rainbow, in the chapter called 'The Balloon of the Mind', I developed this inflationary theme in a general theory of what I called 'software-hardware co-evolution'. The computer analogy is with software innovations and hardware innovations triggering each other in an esca- lating spiral. Software innovations demand an escalation in hardware, which in turn provokes an escalation in software, and so the inflation gathers pace. In the brain, my candidates for the kind of thing I meant by a software innovation were language, spoor-tracking, throwing, and memes. One theory of brain inflation that I didn't do justice to in my earlier book was sexual selection, and it is for this reason alone that 1 shall give it special prominence later in this book.

Could the enlarged human brain, or rather its products such as body painting, epic poetry and ritual dances, have evolved as a kind of mental peacock's tail? l have long had a soft spot for the idea, but nobody developed it into a proper theory until Geoffrey Miller, a young Ameri- can evolutionary psychologist working in England, wrote his book, The Mating Mind. We shall hear this idea in the Peacock's Tale, after the bird pilgrims join us at Rendezvous 16.

APE-MEN 87

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APE-MEN

THE POPULAR LITERATURE on human fossils is

hyped up with alleged ambition to discover the

'earliest' human ancestor. This is silly. You can ask a specific question like 'Which was the earliest human ancestor to walk habitually on two legs?' Or 'Which was the first creature to be our ancestor and not the ancestor of a chimpanzee?' Or Which was the earliest human ancestor to have a brain volume larger than 600 cc?' Those questions at least mean some- thing in principle, although they are hard to answer in practice and some of them suffer from the vice of erecting artificial gaps in a seamless continuum. But 'Who was the earliest human ancestor?' means nothing at all.

More insidiously, the competition to find human ancestors n1eans that new fossil discoveries are touted as on the 'main' human line when- ever remotely possible. But as the ground yields up more and more fos- sils it becomes increasingly clear that, during most of hominid history, Africa housed several species of hominid simultaneously. This has to mean that many fossil species now thought of as ancestral will turn out to be our cousins.

At various times since Homo first appeared in Africa, it shared the continent with more robust hominids, perhaps several different species of them. As usual their affinities, and the exact number of species, are hotly disputed. Names that have been attached to various of these creatures (we met them in the graph at the end of the Handyman's Tale) are Australopithecus (or Paranthropus) robustus, Australopithecus (or Paranthropus or Zinjanthropus) boisei, and Australopithecus (or Par-

anthropus) aethiopicus. They seem to have evolved from more 'gracile' apes (gracile being the opposite of robust). The gracile apes are also placed in the genus Australopithecus, and we too almost certainly emerged from among' gracile australopithecine ranks. Indeed, it is often dif- ficult to distinguish early Homa from gracile australopiiliecines -- which prompted my diatribe on ilie naming conventions that place them in sepa- rate genera.

The immediate ancestors of Homo would be classified as some kind of gracile australopithecine. Let's look at some of the gracile fossils. Mrs Pies is one for whom I have had special affection ever since the Transvaal Museum in Pretoria presented me with a beautiful cast of her skull, on the fiftieth anniversary of her discovery at Sterkfontein nearby, when I gave the Robert Broom Memorial Lecture in honour of her discoverer. She lived about 2.5 million years ago. Her nickname comes from the genus Plesianthropus, to which she was originally assigned before peo- ple decided to incorporate her into Australopithecus; and from the fact that she was thought (perhaps erroneously as is now suspected) to be fe- male. Individual fossil hominids often pick up pet names like this. 'Mr Pies', naturally, is a more recently discovered fossil from Sterkfontein who is in the same species as Mrs Pies, Australopithecus africanus. Other fossils with nicknames include 'Dear Boy', a robust australopithecine also known as 'Zinj' because he was originally named Zinjrmthropus boisei, 'Little Foot' (see below) and the famous Lucy, to whom we now turn.

VVe meet Lucy as our time machine's odometer touches 3.2 million years. Another gracile australopithecine, she is often mentioned because her species, Australopithecus afCJrensis, is a hot contender for a hmnan an- cestor. Her discoverers, Donald Johanson and his colleagues, also found fossils of 13 si1nilar individuals in the same area) knm,vn as the 'First Family'. Other 'Lucys' have since been found between about 3 and 4 mil- lion years ago in other parts of East Africa. The 3.6-million-year-old footprints discovered by Mary Leakey at Laetoli (page 73) are attributed to A. afarensis. Whatever the Latin name, evidently somebody was walk- ing bipedally at that time. Lucy is not greatly different from Mrs Pies, and some people think of Lucys as an earlier version of Mrs Pies. They are anyway more like each other than either is like the robust australopith- ecines. Early East African Lucys are said to have a slightly smaller brain than later South African Mrs Pleses, but there isn't much in it. Their brains were no n1ore different from each other than some modern hu- man brains are from other modern human brains.

As we have come to expect, the more recent afarensis individuals such as Lucy are slightly different from the earliest 3.9-million-year-old

APE-MEN 88

afarensis forms. Differences collect over time and, as we emerge from our time 1nachine 4 1nillion years ago, we find more creatures who might well be ancestral to Lucy and her kin, hut who are sufficiently differ- ent, in the direction of being more chimpanzee-like, to m.erit a different species nan1e. Discovered by Meave Leakey and her team, these Aus- tralopithecus anamensis consist of more than 80 fossils from two differ- ent sites near Lake Turkana. No intact skull has been found, but there is a splendid lower jaw which plausibly could belong to an ancestor of

ours. But the most exciting discovery from this time period, and a good

reason for calling a temporary halt here, is a fossil yet to be fully de- scribed in print. Affectionately known as Little Foot, this skeleton from the Sterkfontein caves of South Africa was originally dated to about three million years ago, but has recently been redated to just over four million. Its discovery is a piece of detective work worthy ofa Conan Doyle story. Bits of Little Foot's left foot were dug up from Sterkfontein in 1978, but the bones were stored away, unremarked and unlabelled, un- til 1994 when the palaeontologist Ronald Clarke, working under the di- rection of Phillip Tobias, accidentally rediscovered them ina box in the shed used by workers at the Sterkfontein cave. Three years later, Clarke chanced upon another box of bones from Sterkfontein, ina store room at Witwatersrand University. This box was labelled 'Cercopitbecoids'. Clarke bad an interest in this kind of monkey, so he looked in the box and was delighted to notice a hominid foot bone in amongst the monkey bones. Several foot and leg bones in the box seemed to match the bones previously found in the Sterkfontein shed. One was half a right shin- bone, broken across. Clarke gave a cast of the shinbone to two African assistants, Nkwane Molefe and Stephen Motsumi, and asked them to re- turn to Sterkfontein and look for the other half.

The task I had set them was like looking for a needle in a haystack as the grotto is an enormous, deep, dark cavern with breccia exposed on the walls, floor and ceiling. After two days of searching ·with the aid of hand-held lamps, they found it on 3 July 1997.

Molefe and Motsumi's jigsaw feat was the more astonishing because the bone that fitted their cast was

at the opposite end to where we had previously excavated. The fit was perfect, despite the bone having been blasted apart by lime workers 65 or more years previously. To the left of the exposed end of the right

APE-MEN 89

tibia could be seen the section of the broken-off shaft of the left tibia, to which the lower end of the left tibia '""'ith foot bones could be joined. To the left of that could be seen the broken-off shaft of the left fibula. From their po,Sitions with the lower limbs in correct anatomical rda- tionship, it seemed that the whole skeleton had to be there, lying face downwards,

Actually, it wasn't quite there but, after pondering the geological col- lapses in the area, Clarke deduced where it must be and, sure enough, Motsumi's chisel found it there. Clarke and his team were indeed lucky, but here we have a first-class example of that maxim of scientists since Louis Pasteur: 'Fortune favours the prepared mind.'

Little Foot is still to be fully excavated, described and formally named, but preliminary reports suggest a spectacular find, rivalling Lucy in com- pleteness but older. Although more human-like than chimpanzee-like, the big toe is more divergent than our toes. This might suggest that Little Foot grasped tree boughs with its feet in a way that we cannot. Although it almost certainly walked bipedally, it probably climbed too and walked with a different gait from us. Like other australopithecines, it may have spent time in trees, perhaps bivouacking in them at night like modern chimpanzees.

