CATHERINE OWENS ONLY
A L S O B Y M A R L E N E Z U K
Sex on Six Legs
Riddled with Life
Sexual Selections
Paleofantasy
WHAT EVOLUTION REALLY TELLS US
ABOUT SEX, DIET, AND
HOW WE LIVE
Marlene Zuk
sr W . W . N O R T O N & C O M P A N Y
New York • London
7
Paleofantasy Love
What is our t rue sexual nature? Humans have highly demanding, precociously born babies; although a mother chimp or gorilla can raise an infant on her own, in all human societies women usually have help with their off- spring, of ten in the form of the father. One could take this to mean that monogamy is both natural and necessary. But a great many popular and scientific sources disagree. Some suggest that men and women are in conflict, with naturally fai thful women fighting a losing battle to keep men f rom straying. As one commenter on an article titled "Is Infidelity Natural? Ask the Apes"1 says:
Men are wired to spread seed far and wide. All men know this, we get hormones that run through our bodies that do this. There is no denying that at all, and it would be stupid to deny it. Men are also logical and can hold those feeling [sic] back. It doesn't always happen.1
Alternatively, the best-selling book Sex at Dawn would have it tha t both men and women are sexual gourmands, with multiple
Paleofantasy Love 165
' ^ .> WAX f
(© Kim Warp/The New Yorker Collection/www.cartoonbank.com)
partners the real norm and monogamy a miserably failed experi- ment . According to authors Christopher Ryan and Cacilda Je tha , "The campaign to obscure the t rue nature of our species' sexual- ity leaves half our marriages collapsing under an unstoppable tide
of swirling sexual frustrat ion, libido-killing boredom, impulsive betrayal, dysfunction, confusion, and shame."3
On Cavemanforum.com, readers chime in:
It's actually also true than [sic] girls are more likely to settle for the less alpha-type guys, but if they are going to cheat they do it with the more macho guys. We're biologically wired that way. Blame evolution.4
The paleo norm was for less than half the men to reproduce. Having 80-90% of men reproduce is a very recent phenom- enon (even closer in time than agriculture). Lifetime Monog- amy is as artificial as bread or corn syrup.5
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What all of these views have in common is an evolutionary per- spective. They seek to explain our modern behavior in terms of our past, recognizing tha t how we mate and have babies is at the heart of who we are. Sex is the way in which we can seem most like
our animal ancestors and relatives; all creatures reproduce in some fashion, and we obviously are descended f rom primates that chose mates, gave birth, and raised their children. What is not so clear is how they did it, and by extension what behaviors and roles we have inherited from our ancestors.
Perhaps not surprisingly, many of the paleo-lifestyle advocates point wistfully to a time when "sex was a lot more egalitarian and promiscuous than you'd think it was."6 One woman confessed that , since eating a paleo diet, "I'm VERY attracted to more aggressive (but still respectful), more capable men, strong of intellect and body, physically larger, hair on their chests, who would make good protectors and providers—not so much financially, but in the
home."7 Exactly how a change in diet leads to a preference for more hirsute partners is not clear.
We may not really believe that men are f rom Mars and women from Venus, but we have a lot of opinions about what each sex can do or is likely to enjoy, f rom reading maps to watching tearjerker movies. Men and women are of ten supposed to differ even more in their sexuality, whether that means the kind of par tner they want, their sexual appetites, or what they think a good marriage looks like. Evolution has been invoked to explain all of these characteristics—
maybe not the movies per se, but supposed sex differences in traits,
ranging f rom the capacity to appreciate nuances in relationships or have empathy for the travails of others, to the ability to perform spatial reasoning tasks, to a love of shopping for shoes, have been attributed to our history as hunter-gatherers.
As with the question regarding other aspects of human behavior, however, how much do we know about the sex lives of our ances- tors? And how much of that behavior is still manifested today, in a
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world with speed dating and sperm banks? Is it instructive to look at our ancestry, or will we simply see what we want to see?
Darwin, peacocks, and pipefish
To unders tand the evolutionary basis for sex differences, we need to pay a brief visit to Charles Darwin. Although he is best known for his theory of natural selection as it applies to the origin and diversity of species on Earth, Darwin was also extremely interested in sex. One might almost say he was troubled by it, not necessarily
in his own life (though some biographers have speculated about problems in his relationship with his wife, Emma, as well as his Victorian-era prudery), bu t because many things about sex in ani- mals seem perplexing.
