Anthropology 150

Lecture 5: The First Hominids

This week and next we will be discussing the evolution of the hominids, which is the line of primates that

leads to us, Homo sapiens sapiens. Please print out and refer to the chart of hominid relationships

(hominids.jpg) which you can download from the Moodle site. This chart is my view of how the fossil

hominids are related to one another. You can see alternative views in your texts.

A note on terminology: Both Zimmer and I use the word “hominid” to refer to the line of apes that leads

to humans. Kottak uses the term “hominins.” The words are synonymous, and simply reflect different

terminology driven by arguments over classification that need not concern us. You may use either term.

Fossil Record of Early Hominids

Notes on differing opinions: My chart of hominid evolution differs from that in both of your texts. There are

substantial disagreements among physical anthropologists as to how to interpret some fossil specimens.

Here are some examples:

Sahelanthropus Tchadensis: Zimmer (Smithsonian Guide) believes this species is a hominid. I do not. The

best analysis of the specimen, done by Milford Wolpoff, suggests that is an ape closely related to hominids,

but not within the hominid line. Wolfpoff’s analysis of the skull (which was very badly shattered) suggests

that it was not bipedal. So, I don’t consider it to be a hominid and I’m not going to discuss it.

Orrorin tugensis: I side with Tim White (UC Berkeley) on this one and say that it’s a hominid, but

probably falls within the parameters of other known hominid species (the genus Ardipithecus). I’ll

discuss Ardipithecus below, after a couple of background statements.

Hominid Species and the Evolutionary Model Most

discussions of hominid evolution focus on a series of

species that develop one after another, as seen in the

picture below.

This model isn’t wrong, but it is incomplete. The

process illustrated in the picture is called anagenesis.

Anagenesis simply describes the origin of one

species from another: over time a population will

accumulate enough mutations that it is biologically

distinct from its ancestral population. In that sense,

every mammalian species living today can be traced back in time to ancestors classified as a distinct species,

and that process can be repeated back to the origins of the mammals. If we examine hominid evolution in

this manner, we can start with the first hominids, and then the first members of the genus Homo, the first

Homo sapiens, and the first Anatomically modern humans (AMH or Homo sapiens sapiens).

That line of descent, however, only tells part of the story. At most points in the past, since the

divergence of hominids from the other apes, there have been multiple populations of hominids on the planet.

Some of them were successful and underwent adaptive radiation, leading to new populations and species.

Other hominid populations were not successful and they went extinct. Those episodes of adaptive radiation,

followed by selective extinction are the stuff of evolution: natural selection works by death (or at least the

failure to reproduce). The populations and species of hominids that became extinct are just as important as

those that survived if we are to tell the complete story of hominid evolution.

So, remember that evolution produces lots of branches, not just branch. Adaptive radiation is

branching. Some branches live, some die.

Hominoids (The Apes) and Hominids

There are disputes about the phylogenetic terms that should be used to describe apes and humans. In

some schemes apes are homidae and humans are hominae; in others apes are hominoidae and the line

leading to humans is called “hominidae.” Both schemes are recognized by the Journal of Human Evolution.

I will follow your text and use the word “hominid” to describe the line separate from the other African apes

(chimps and Gorillas) 6-8 mya and that leads to modern humans.

There are several important physical characteristics that define hominids. Relative to the other apes,

we can note increased brain size, a more opposable thumb and in general greater manual dexterity. A

reduction in the size of teeth and the thickness of tooth enamel is also present.

But the single most important trait that separates our line from the other apes is bipedalism: Full and

conventional upright posture. Convention in this sense means that it is the normal means of locomotion.

Remember that chimps and gorillas and (baboons) for that matter, can go vertical for brief spans of time.

So what brought about this adaptation? Let’s go back to the usual scenario for evolutionary change:

the isolation of a population from others of its family or genus, and location adaptation to changing

environmental condition.

If you look at the modern distribution of apes, both chimps and gorillas occur principally in western

Africa. Although there is some overlap in the range of chimps and gorillas, chimp population live largely to

the south of the gorillas.

Hominids, by all available evidence, developed in eastern Africa. We can see the split between

chimps, humans, and gorillas being the result of an adaptive radiation followed by geographic isolation.

In short, humans are the east African apes.

The divergence of the three African ape groups (gorillas, apes, and hominids) occured at the

boundary between the Miocene and the Pliocene. The middle miocene was pretty wet and warm, but at the

end of the period there is another cooling and drying period. As we’ve seen in other cases, this causes a

shrinkage of the forest, bringing about stands of forest separated by dry savannas. This is what brings about

the isolation population from each other and speciation. Because of wind and ocean currents, the drying

conditions of the early Pliocene were more intense in eastern Africa than in the west. West Africa

maintained large areas of tropical forest, which is where the chimps and gorillas lived, while in eastern Africa

the forest retreated and the savannas that characterize much of the region today began to emerge.

Long ago, it was thought that bipedalism came about because it was adaptive on the savanna

landscape. However, the reconstructed landscape associated with the earliest hominid fossils suggest that

tropical forest was still dominant when bipedalism emerged. Why then, did our ancestors start walking on

two legs?