Having paused at the 4-million-year milestone, let's take a quick peek at the journey yet to unfold. There are some fragmentary remains of a possibly bipedal Australopithecus-like creature even further back in time, about 4.4 million years ago. Tim -white and his colleagues discov- ered it in Ethiopia, quite close to Lucy's last resting place. They named it Ardipithecus ramidus,' although some prefer to keep it in the genus Australopithecus. No skull of Ardipithecus has so far been found, but its teeth suggest that it was more chimpanzee-like than any later humans. Its tooth enamel was thicker than that of chimpanzees, but not as thick as ours. A few isolated cranial bones have been found, and these indicate that the skull rested on top of the vertebral column, as iri us, rather than in front of it, as in chimpanzees. This suggests a vertical stance, and such foot bones as have been found support the idea that Ardipithecus was bipedal.

Bipedality separates humans from the rest of the mammals so dra- - matically that I feel it deserves a tale to itself. And who better fitted to tell

it than Little Foot?

• Some people distinguish a second species, Ardipithecus kadahha.

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90 APE-MEN

LITTLE FOOT'S TALE

T rsN'T PARTICULARLY helpful to dream up reasons why walking on two legs might be generally a good thing. If it were, the chimps would

do it too, to say nothirig of other mam1nals. There is no obvious reason for saying that either bipedal or quadrupedal running is faster or more efficient than the other. Galloping mammals can be astonishingly fleet, using the up-and-down flexibility of the backbone to achieve - among other benefits - a lengthened effective stride. But ostriches show that a man-like bipedal gait can be a match for a quadrupedal horse. Indeed a top human sprinter, though noticeably slower than a horse or dog (or os- trich or kangaroo, for that matter), is not disgracefully slow. Quadrupe- dal monkeys and apes are generally undistinguished runners, perhaps because their bodily designs have to compromise with the needs of a climber. Even baboons, which normally forage and run on the ground, resort to the trees to sleep and as a defence against predators, but ba- boons can run fast when they need to.

So, when we ask why our ancestors rose up on their hind legs, and when we imagine the quadrupedal alternative that we forsook, it is un- fair to 'think cheetah', or anything like it. When our ancestors first stood up, there was no overwhelmingly strong advantage in efficiency or speed. We should look elsewhere for the natural selection pressure which drove us to this revolutionary change in gait.

Like some other quadrupeds, chimpanzees can be trained to walk bipedally, and they often do it anyway over short distances. So it proba- bly wouldn't be insuperably difficult for them to make the switch if there were strong benefits to doing so. Orang utans are even better at it. Wild gibbons, whose fastest method of locomotion is brachiation - swinging under the boughs by their arms - also run across clearings on their hind legs. Some monkeys rise upright, to peer over long grass or to wade through water. A lemur, Verreaux's sifaka, although it lives mainly in trees where it ls a spectacular acrobat, 'dances' across the ground be- tween trees on its hind legs, the arms held up with balletic grace.

Doctors sometimes ask us to run on the spot in a mask, so they can measure our oxygen consumption and other metabo1ic indices when we are exerting ourselves. In 1973 some American biologists, C. R. Taylor and V. J. Rowntree, did this with trained chimpanzees and capuchin monkeys, running on a treadmill. By making the animals run the tread- mill either on four legs or on two (they were given something to hold on to), the researchers could compare the oxygen consumption and ,f. ficiency of the two gaits. They expected that quadrupedal running would

LITTLE FOOT'S TALE 91

be more efficient. This, after all, is what both species naturally do, and it is what their anatomy fits them for. Maybe bipedalism was helped by the fact that they had something to hold on to. In any case, the resnlt was otherwise. There was no significant difference between the oxygen con- sumption of the two gaits. Taylor and Rowntree concluded that:

The relative energy cost of bipedal versus quadrupedal running should not be used in arguments about the evolution of bipedal locomotion in man.

Even if this is an exaggeration, it should at least encourage us to look elsewhere for possible benefits of our unusnal gait. It arouses the suspi- cion that, whatever non-locomotor benefits of bipedality we might pro·· pose as drivers of its evolution, they probably did not have to fight against strong locomotor costs.

What might a non-locomotor benefit look like? A stimulating sugges- tion is the sexual selection theory of Maxine Sheets-Johnstone, of the University of Oregon. She thinks we rose on our hind legs as a means of showing off our penises. Those of us that have penises, that is. Females, in her view, were doing it for the opposite reason: concealing their geni- tals which, in primates, are more prominently displayed on all fours. This is an appealing idea but I don't carry a torch for it. I mention it only as an example of the kind of thing I mean by a non-locomotor theory. As with so many of these theories, we are left wondering why it would apply to our lineage and not to other apes or monkeys.

A different set of theories stresses the freeing of the hands as the really important advantage ofbipedality. Perhaps we rose on our hind legs, not because that is a good way of getting about, but because of what we were then able to do with our hands - carry food, for instance. Many apes and monkeys feed on plant matter that is widely available but not partic- ularly rich or concentrated, so you 1nust eat as you go, more or less con- tinuously like a cow. Other kinds of food snch as meat or large under- ground tubers are harder to acquire but, when you do find them, they are valuable worth carrying home in greater quantity than you can eat. When a leopard makes a kill, the first thing it normally does is drag it up a tree and hang it over a branch, where it will be relatively safe from ma- rauding scavengers and can be revisited for meals. The leopard uses its powerful jaws to hold the carcass, needing all four legs to climb the tree. Having much smaller and weaker jaws than a leopard, did our ancestors benefit from the skill of walking on two legs because it freed their hands for carrying food - perhaps back to a mate or children, or to trade fa-

92 APE-MEN

vours with other companions, or to keep in a larder for future needs? Incidentally the latter two possibilities may be closer to each other than

they appear. The idea (I attribute this inspired way of expressing it to Steven Pinker) is that before the invention of the freezer the best lar- der for meat was a companion's belly. How so? The meat itself is no longer available, of course, but the goodwill it buys is safe in long-term storage in a companion's brain. Your cmnpanion will remember the fa- vour and repay it when fortunes are reversed.* Chi1npanzees are known to share meat for favours. In historic times, this kind of 1. o. u. became tokenised as money.

A particular version of the 'carrying food home' theory is that of the American anthropologist Owen Lovejoy. He suggests that females would often have been hampered in their foraging by nursing infants, there- fore unable to travel far and wide looking for food. The consequent poor nutrition and poor milk production would have delayed weaning. Suck· ling females are infertile. Any male who feeds a nursing female acceler- ates the weaning of her current child and brings her into receptiveness earlier. When this happens, she might make her receptiveness especially available to the male whose provisioning accelerated it. So, a male who can bring lots of food home might gain a direct reproductive advantage over a rival male who just eats where he finds. Hence the evolution of bipedalism to free the hands for carrying.

Other hypotheses of bipedal evolution invoke the benefits of height, perhaps standing upright to look over the long grass; or to keep the head above water while wading. This last is the imaginative 'aquatic ape' the- ory of Alister Hardy, ably championed by Elaine Morgan. Another the- ory, favoured by John Reader in his fascinating biography of Africa, sug- gests that upright posture minimises exposure to the sun, lilniting it to the top of the head which is consequently furnished with protective hair. Moreover, when the body is not hunched close to the ground, it can lose heat more rapidly.

My colleague the distinguished artist and zoologist Jonathan King· don has centred a whole book, Lowly Origin, around the question of the evolution of human bipedality. After a lively review of 13 more-or-less distinct hypotheses, including the ones I have mentioned, Kingdon ad- vances his own sophisticated and multifaceted theory. Rather than seek

"' There is a well-developed theory of reciprocal altruism in Darwinism, beginning with the pioneering work of Robert 1i_.ivers and continuing with the modelling of Rober! Axelrod and others. Trading favours, with delayed repayment, really works. My ov.:n ex- position of it is in The Selfish Gene, especially the second edition.

LITTLE i:ooT's TALE 93

an immediate benefit of walking upright, Kingdon expounds a complex of quantitative anatomical shifts which arose for some other reason, but which then J:!lade it easier to become bipedal (the technical term for this kind of thing is pre-adaptation). The pre-adaptation that Kingdon pro- poses is what he calls squat feeding. Squat feeding is fan1iliar fron1 ba~ boons in·open country, and Kingdon visualises something siinilar in our ape ancestors. in the forest, turning over stones or leaf litter for insects, worms, snails and other nutritious morsels. To do this effectively they would have had to undo some of their adaptations to living up trees. Their feet, previously hand-like for gripping branches, would have be- come flatter, forming a stable platform for squatting on the haunches. You will already be getting a glimmering of where the argument is going. Flatter, less hand- like feet for squatting are later going to serve as pre- adaptations for upright walking. And you will, as usual, understand that this apparently purposeful way of talking - they had to 'undo' their tree- swinging adaptations, etc. - is a shorthand which is easily translated into Darwinian terms. Those individuals whose genes happened to make their feet more suitable for squat feeding survived to pass on those genes because squat feeding was efficient and aided their survival. 1 shall con- tinue to employ the shorthand because it chimes with the way humans naturally think.

A tree-swinging, 'brachiating' ape could fancifully be said to walk up- side down under the branches - run and leap in the case of an athletic gibbon - using the arms as its 'legs' and the shoulder girdle as its 'pelvis'. Our ancestors probably passed through a brachia ting phase, and the true pelvis consequently became rather inflexibly bound to the trunk by long blades of bone, which form a substantial part of a rigid trunk that can be swung as a single unit. Much of this, according to Kingdon, would have needed to change, to make an efficient squat feeder out of an ancestral brachiator. Not all, however. The arms could have remained long. In- deed, long brachia ting arms would have been a positively beneficial 'pre- adaptation', increasing the reach of the squat feeder and decreasing the frequency with which it had to shuffle to a new squatting position. But the massive, inflexible, top-heavy ape trunk would have been a disadvan- tage in a squat feeder. The pelvis would have needed to free itself and be- come less rigidly tied to the trunk, and its blades would have shrunk - to more human proportions. This, to anticipate the later stages of the argu- ment again (you might say that anticipation is what a pre-adaptation ar- gument is all about) just happens to make a better pelvis for bipedal walking. The waist becaine more flexible, and the spine was held n1ore vertically, to allow the squat-feeding ani1nal to search all around with its

W

APE-MEN 94

arms, turning on the platform of the flat feet and the squatting haunches. The shoulders became lighter and the body less top-heavy. And the point is that these subtle quantitative changes, and the balancing and compen- sating shifts that went with them, incidentally had the effect of 'prepar- ing' the body for bipedal walking.

Not for a moment is Kingdon proposing any kind of anticipation of the future. It is just that an ape whose ancestors were tree-swingers, but which has switched to squat feeding on the forest floor, now has a body which feels relatively comfortable walking on its hind feet. And it would have begun to do this while squat feeding, shuffling to a new squatting

position as the old one became depleted. Without realisiug what was happening, squat feeders were, over the generations, preparing their

hodies to feel more comfortable when upright and on two legs; to feel more awkward on four. I use the word comfortable deliberately. It is not a trivial consideration. We are capable of walking on all fours like a typi- cal mammal, hut it is uncomfortable: hard work, because of our altered body proportions. Those proportional changes which now make us feel comfortable on two legs originally came about, Kingdon suggests, in the service of a minor shift in food habits - to squat feeding.

There is much more in Jonathan Kingdon's subtle and complex the- ory, hut l will now recommend his hook, Lowly Origin, and move on. My own slightly wayout theory of bipedality is very different hut not incom- patible with bis. Indeed, most of the theories of hnman hipedality are mutually compatible, with the potential to assist rather than oppose one another. As in the case of the enlargement of the human brain, my tenta- tive suggestion is that bipedality may have evolved through sexual selec- tion, so again I postpone the matter to the Peacock's 1ale.

Whatever theory we believe about the evolutionary origins of human hipedality, it subsequently turned out to he an extremely important event. In former times it was possible to believe, as respected anthropol- ogists did up to the 1960s, that the decisive evolutionary event that first separated us from the other apes was the enlargement of the brain. Ris- ing up on the hind legs was secondary, driven by the benefits of freeing the hands to do the kind of skilled work which the enlarged brain was now capable of controlling and exploiting. Recent fossil finds point deci- sively towards the reverse sequence. Bipedality came first. Lucy, who lived long after Rendezvous 1, was bipedal, nearly or completely as bipedal as we are, yet her brain was approxi1nately the smne size as a chimpanzee's, The enlargement of the brain could still have been associated with the freeing of the hands, but the sequence of events was reversed. If anything

EPILOGUE TO LITTLE FOOT'S TALE 95

it would he the freeing of the hands by bipedal walking that drove the en- largement of the brain. The manual hardware came first, then the con- trolling brainware evolved to take advantage of it, rather than the other way around. ,.

EPILOGUE TO LITTLE FOOT'S TALE

H A T E VE R THE REASON for the evolution of bipedality, recent

fossil discoveries seem to indicate that hominids were already bipedal

at a date which is pushing disconcertingly close to Rendezvous 1, the fork between ourselves and chimpanzees (disconcerting because it

seems to leave little time for hipedality to evolve). In the year 2000, a French team led by Brigitte Senut and Martin Pickford announced a

new fossil from the Tugen Hills, east of Lake Victoria in Kenya. Dubbed 'Millennium Man'. dated at 6 million years and given yet another new ge- neric name, Orrorin tugenensis was also, according to its discoverers, bipedal. Indeed, they claim that the top of its femur, near the hip joint, was more human-like than that of Australopithecus. This evidence, sup- plemented by fragments of skull bones, suggested to Senut and Pickford

that orrorins are ancestral to later hominids and that Lucys are not. These French workers go further and suggest that Ardipithecus might he

ancestral to modern chimpanzees rather than to us. Clearly we need more fossils to settle these arguments. Other scientists are sceptical of

these French claims, and some doubt that there is enough evidence to show whether Orrorin was or was not bipedal. If it was, since 6 million years is approximately the time of the split from chimpanzees according to molecular evidence, this raises difficult questions about the speed with which hipedality must have arisen.

If a bipedal Orrorin pushes hack alarmingly close to Rendezvous I, a newly discovered skull from Chad in southern Sahara, found by another French team led by Michel Brunet, is even more disturbing to accepted ideas. This is partly because it is so old, and partly because the site is far to the west of the Rift Valley (as we shall see, many authorities had thought early hominid evolution confined to the east of the Rift). Nick- named Toumai (Hope of Life in the local Goran language) its official name is Sahelanthropus tchadensis, after the Sahel region of the Sahara in Chad where it was found. It is an intriguing skull, looking rather human from in front (lacking the protruding face of a chimpanzee or gorilla) but chimpanzee-like from behind, with a chimpanzee-sized hraincase. It

96 APE-MEN

has an extremely well-developed brow- ridge, even thicker than a gorilla's, which is the main reason for thinking 1oumai was male. The teeth are rather human-like, es- pecially the thickness of the enamel which is intennediate between a chimpanzee's and our own. The foramen magnum (the big hole through which the spinal cord passes) is placed further forward tban in a chimpanzee or gorilla, suggesting to Bru- net himself, though uot to some others, that Toumai was bipedal. Ideally, this

E P l L O G U E T O I. I T T I. E F O O T ' S T A L E 97

to be four ways (or some combination from among the four) in which we might respond to Orrorin and Toumai.

1. Orrorin and/or Toumai walked on all fours. This is not un- likely, but the remaining three possibilities assume, for the sake of argument, that it is wrong. lf we accept option 1, the prob- lem just goes away.

2. An extremely rapid burst of evolution occurred immediately afrer Concestor I, which itself walked on all fours likea chim- panzee. The more humanoid Toumai and Orrorin evolved their bipedality so swiftly after Concestor I that the separation

Hope of Life. Skull of Sahelantlirop115 tchadensis, or '1bumai', discovered in the Sahel region of Chad by Michel Brunet and colleagues in 2001.

should be confirmed by pelvis and leg bones but, unfortunately, nothing but a skull has so far been found.

There are no volcanic remains in the

in dates cannot easily be resolved.