You can read one good explanation in the Smithsonian Intimate Guide to Human Evolution (see

pages 58-59). The idea is that hominid bipedalism initially developed as a way to move through trees (in a

manner used by orangutans) and only later was used as a way of getting around on the ground.

Other ideas:

Bipedalism is an odd adaptation. It has a lot of drawbacks. First, its slow. Quadrapeds are faster.

Quadrapeds are also able to change directions faster and jump farther. Standing upright makes you more

vulnerable to predators especially because it exposes the unprotected abdomen to attack.

On the other hand, being bipedal does some good things. First, you can see better when you’re in

grasslands, simply because your eyes are higher. Second, you can carry things. Most importantly, it helps you

stay cool. Being upright exposes less of the body to both solar radiation and reflected ground heat (reducing

energy absorption) and exposes more of the body to breezes, which speeds cooling. These factors may have

been critical in favoring bipedalism as Pliocence drying increased and the savanna spread more widely.

The scenario is, to summarize, that bipedalism originally emerged as a way to move and feed in the

trees. As the savannas of east Africa expanded during the Pliocence period, however, this adaptation may

have proved very well-suited to the new environment, leading to increasing populations and an adaptive

radiation of early hominids.

Earliest Hominid Fossils:

Arditpithecus kadabba and Ardipithecus ramidus

If we exclude Sahelanthropus tchadensis from the hominid line, the earliest documented hominid

belongs to the genus Ardipithecus. You may have read about the fossil nicknamed “Ardi” that was in the

news in late 2009. “Ardi” was discovered ikn 1994, but has only now been fully reconstructed so that details

of its adaptation could be published. “Ardi” is a remarkably complete specimen and belongs to the species

Ardipithecus ramidus.

There is an earlier species of Ardipithecus, called Ardipithecus kadabba Ardipithecus kadabba is

known only from only fragmentary fossils. The genus Ardipithecus was originally defined on the basis of the

better-preserved Arditpithecus ramidus, but Ardipithecus kadabba is dated earlier than that species and it

seems clear that it is ancestral to Ardipithecus ramidus. Most importantly for our purposes, Ardipithecus

kadabba fossils include a largely complete femur (upper leg), which is clearly from a bipedal ape. If it’s

bipedal, then it’s a hominid. Dated to 5.6 million years ago (mya), A. kadabba is the earliest hominid known

at the present time.

Also very significant is the paleo-environmental reconstruction, which indicates that A. kadabba

lived in an environment made up of a mosaic of woodland and grasslands. There were lakes, swamps and

springs within the broader region. Animal remains found in association with Ardipithecus suggest that it

lived in the forest. So, the environment was not the expansive savanna that characterizes eastern Africa

today and bipedalism precedes the expansion of the savannas.

Ardipithecus ramidus is known from a number of fossils, all dated to a period about 4.6

4.6 mya. Except for being bipedal, Ardipithecus ramidus has a lot in common with chimps. Its 3

cranial capacity was between 300 and 350 cm and its teeth had thin enamel, also like a chimp.However, the

placement of the foramen magnum (the opening at the base of the skull) well under the skull, makes it clear

that Ardipithecus ramidus was bipedal.

Ardipithecus ramidus was an omnivore, and less specialized than either chimps or gorillas (although

chimps are more omnivorous than gorillas). The teeth also indicate some social differences between A.

ramidus and other apes. Two points are important: 1) the canine teeth of males and females are not greatly

different 2) the canines of males are on the whole much smaller than those of the living chimps and gorillas.

These two traits indicate that competition between A. ramidus males was less than that seen today in chimps

and gorillas. Remember from last week that modern gorillas show a great deal of sexual dimoprhism, which

is a product of competition between males. Chimps show less competition, because they don’t have the

alpha-male type of social organization. Nonetheless chimps do show competition. Both chimp and gorilla

males have large canines, which they use in fights with other males. What these data indicate is that chimps

and gorillas may not be good analogues for what the last common ancestor of the three species was like. That

is, heightened male competition may be a derived trait for both chimps and gorillas.

The Australopithecines

Some population of the genus Ardipithecus probably gave rise to the next major group of hominids,

who are classed as the genus Austalopithecus. We cannot be certain that the Australopithecines were derived

from Ardipithecus ramidus, because it is always possible that haven’t found the fossils of other populations of

Ardipithecus.

The Australopithecines were a very, very successful group. There were many adaptive radiations,

which resulted in a great number of species within the genus. The genus existed for over 3 million years,

and didn’t die out until around 1 million years ago.

Australopithecus anamensis

The earliest known member of the genus Australopithecus is Australopithecus anamensis, known

from fossils in the Lake Turkana basin of Kenya, dated to 3.7-4.2 mya. Although there are now over 20

known fossils assigned to the species, all the remains are fragmentary, so that many aspect of its make-up

remain unclear. In many respects, Australopithecus anamensis does not appear to be terribly different from

Ardipithecus ramidus. It probably still spent a lot of time in the trees (especially at night, when trees are the

safest place to be). It was not an efficient walker, so the bipedal adaptation was still in the works, so to

speak. It has not been possible to reconstruct a complete skull, so its cranial capacity is not known.