3. Humanoid features such as bipedality have evolved more than once, maybe many times. Orrorin and Toumai could represent earlier occasions when African apes experimented with

area to provide radiometric dates, and Brunel's team had to use other fossils in the area as an indirect clock. These are compared with already known faunas from other parts of Africa which can be dated absolutely. The comparison yields a date for Toumai of between 6 and 7 million years. Brunet and his colleagues claim it as older than Orrorin, which has predictably elicited indignant ripostes from Orrorin's discoverers. One of them, Brigitte Senut, of the Natural History Museum in Paris, has said that Toumai is 'a female gorilla', while her colleague Martin Pickford de- scribed Toumai's canine teeth as typical 'of a large female monkey'. These were the two, remember, who (perhaps rightly) wrote off the human credentials of Ardipithecus, another threat to the priority of their own baby, Orrorin. Other authorities have bailed Toumai more generously: 'Astonishing.' 'Amazing.' 'This will have the impact of a small nuclear bomb.'

If their discoverers are right that Orrorin and Toumai were bipedal, this poses problems to any tidy view of human origins. The naive expec- tation is that evolutionary change spreads itself uniformly to fill the time available for it. If 6 million years elapsed between Rendezvous 1 and modern Homo sapiens, the qnantity of change ought to be spun out, rata one might naively think, through the 6 million years. But Orrorin and Toumai both lived very close to the date identified from molecular evidence as that of Concestor 1, the split between our line and that chi1npanzees. These fossils even pre-date Concestor 1 according to

datings. Assuming that the molecular and fossil dates are correct, there

bipedality, and perhaps other human features too. On this hy- pothesis, they could indeed pre-date Concestor 1 while being bipedal, and our own lineage would constitutea later foray into bipedality.

4. Chimpanzees and gorillas descend from more human-like, even bipedal ancestors, and have reverted to all fours more re- cently. On this hypothesis, Toumai, say, could actually be Concestor I.

The last three hypotheses all have difficnlties, and many authorities are driven to doubt either the dating, or the supposed bipedality, of 1oumai and1 Orrorin. But if we accept these for the moment and look at the three

hn o t hes es that assume ancient bipedality, there is no strong theoretical reason to favour or disfavour any particular one of them. We shall learn from

the Galapagos Finch's Tale and the Lungfish's Tale that evolution can be extremely rapid or can be extremely slow. So Theory2 is not im- plausible. The Marsupial Mole's Tale will teach us that evolution can fol- low the same path, or

strikingly parallel paths, on more than one occa- sion. There's nothing particularly implausible, then, about Theory 3. 'theory 4, at first sight, seems the most surprising. We are so used to the idea that we have risen 'up' from the apes

that Theory4 seems to put the ·cart before the horse, and may even insult human dignity into the bar-

gttin (often good fora laugh in my experience). Also there isa so-called law, Dollo's Law, which states that evolution never reverses itself, and it

,Jilight seem that Theory 4 violates it.

98 APE-!v1EN

The Blind Cave Fish's Tale, which is about Dollo's Law, will reassure us that this last is not the case. There is nothing in principle wrong with Theory 4. Chimpanzees really could have passed through a more hu- manoid, bipedal stage before reverting to quadrupedal apehood. As it happens, this very suggestion has been revived by John Gribbin and Jeremy Cherfas, in their two books, The Monkey Puzzle and The First Chimpanzee. They go so far as to suggest that chimpanzees are descended from gracile australopithecines (like Lucy), and gorillas from robust aus- tralopithecines (like 'Dear Boy'). For such an in-your-face radical sug- gestion, they make a surprisingly good case. It centres on an interpreta- tion of human evolution which has long been widely accepted, although not without controversy: people are juvenile apes who have become sex- ually mature. Or, putting it another way, we are like chimpanzees who have never grown up.

1be Axolotl's Tale explains the theory, which is known as neoteny. To summarise, the axolotl is an overgrown larva, a tadpole with sex organs. In a classic experiment by Vilern Laufberger in Gennany, honnone injec- tions persuaded an axolotl to grow into a fully adult salamander of a spe- cies that nobody had ever seen. More famously in the English-speaking world, Julian Huxley later repeated the experiment, not knowing it had already been done. In the evolution of the axolotl, the adult stage had been chopped off the end of the lifo cycle. Under the influence of experi- mentally injected hormone, the axolotl finally grew up, and an adult sala- mander was recreated, presumably never before seen. The missing last stage of the life cycle was restored.

The lesson was not lost on Julian's younger brother, the novelist Aldous Huxley. His After Many a Summer* was one of my favourite nov- els when I was a teenager. It is about a rich man, Jo Stoyte, who resembles William Randolph Hearst and collects objets d'art with the same vora- cious indifference. His strict religious upbringing has left him with a ter- ror of death, and he employs and equips a brilliant but cynical biologist, Dr Sigismund Obispo, to research how to prolong life in general and Jo Stoyte's life in particular. Jeremy Pordage, a (very) British scholar, has been hired to catalogue some eighteenth-century manuscripts recently acquired as a job lot for Mr Stoyte's library. In an old diary kept by the Fifth Earl of Gonister, Jeremy makes a sensational discovery which he imparts to Dr Obispo. The old Earl was hot on the trail of everlasting life (you have to eat raw fish guts), and there is no evidence that he ever died. Obispo takes the increasingly fretful Stoyte to England in quest of the

EPILOGUE TO LITTLE FOOT'S TALE 99

Fifth Earl's remains ... and finds him still alive at 200. The catch is that he has finally matured from the juvenile ape which all the rest of us are intoa fully adult ape: quadrupedal, hairy, repellent, nrinating on the floor while humming a grotesquely distorted vestige of a Mozart aria. The diabolical Dr Obispo, beside himself with gleeful laughter and evi- dently acquainted with Julian Huxley's work, tells Stoyte he can start on the fish guts tomorrow.

Gribbin and Cherfas are in effect suggesting that modern chimpan- zees and gorillas are like the Earl of Gonister. They are humans (or aus- tralopithecines, orrorins or sahelanthropes) who have grown up and be- come quadrupedal apes again, like their, and our, more distant ancestors. I never thought the Gribbin/Cherfas theory was obviously silly. The new findings of very ancient hominids like Orrorin and Tonmai, whose dates push up against onr split with chimpanzees, could almost justify them in a sotto voce 'vVe told you so'.

Even if we accept Orrorin and Toumai as bipedal, I would not choose with confidence between Theories 2, 3 and 4. And we mustn't forget Theory I, that they walked on all fours and the problem goes away, which many people think is the most plausible. But of course these dif- ferent theories make predictions about Concestor 1, our next stopping point. Theories l, 2, and 3 agree in assuming a chimpanzee-like Concestor 1, walking on all fours, but occasionally rising on the hind legs. Theory4 by contrast differs in assuming a more humanoid Concestor 1. In narrat- ing Rendezvous 1, I have been forced to make a decision hetween the the- ories. Somewhat reluctantly, I'll go with the majority, and assumea chimpanzee-like concestor. On to meet it.

* The American edition rounds off the 1E'nnyson quotation: 'Dies the Swan.'

B

RENDEZVOUS I

CHIMPANZEES

ET W EE N 5 AND 7 1nillion years ago, somewhere in Africa, we human pilgrims enjoy a momentous

encounter. It is Rendezvous 1, our first meeting with pilgrims from another species. Two other species to be precise, for the common chim- panzee pilgrims and the pygmy chimpanzee or bonobo pilgrims have already joined forces with each other some 4 million years 'before' their rendezvous with us. The common ancestor we share with them, Con- cestor l, is our 250,000-greats-grandparent an approximate guess this, of course, like the comparable estimates that I shall be making for other concestors.

As we approach Rendezvous 1, then, the chimpanzee pilgrims are ap- proaching the same point from another direction. Unfortunately we don't know anything about that other direction. Although Africa has yielded up some thousands of hominid fossils or fragments of fossils, not a single fossil has ever been found which can definitely be regarded as along the chimpanzee line of descent from Concestor 1. This may be be- cause they are forest animals, and the leaf litter of forest floors is not friendly to fossils. Whatever the reason, it means that the chimpanzee pilgrims are searching blind. Their equivalent contemporaries of the Turkana Boy, of 1470, of Mrs Pies, Lucy, Little Foot, Dear Boy, and the rest of 'our' fossils - have never been found.

Nevertheless, in our fantasy the chimpanzee pilgrims meet us in some Pliocene forest clearing, and their dark brown eyes, like our less predict- able ones, are fixed upon Concestor 1: their ancestor as well as ours. In

Chimpanzees join. White lines depict the evolutionary tree (or 'phylogeny') of chimps and humans, branching apart at Concestor I (marked by a numbered circle). The vertical right branch represents the current set of pilgrims: in this case, only humans. The left hranch shows chimps split- ting into two species about 2 million years ago.

If we were to zoom in on any of the lines, we would find them not solid, but crisscrossing networks of inter- breeding, as depicted in the hu man- kind diagram at Rendezvous 0. From now on we'll continue to use this wlid line representation.

Images, left to right: common chim- panzee (I'an trogfodytes); bonoho (I'i111 panisrns)

l02 RENDEZVOUS 1

trying to imagine the shared ancestor, an obvious question to ask is, is it more like modern chimpanzees or modern humans, is it intermediate, or completely different from either?

Notwithstanding the pleasing speculation that ended the previous section - which I would by no means rule out - the prudent answer is that Concestor 1 was more like a chimpanzee, if only because chimpan- zees are more like the rest of the apes than humans are. Hu1nans are the odd ones out among apes, both living and fossil. Which is only to say that more evolutionary change has occurred along the human line of de- scent from the common ancestor, than along the lines leading to the chimpanzees. \Ve must not assume, as many layn1en do, that our ances- tors were chimpanzees. Indeed, the very phrase 'missing link' is sugges- tive of this misunderstanding. You still hear people saying things like, 'Well, if we are descended from chimpanzees, why are there still chim- panzees around?'

So, when we and the chimpanzee/bonobo pilgrims meet at the ren- dezvous point, the likelihood is that the shared ancestor that we greet in that Pliocene clearing was hairy like a chimpanzee, and had a chimpan- zee-sized brain. Reluctantly to set aside the speculations of the previous chapter, it probably wall,ed on its hands (knuckles) like a chimp, as well as its feet. It probably spent some time up trees, but also lots of time on the ground, maybe squat feeding as Jonathan Kingdon would say. All availahle evidence suggests that it lived in Africa, and only in Africa. lt probably used and made tools, following local traditions as modern chimpanzees still do. It probably was omnivorous, sometimes hunting, but with a preference for fruit.

Bonobos have been seen to kill duikers, but hunting is more fre- quently documented for common chimpanzees, including highly co- ordinated group pursuits of colobus monkeys. But meat is only a supple- ment to fruit, which is the main diet of both species. Jane Goodall, who first discovered hunting and intergroup warfare in chimpanzees, was also the first to report their now famous habit of termite fishing, using tools of their own construction. Bonobos have not been seen to do this, but that may be because they have been studied less. Captive bonobos readily use tools. Common chimpanzees in different parts of Africa develop lo- cal traditions of tool use. Where Jane Goodall's animals on the east side of the range fish for termites, other groups to the west have developed lo- cal traditions of cracking nuts using stone or wood hammers and anvils. Some skill is required. You have to hit hard enough to break the kernel but not so hard as to pulp the nut itself.

CHIMPANZEES 103

Although often spoken of as a new and exciting discovery, by the way, nutcracking was mentioned by Darwin in Chapter 3 of The Descent o( ivlan (1871):

It has often been said that no animal uses any tool; but the chimpanzee in a state of nature cracks a native fruit, somewhat like a lvalnut, with a stone.

The evidence cited by Darwin (a report by a missionary in Liberia in the 1843 issue of the Boston Journal of Natural History) is brief and non-spe- cific. lt simply states that 'the Troglodytes nige1; or Black Orang of Africa' is fond of a species of unidentified nut, which 'they crack with stones precisely in the 1nanner of human beings'.

The especially interesting thing about nut cracking," termite fishing and other such chimpanzee habits is that local groups have local cus- toms, handed down locally. This is true culture. Local cultures extend to social habits and manners. For example, one local group in the Mahale Mountains in Tanzania has a particular style of social grooming known as the grooming hand clasp. The same gesture has been seen in another population in the Kibale forest in Uganda. But it has never been seen in Jane Goodall's intensively studied population at Gombe Stream. Inter- estingly, this gesture also spontaneously arose and spread among a cap- tive group of chimpanzees.

lfboth species of modern chimpanzee used tools in the wild as we do, this would encourage us to think that Concestor 1 probably did too. I think it probably did -- even though bonobos have not been seen using tools in the wild, they are adept tool-users in captivity. The fact that common chimpanzees use different tools in different areas, following lo- cal traditions, suggests to me that lack of such a tradition in a particular area should not be taken as negative evidence. After all, Jane Goodall's Gombe Streain chimpanzees haven't been seen to crack nuts. Presumably they would, if the West African nut-cracking tradition were introduced to them. I suspect that the same might be true of bonobos. Maybe they just haven't been studied enough in the wild. In any case, I think the in- dications are strong enough that Concestor 1 made and used tools. This idea is strengthened by the fact that tool use aJso occurs in wild orang utans, local populations again differing in ways that suggest local traditions.*

The present-day representatives of the chi1npanzee lineage are both forest apes, whereas we are savannah apes, more like baboons except, of

*1tool USE' is, in any case, widespread among mammals and birds, as Jane Goodall her- self (among others) has documented.

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104 RENDEZVOUS 1

course, that baboous are not apes at all but monkeys. Bonobos today are confined to the forests south of the great curve of the River Congo and north of its tributary the Kasai. Common chimpanzees inhabit a wider belt of the continent, north of the Congo, westward to the coast, and ex- tending as far as the Rift Valley in the east.

As we shall see in the Cichlid's Tale, current Darwinian orthodoxy suggests that usually, in order for an ancestral species to split into two daughter species, there is an initial, accidental geographical separation between them. Without the geographical barrier, sexual mixing of the two gene pools keeps them together. It is plausible that the great Congo river provided the barrier to gene flow which assisted the evolutionary divergence of the two chimpanzee species from each other, two or three million years ago. In the same way, it has been suggested that the Rift Val- ley, in the throes of its formation at the time, may have provided the bar- rier to gene flow which, further in the past, allowed our line to separate from that which gave rise to the chimpanzees.

This Rift Valley theory was proposed and supported by the distin- guished Dutch primatologist Adriaan Kortlandt. It became better known when it was later espoused by the French palaeontologist Yves Coppens, and it is now widely called by tbe name Coppens gave it, East Side Story. Incidentally, I don't know what to make of the fact that, in his native France, Yves Coppens is widely cited as the discoverer of Lucy, even as the 'father' of Lucy. ln the English-speaking world, this important dis- covery is universally attributed to Donald Johanson. East Side Story has a hard time dealing with Sahelanthropus ('Toumai') from Chad, thousands of miles to the west of the Rift Valley. Australopithecus bahrelghazali, a poorly known australopithecine also discovered in Chad, adds to the problem, although it is younger.

Vvhatever I say on this matter will soon be out of date when new fossils are discovered, so I'll hand over at this point to the bonobo and his tale.

THE BONOBO'S TALE

HE BONOBO, Pan paniscus, looks pretty 1nuch like a com1non chim- panzee, Pan troglodytes, and before 1929 they were not recognised as

separate species. The bonobo, despite its other name of pygmy chimpan- zee, which should be abandoned, is not noticeably smaller than the com- mon chimpanzee. Its body proportions are slightly different, and so are

its habits, and that is the cue for its brief tale. The primatologist Frans de Waal put it neatly: 'The chimpanzee resolves sexual issues with power:

THE BONOBO S TALE 105

the bonobo resolves power issues with sex ' Bonobos use sex as a cur- rency of social interaction) somewhat as we use money. They use copula- tion, or copulat0ry gestures, to appease, to assert do1ninance, to ce1nent bonds with other troop members of any age or sex) including small in- fants. Paedophilia is not a hang-up with bonobos; all kinds of philia seem fine to them. De Waal describes how, in a group of captive bonobos that he watched, the males would develop erections as soon as a keeper approached at feeding time. He speculates that this is in preparation for sexually mediated food-sharing. Female bonobos pair off to practise so- called GG (genital-genital) rubbing.