Australopithecus afarensis

Australopithecus afarensis is almost certainly directly descended from Austalopithecus anamensis.

There is a wide range of dates for fossils assigned to this species, ranging from 3.8 to

2.9 mya, but most cluster on either side of 3.4 mya.

Australopithecus afarensis is an extremely well-known species, due to the surprisingly large

number of specimens in which a great deal of the body is present. The famous fossil Lucy, in which over

60% of the body is present, along with the “First Family” (remains of 13 individuals found together),

belong to this species.

In some respects its tempting to say that A. afarensis is just a bipedal chimp, but that would be

cynical. A. afarensis was clearly bipedal, and well-adpated for walking, albeit not as efficiently as later

hominids. The Laetoli footprints indicate striding bipedalism, and a big toe that is in line with the other

toes (very distinct from Ardipithecus). A. afarensis, however, was still well adapted for climbing smaller

trees, with shorter legs than modern humans, and hands well-developed for grasping branches.

Australopithecus anamensis shows some selective pressure for a large cranium, with a 3

cranial capacity in the range of 380 to 530 cm , slightly larger than modern chimpanzees (282 3

300 cm ). Height estimates for females are 3.5-4 feet and for males as high as 5 feet. This degreeof sexual

dimorphism is comparable to that found in modern gorillas or baboons.

The characteristics of Australopithecus anamensis indicate that the development of bipedalism

preceded expansion of the brain.

After Lucy

The development of the genus Australopithecus after A. afarensis is the subject of great dispute. In

once scenario, A. afarensis was ancestral to all later australopithecines. In others, it is ancestral only to a

group a large-jawed creatures called the “robust australopithecines.” In even reconstruction, however, the

Australopithecines split into two distinct lines, known as the gracile and the robust Australopithecines.

The Gracile Australopithecines

The gracile group includes species with relatively small jaws and teeth. Within this group are

Australopithecus africanus, Australopithecus garhi, and the poorly documented A. bahrelghazali. A.

bahrelghazali is so poorly documented that I’ll not mention it again.

Australopithecus africanus

Dated to a broad range of 2-3 mya. The princiapl features of Australopithecus africanus are 1) Its

cranium is small and rounded 2) It has a significant degree of facial prognathism with a "dished out" facial

profile 3) It has a larger cranial capacity than A. afarensis, with an average around 450 to 500 cc 4) 4)

Studies of casts of the braincase indicate an expansion of areas associated with higher cognitive functions.

Whether or not lateralization was present is unclear from available material 5) The placement of the foranem

magnum, and its pelvic and femoral anatomy all indicate full bipedalism

Australopithecus garhi

Dated to about 2.5 mya old in the Middle Awash Valley, Ethiopia,)It has larger postcanine teeth

than A. afarensis and its face retains some primitive features not seen in A. africanus. xxv) Contemporary

postcranial remains feature a derived humanlike humeral/femoral ratio and an apelike upper arm-to-lower

arm ratio.Near the site where A. garhi was discovered, Desmond Clark discovered the earliest known traces

of stone tools used to butcher animals.

The stone tools were in association with animal bones bearing cut marks on some ancient. It cannot

be stated conclusively that Australopithecus garhi made the tools and butchered the animals, but the

circumstantial evidence is there.

The Robust Australopithecines

The robust Australopithecines represent a very distinctive adaptation to a very specialized diet. The

robust Australopithecines are so distinctive that many favor giving them a separate genus designation, calling

them Paranthropus.

At present, there are three species assigned to the robust Australopithecine group: A.

aethiopicus, A. robustus, and A. Boisei. This group appears to have split from the gracile

Australopithecines at around 2.7 mya. A. aethiopicus appears to be ancestral to the others.

The robust australopithecines have four traits in particular that distinguish them from the gracile

group 1) very large teeth, and enormous molars 2) a very robust jar to hold those teeth 3) a sagittal crest, or a

ridge of bone on top of the skull 4) a wide zymogatic process ( the cheekbones). The big teeth are there to

chew tough vegetable matter. The big jars hold the big teeth. The sagittal crest and the wide cheek bones

serve to anchor the large muscles than moved the jaws that held the teeth that chewed the tough vegetable

matter.

In short, the robust Australopithecines had a specialized diet of tough and fibrous material. Like

modern gorillas, their diet may have included things like bamboo and bark. They are often described as

“chewing machines.”

The Big Picture

The split between the gracile and robust Australopithecines represents a fantastic example of

adapative radiation. Out of one ancestral population, there was a radiation into distinctive environments, and

natural selection favored very distinctive traits in each of those zones.

The gracile Australopitchecines were omnivorous, eating a wide variety of food. The robust

Australopithecines became highly specialized, eating only very tough vegetable matter.

The difference between these two adaptive strategies becomes much more clear after 2.5 mya. At

that juncture, one of the gracile species develops into a new species that we classify within a different genus,

the genus Homo. If we were to guess, we’d probably say that Homo developed out of some population of

Australopithecus garhi.

Next up: early members of the genus Homo