One female facing another clings with arms and legs to a partner that, standing on both hands and feet, lifts her off the ground. The two fe- males then rub their genital swellings laterally together, emitting grins and squeals that probably reflect orgasmic experiences.

The 'Haight-Ashbury' image of free-loving bonobos has led to a piece of wishful thinking among nice people, who perhaps came of age in the 1960s - or maybe they are of the 'medieval bestiary' school of thought, in which animals exist only to point moral lessons to us. The wishful thinking is that we are n1ore closely related to bonobos than to con1n1011 chimpanzees. The Margaret Mead in us feels closer to this gentle role- model than to the patriarchal, monkey-butchering chimpanzee. Unfor- tunately, however, like it or not, we are exactly equally close to both spe- cies. This is simply because P troglodytes and P paniscus share a con1111011 ancestor which lived more recently than the ancestor they share with us. By the same token, molecular evidence suggests that chimpanzees and bonobos are more closely related to humans than they are to gorillas. Ftom this it follows that humans are exactly as close to gorillas as chim- panzees and bonobos are. And we are exactly as close cousins of orang ulans as chimpanzees, bonobos and gorillas are.

It does not follow from this that we resemble chimpanzees and bonobos equally. If chimpanzees have changed more than bonobos since the shared ancestor, Concestor l, we might be more like bonobos than chimpanzees, or vice versa - and we shall probably find different things in common with both our Pan cousins, perhaps in roughly equal meas- ure. They are equally closely related to us becanse they are linked to us via the same shared ancestor. This is the moral of the Bonobo's Tale, a simple moral and a very genera] one, which we shall meet again and again at other junctures of our pilgrimage.

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RENDEZV US 2

GORILLAS

i E MOLECULAR CLOCK tells us that Rendezvous 2, where the gorillas join us, again in Africa, is only

a million years further into our pilgrimage than Rendezvous 1. Seven million years ago, North and South America were not joined, the Andes had not undergone their major uplift and the Himalayas only just so. Nevertheless the continents would have looked pretty much as now and the African climate, while less seasonal and slightly wetter, would have been similar. Africa was more thoroughly forested then than now even the Sahara would have been wooded savannah at the time.

Unfortunately there are no fossils to bridge tl1e gap between Con- cestors 2 and 1, nothing to guide us in deciding whether Concestor 2, which is perhaps our 300,000-greats-grandparent, was more like a go- rilla or more like a chimpanzee or, indeed, more like a human. My guess would be chimpanzee, but this is only because the huge gorilla ·seems more extreme, and less like the generality of apes. Don't let's exaggerate the unusualness of gorillas, however. They are not the largest apes that have ever lived. The Asian ape Gigantopithecus, a sort of giant orang utan, would have stood head and massive shoulders over the largest go- rilla. It lived in China, and went extinct only recently, about half a mil- lion years ago, overlapping with Homo erectus and archaic Homo sapiens. This is so recent that some enterprising fantasists have gone so far as to suggest that the Yeti or Abominable Snowman of the Himalayas ... but I digress. Gigantopithecus presumably walked like a gorilla, probably on the knuckles of its hands and the soles of its feet as gorillas and chimpan- zees do, and as orang utans, committed as they are to life up trees, do not.

Gorillas join. Phylogeny showing the gorillas diverging from the other Afri- can apes around 7 million years ago, as suggested by genetics. The right branch now represents the chimpanzees and humans (Concestor 1 is marked on the branch ·with a dot at 6 million years ago). The left branch represents the sin- gle genus of gorillas, now thought to comprise two species.

Image: western gorilla ( Gorilla gorilla).

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108 RENDEZVOUS 2 THE GORILLA'S TALE 109

It is a reasonable guess tbat Concestor 2 was also a knuckle-walker but that, like chimpanzees, it spent time in trees as well, especially at night. Natural selection under a tropical sun favours dark pigmentation as protection against ultraviolet rays, so if ,ve had to guess at Concestor 2's colour we would presumably say black or dark brown. All apes except humans are hairy, so it would be surprising if Concestors 1 and 2 were not. Since chimpanzees, bonobos and gorillas are inhabitants of deep forest, it is plausible to locate Rendezvous 2 in a forest, in Africa, but there is no strong reason to guess any particular part of Africa.

Gorillas are not just giant chimpanzees, they are different in other re- spects which we need to think about in trying to reconstruct Concestor 2. Gorillas are entirely vegetarian. The males have harems of females. Chimpanzees are more promiscuous, and the differences in breeding syste111s have interesting consequences on the size of their testes as we shall learn from the Seal's Tale. I suspect that breeding systems are evolutionarily labile, meaning easily changed. I don't see any obvious way to guess where Concestor 2 stood in this respect. Indeed, the fact that different human cultures today show a large range of breeding sys- tems, from faithful monogamy to potentially very large harems, rein- forces my reluctance to speculate about such matters for Concestor 2, and persuades me to bring my speculations as to its nature to a swift end.

Apes, perhaps especially gorillas, have long been potent generators - and victims - of human myths. The Gorilla's Tale considers our chang- ing attitudes to our closest cousins.

THE GORILLA'S Ti\LE

H E RISE of Darwinism in the nineteenth century polarised atti- tudes towards the apes. Opponents who might have stomached evo- lution itself balked with visceral horror at cousinship with what they per- ceived

as low and revolting brutes, and desperately tried to inflate our differences from them. This was nowhere n1ore true than with gorill8s. Apes were 'animals'; we were set apart. Worse, ,,vhere other ani1nals such as cats or deer could be seen as beautiful in their own way, gorillas and

other apes, precisely because of their similarity to ourselves, seemed like caricatures, distortions, grotesques.

Darwin never missed an opportunity to put the other side, some- times in little asides such as his charming observation in The Descent of Man that monkeys 'smoke tobacco with pleasure'. T H. Huxley, Darwin's formidable ally, had a robust exchange with Sir Richard Owen, the lead-

ing anatomist of the day, who claimed (wrongly as HlL'<ley showed) that the 'hippocampus minor' was uniquely diagnostic of the human brain. Nowadays, scientists not only think we resemble apes. We include our- selves within the apes, specifically the African apes. We emphasise, by contrast, the distinctness of apes, including hmnans, from 1nonkeys. To call a gorilla or a chimpanzee a monkey is a solecism.

It has not always been so. In forn1er times, apes were frequently lumped with monkeys, and some of the early descriptions confused apes with baboons, or with Barbary macaques, which indeed are still known as Barbary apes. More surprisingly, long before people thought in terms of evolution at all, and before apes were dearly distinguished from each other or from monkeys, great apes were often confused with humans. Agreeable as it would be to approve this apparent prescience of evolu- tion, it unfortunately may owe more to racism. Early white explorers in Africa saw chimpanzees and gorillas as close kin only to black humans, not to themselves. Interestingly, tribes in both South East Asia and Africa have traditional legends suggesting a reversal of evolution as convention- ally seen: their local great apes are regarded as humans who fell from grace. Orang utan means 'man of the woods' in Malay.

A picture of an 'Ourang Outang' by the Dutch doctor Bontius in 1658 is, in T. H. Huxley's words, 'nothing but a ve,y hairy woman of rather comely aspect and with proportions and feet wholly human'. Hairy she is except, oddly, in one of the few places where a real woman is: her pubic region is conspicuously naked. Also very human are the pic- tures made, a century later, by Linnaeus's pupil Hoppius (1763). One of his creatures has a tail, but is otherwise wholly human, bipedal, and car- riesa walking stick. Pliny the Elder says that 'the tailed species have even been known to play at draughts' (American 'checkers').

One might have thonght such a mythology would have prepared our civilisation for the idea of evolution when it arrived in the nineteenth century, and might even have accelerated its discovery. Apparently not. Instead, the picture is one of confusion between apes, monkeys and humans. This makes it hard to date the scientific discovery of each spe- cies of great ape, and it i-; often unclear which one is being discovered. The exception is the gorilla, which became known to science the most recently.

In 1847 an A1nerican missionary, Dr Thon1as Savage, saw in the house of another missionary on the Gaboon river 'a skull represented by the natives to be a monkey-like animal, remarkable for its size, ferocity and habits'. The unjust reputation for ferocity, later to be hyperbolised in the story of King Kong, comes through loud and clear in an article about

110 RENDEZVOUS 2

the gorilla in the Illustrated London News published in the same year as the Origin of Species. This piece is replete with falsehoods of a quautity and magnitude that try even the high standards set by travellers' tales of the time:

... a close inspection is almost an impossibility, especially as the mo- ment it secs a man it attacks him. The strength of the adult male being prodigious, and the teeth heavy and powerful, it is said to watch, con- cealed in the thick branches of the forest trees, the approach of any of the human species, and, as they pass under the tree, let down its terrible hind feet, furnished with an enormous thumb, grasp its victim round the throat, lift him from the earth, and, finally, drop him on the ground dead. Sheer malignity prompts the animal to this course, for it does not eat the dead man's flesh, but finds a fiendish gratification in the mere act of killing.

Savage believed the skull in the missionary's possession belonged 'toa new species of Orang'. He later decided that his new species was none other than the 'Pongo' of earlier travellers' tales in Africa. In naming it formally, Savage, with bis anatomist colleague Professor Wyman, avoided Pongo and revived Gorilla, the name used by an ancient Carthaginian ad- miral for a race of wild hairy people which he claimed to have found on an island off the African coast. Gorilla has survived as both the Latin and common nan1e for Savage's animal, while Pongo is now the Latin name of the orang utan of Asia.

Judging from its location, Savage's species must have been the \vestern gorilla, Gorilla gorilla. Savage and Wyman put it in the same genus as the chimpanzees, and called it Troglodytes gorilla. By the rules of zoological nomenclature, Troglodytes had to be relinquished by both chimpanzee and gorilla because it had already been used for - of all things - the tiny wren. It survived as the specific name of the common chimpanzee, Pan troglodytes, while the former specific name of Savage's gorilla was pro- moted to become its generic name, Gorilla. The 'mountain gorilla' was 'discovered' - he shot it! - by the German Robert von Beringe as late as 1902. As we shall see, it is now regarded as a subspecies of the eastern go- rilla, and the whole eastern species now - unfairly, one might think·- bears his name: Gorilla beringei.

Savage did not believe his gorillas really were the race of islanders reported by the Carthaginian sailor. But the 'pygmies', originally men- tioned by Homer and Herodotus as a legendary race of very small hu- mans, were later assumed by seventeenth- and eighteenth-century ex-

THE GORILLA'S TALE l[]

plorers to be none other than the chimpanzees then heing discovered in Africa. Tyson (1699) shows a drawing of a 'Pygmie' which, as Huxley says, is plainly a young chimpanzee although it, too, is depicted walking upright and carrying a walking stick. Now, of course, we use the word pygmy for small humans again.

This leads us back to the racism which, until relatively late in the twentieth century, was endemic in our culture. Early explorers often as- signed the native peoples of the forests a closer affinity with chimpan- zees, gorillas or orangs than with the explorers the1nsel.ves. In the nine- teenth century, after Darwin, evolutionists often regarded African peoples as intermediate between apes and Europeans, on the upward path to white supremacy. This is not only factually wrong. It violates a funda- mental principle of evolution. Two cousins are always exactly equally re- lated to any outgroup, because they are connected to that outgroup via a shared ancestor. For the reasons given in the Bonobo's Tale, all humans are exactly equally close cousins to all gorillas. Racism and speciesism, and our perennial confusion over how inclusively we wish to cast our moral and ethical net, are brought into sharp and sometimes uncom- fortable focus in the history of our attitudes to our fellow humans, and our attitudes to apes - our fellow apes.*

)! The Great Ape Project, dreamed up by the distinguished moral philosopher Peter Singer, goes to the heart of the matter by proposing that great apes should be granted, as far as is practically possible, the same moral status as humans. My own contribution to the book The Great Ape Project is one of the essays reprinted in A Devil's Chaplain.

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RENDEZVOUS 3

ORANG UTANS

oLECULAR EVIDENCE puts Rendezvous 3- where our ancestral pilgrimage is joined by

the orang utans - at 14 million years ago, right in the middle of the Miocene Epoch. Although the world was starting to enter its current cool phase, the climate was warmer and the sea levels higher than at present. Coupled with minor differences in the positions of the continents, this led to the land between Asia and Africa, as well as much of south-east Europe, being intermittently submerged by sea. This bears, as we shall see, on our calculation of where Concestor 3, perhaps our two-thirds-of• a-million- greats-grandparent, might have Jived. Did it live in Africa like l and 2, or Asia? As the common ancestor of ourselves and an Asian ape, we should be prepared to find it in either continent, and partisans of both are not hard to find. In favour of Asia is its richness of plausible fossils from around the right time, the mid-to-late Miocene. Africa, on the other hand, seems to be where the apes originated, before the begin· ning of the Miocene. Africa witnessed a great flowering of ape life in the early Miocene, in the form of proconsulids (several species of the early ape genus Proconsul) and others such as Afropithecus and Kenyapit/,erns. Our closest living relatives today, and all our post-Miocene fossils) are African.

But our special relationship to chimpanzees and gorillas has known only for a few decades. Before that, most anthropologists we ..were the sister group to all the apes1 and therefore equally close to rican and Asian apes. The consensus favoured Asia as the home of

Orang utans join. The mo species of Asian orang utan are generally accepted to have diverged from the rest of the great apes approximately 14 million years ago. As with all our rendezvous phylogenies, the right branch represents the species which have already joined the pilgrimage, with the positions of previous concestors marked with dots.

lmage: Ilorneo orang utan (Pongo pygnweus).

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114 RENDEZVOUS 3 THE ORANG UTAN S TALE 115

late Miocene ancestors, and some authorities even picked out a particu- lar fossil 'ancestor', Ramapithecus. 111is animal is now thought to be the same as one previously called Sivapithecus which therefore, by the laws of zoological nomeuclature, takes precedence. Ramapithecus should no longer be used a pity because the name had become familiar. Whatever one feels about Sivapithecus/Ramapithecus as a human ancestor, many authorities agree that it is base to the line that gave rise to the orang utan and might even he the orang utan's direct ancestor. Gigantopithecus could be regarded as a kind of giant, ground-dwelling version of Sivapithecus. Several other Asian fossils occur from about the right time. Ourcmopithecus and Dryopithecus seem almost to be jostling for the title of most plausible human ancestor of the Miocene. If only, it is tempting to remark, they were in the right continent. As we shall see, this 'if only' just might turn out to be true.

If only the late Miocene apes were in Africa instead of Asia, we'd have a smooth series of plausible fossils linking the modern African apes all the way back to the early Miocene and the rich proconsnlid ape fauna of Africa. vVhen molecular evidence established beyond any doubt our affinities with the African chimpanzees and gorillas, rather than with the Asian orangs, seekers of human ancestors relnctantly turned their backs on Asia. They assumed, in spite of the plausibility of the Asian apes themselves, that our ancestral line must lie in Africa right through the Pleiocene and concluded that, for some reason, our African ancestors had not fossilised after tbe early burgeoning of proconsnlid apes in the early Niiocene.

That's where things stood until 1998, when an ingenious piece oflat- eral thinking appeared in a paper called 'Primate evolution - in and out of Africa' by Caro-Beth Stewart and Todd R. Disotell. This tale, of back and forth traffic between Africa and Asia, will be told by the orang utan. lts conclusion will be that Concestor 3 probably lived in Asia after all.

But never mind, for the moment, where it lived. What did Concestor 3 look like? It is the common ancestor of the orang utans and all today's African apes, so it might resemble either or both of them. Which fossils might give us helpfnl clues? Well, looking at the family tree, the fossils known as Lufengpithecus, Oreopithecus, Sivapithecus, Dryopithecus and Ouranopithecus all lived around the right time or slightly later. Our best- guess reconstruction of Concestor 3 might combine elements of all five of these Asian fossil genera• butit would help if we could accept Asia as the location of the concestor. Let's listen to the Orang Uta n's Tale and see what we think.

THE ORANG UTAN'S TALE

E RH AP S WE HAVE BEEN too ready to assume that our links with Africa go hack a very long way. What if, instead, our ancestral lineage

hopped sideways out of Africa around 20 million years ago, flourished in Asia until around 10 million years ago, and then hopped back to Africa?

On this view, all the surviving apes, including the ones that ended up in Africa, are descended from a lineage that migrated out of Africa into Asia. Gibbons and orang utans are descendants of these migrants who stayed in Asia. Later descendants of tbe migrants returned to Africa, where the earlier Miocene apes had gone extinct. Back in their old ances- tral home of Africa, these migrants tben gave rise to gorillas, chimpan- zees and bonobos, and us.

The known facts about the drifting of the continents and the fluctua- tions of sea levels are compatible. There were land bridges available across Arabia at the right times. The positive evidence in favour of the theory depends upon 'parsimony': an economy of assumptions. A good theory is one that needs to postulate little, in order to explain lots. (By this criterion, as I have often remarked elsewhere, Darwin's theory of natural selection may be the best theory of all time.) Here we are talking about minimising our assumptions about migration events. The theory that our ancestors stayed in Africa all along (no emigrations) seen1ed, on the face of it, more economical with its assumptions than the theory that our ancestors moved from Africa into Asia (a first migration) and later moved back to Africa (a second migration).

But that parsimony calculation was too narrow. It concentrated on uur own lineage and neglected all the other apes, especially the many fos- sil species. Stewart and Disotell did a recount of the migration events, but they counted those that would be needed to explain the distribution of all the apes including fossils. In order to do this, you first have to con- struct a family tree on which you mark all the species about which you have sufficient information. The next step is to indicate, for each species on the family tree, whether it lived in Africa or Asia. In the diagram on the following page, which is taken from Stewart and Disotell's paper, Asian fossils are highlighted in black, African ones are in white. Not all the known fossils are there, but Stewart and Disotell did include all whose position on the family tree could be clearly worked out. They also drew in the Old World monkeys, who diverged from the apes around 25 million years ago (the most obvious difference between monkeys and apes, as we shall see, is that the monkeys retained their tails). Migration events are indicated by arrows.

116 RENDEZVOUS

THE ORANG UTAN'S TALE 117

L Gibbons, around 18 million years ago 2. Oreopitherns, around 16 million years ago 3. Lufengpithecus, around 15 million years ago 4. Sivapithecus and orang utans, around 14 million years ago 5. Dryopithecus, around 13 million years ago 6. Ouranopithecus, around 12 million years ago

Of course all these migration counts are valid only if Stewart and Diso- tell have got the family tree right, based on anatomical comparisons. They think, for example, that among the fossil apes, Ouranopithecus is the closest cousin to the modern African apes (its branch is the last to come off the family tree in the diagram before the African apes). The

In and out of Africa. Stewart and Disotell's family tree of African and Asian apes. Swol· Jen areas represent dates known from fossils, while the lines linking these to the tree are inferred from parsimony analysis. Arrows represent inferred migration events. Adapted

from Stewart and Disotell [273].

Taking into account the fossils, the 'hop to Asia and back again' the- ory is now more parsimonious than the 'our ancestors were in Africa all along' theory. Leaving out the monkeys which, on both theories, account for two migration events from Africa to Asia, it need postulate only two ape migrations, as follows:

1. A population of apes migrated from Africa to Asia around 20 million years ago and became all the Asian apes including the living gibbons and orang utans.

2. A population of apes migrated back from Asia to Africa and became today's African apes including us.

Conversely, the 'our ancestors were in Africa all along' theory six migration events to account for ape distributions, all from Africa Asia, by ancestors of the following:

next closest cousins, according to their anat01nical assessments, are all Asian (Dryopithecus, Sivapithecus, etc.). If they have got the anatomy all wrong: if, for instance, the African fossil Kenyapithecus is actually closest to the modern African apes, then the migration counts would have to be done all over again.

The family tree was itself constructed on grounds of parsimony. But it is a different kind of parsimony. Instead of trying to minimise the num- ber of geographical n1igration events we need to postulate, we forget about geography and try to minimise the number of anatomical coinci- dences (convergent evolution) we need to postulate. Having got our family tree without regard to geography, we then superimpose the geo- graphical information (the black and white coding on the diagram) to count migration events. And we conclude that it is most likely that the 'recent' African apes, that is gorillas, chimpanzees and humans, arrived from Asia.

Now here's an interesting little fact. A leading textbook of human evolution, by Richard G. Klein of Stanford University, gives a fine de- scription of what is known of the anatomy of the main fossils. At one point Klein compares the Asian Ournnopithecus and the African Ken- yapitherns and asks which most resembles our own dose cousin (or ancestor) Australopithecus. Klein concludes that Australopithecus re- sembles Ouranopithecus 1nore than it resembles Kenyapithecus. He goes on to say that, if only Ouranopithecus had lived in Africa, it might even make a plausible human ancestor. 'On combined geographic- morphologic grounds', however, Kenyapithecus is a better candidate. You See what is going on here? Klein is making the tacit assumption that Afri- can apes are unlikely to be descended frorn an Asian ancestor, even if

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ll8 RENDEZVOUS 3

the anatomical evidence suggests that they were. Geographical parsi· many is being subconsciously allowed to pull rank over anatomical par· simony. Anatomical parsimony suggests that Ouranopithecus is a closer cousin to us than Kenyapithecus is. But, without being explicitly so called, geographical parsimony is assumed to trump anatornical parsin1ony. Stewart and Disotell argue that, when you take into account the geo· graphy of all the fossils, anatomical and geographical parsimony agree with each other. Geography turns out to agree with Klein's initial ana· tomical judgement that Ouranopithecus is closer to Australopithecus than Kenyapithecus is.

This argument may not be settled yet. It is a complicated business juggling anatomical and geographical parsimony. Stewart and Disotell's paper has unleashed a flourishing correspondence in the scientific jour· nals, both for and against. As the available evidence stands at present, I think we should on balance prefer the 'hop to Asia and back' theory of ape evolution. Two migration events is more parsilnonious than six. And there really do seem to be some telling resemblances between the late Miocene apes in Asia and our own line of African apes such as Australopithecus and chimpanzees. lt is only a preference 'on balance', but it leads me to locate Rendezvous 3 (and Rendezvous 4) in Asia rather than Africa.

The moral of the Orang Utan's Tale is twofold. Parsimony is always in the forefront of a scientist's mind when choosing between theories, but it isn't always obvious how to judge it. And possessing a good family tree is often an essential first prerequisite to powerful further reasoning in evo lutionary theory. But building a good family tree is a demanding exercise in itself. The ins and outs of it will be the concern of the gibbons, in the tale that they will tell us in melodious chorus after they join our pilgrim· age at Rendezvous 4.

4

GIBBONS

ENDEzvous 4, where we are joined by the gib- bons, occurs around 18 million years ago, proba-

bly in Asia, in the warmer and more wooded world of the early Miocene. Depending on which authority you consult, there are up to twelve mod- ern species of gibbons. All live in South East Asia, including Indonesia and Borneo. Some authorities place them all in the genus Hy/abates. The siamang used to be separated off, and people spoke of 'gibbons and siamangs'. With the realisation that they divide into four groups, not two, this distinction has become obsolete, and I shall call them all gibbons.*

Gibbons are small apes, and perhaps the finest arboreal acrobats that have ever lived. In the Miocene there were lots of small apes. Getting smaller and getting larger are easy changes to achieve in evo1ution. Just as Gigantopithecus and Gorilla got large independently of each other, plenty of apes, in the Miocene golden age of apes, got small. The pliopithecids, for instance, were small apes which flourished in Europe in the early Miocene and probably lived in a similar way to gibbons, without being ancestral to them. I suppose, for example, that they 'brachiated'.

Brachia is the Latin for 'arm'. Brachiation means using your arms rather than your legs to get about, and gibbons are spectacularly good at it. Their big grasping hands and powerful wrists are like upside- down seven-league boots, spring-loaded to slingshot the gibbon from branch to branch and from tree to tree. A gibbon's long arms, perfectly in tune with the physics of pendulums, are capable of hurling it across a

Siamangs were separated off because they are larger, and they have a throat s<1c for amplifying their calls.