Take, for example, the peacock, with his splendid shimmering
train of green, blue, and gold. Anyone who has seen a peacock dis- playing at a zoo will have no difficulty imagining tha t the huge fan
of feathers impedes the male's movements , and would make it dif- ficult for h im to escape, say, a tiger creeping up in the forests of the peacock's native India. The females lack this encumbrance, as is the case for many kinds of animals: though exceptions occur, males are the ones with the elaborate plumage, colorful pat terns , and loud
songs—all characteristics that require a fair amount of energy to produce and that render their bearer more conspicuous to preda-
tors. In short , these traits seem disadvantageous, and hence would
not be expected to evolve under natural selection. Darwin's solution was tha t another process besides na tu ra l
selection was at work, called sexual selection. J u s t like na tu ra l
selection, sexual selection causes some organisms to leave more genetic representat ions of themselves t h a n others do, but instead of operating on characteristics such as a greater ability to blend in with the bark of a tree and hence avoid detection by a hungry bird,
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sexual selection is all about who gets more and/or be t ter mates.
If a female peafowl (the correct t e rm for the species; "peacock," strictly speaking, refers only to the male, while "peahen" is t he
word for the female) is more likely to mate with a male bearing an ornamented t ra in t h a n one with more modest plumes, the genes conferring the lovely feathers are more likely to appear in suc- ceeding generations. The process works even if the t ra i ts hamper survival, though if the advantage in mat ing is outweighed by a disadvantage in survival, sexual selection won't continue to act to exaggerate t he trai t .
Sexual selection is thought to account for many of the differences that we see between males and females: male deer with enormous
racks of antlers, brightly colored birds of paradise, and frogs that croak the night away. It also explains why it is mainly the males that sport fancy advertisements, and not the females. Darwin's original idea relied on the aforementioned Victorian notions of coy females needing to be persuaded by macho males, and it has been modified by his successors.
In my opinion, the best framework for understanding sexual selection comes f rom the more modern interpretat ion by Robert Trivers, an evolutionary biologist at Rutgers University. In a book
chapter published in 1972, Trivers pointed out tha t although both sexes devote a great deal of effort, t ime, and energy to perpetuat-
ing their genes, the way in which they do so of ten differs.8 Evolu- tion means changing the genes that occur in the population, which means that individuals who have higher reproductive output are more likely to be represented. What limits tha t output? For females, it's the number of offspring they can produce and, in some cases, rear, because by definition the female is the egg-producing sex and the eggs, once fertilized, are usually (though not always) cared for by the mother. Because eggs, and embryos and fetuses, are expen-
sive and time-consuming to manufacture, even if females operate at maximum capacity their production is fairly small.
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Imagine, for example, the highest number of puppies or ki t tens
a female dog or cat could have in her lifetime, assuming tha t she mated every t ime she came into heat . That number , while obviously
higher than the number of children a human female could bear over her lifetime, is still vastly smaller t han the number of young
that , at least in theory, could be sired by a given male of the spe- cies, since his investment, as Trivers would refer to it, is only the relatively smaller amount of t ime and energy required to mate with
the females. Given the opportunity, a single male could inseminate many if not all the females in a population. Therefore, a male can
win much bigger, e v o l u t i o n a r y speaking, than a female, because her upper limit is smaller. But of course, one male's win is anoth- er's loss, and in many species, most males do no t mate with even a single female. Sexual selection therefore favors males tha t are able to compete with members of their own sex to get the opportuni ty to mate at all, because the stakes are high.
Females, on the other hand, will of ten do bet ter f rom an evo- lutionary perspective if they mate with only the highest-quality partner, because a male tha t either has genes conferring a greater ability to survive or provides for the offspring directly—say, by feeding them—will make it more likely tha t her own genes persist in future generations. With this dichotomy in mind, many evolu-
tionary biologists talk about competitive males and choosy females. Exceptions abound, of course, and Trivers himself emphasized tha t it's the level of investment that is important , not the sex of the individual doing the investing. If, as is the case in some insects, the male mus t present the female with a nutr i t ious package tha t he produces with his own body fluids before she will accept him as a mate, his investment becomes quite high, and males of species
that offer these "nuptial gifts," as they are called, are quite choosy about their mates. Seahorses and their relatives the pipefish are also good examples of this reversal of the usual s tate of affairs, with males receiving the eggs of females in their pouches so t ha t they
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become pregnant and give bir th to tiny replicas of themselves af ter the young develop.
Some researchers and popular writers took the idea of selection favoring different behaviors in the sexes to mean that women are expected to be rather unenthusiastic about sex itself, or at least to want it only as a means to gain a male's investment in children, while men are expected to enjoy sex for its own sake. Presum-
ably, the thinking goes that if men invest only sperm in mat ing (the more the merrier, as it were) while women have to weigh the
consequences of their pleasure in a potential pregnancy, selection should favor men who seek out one encounter a f ter another. Ryan and Jetha, the authors of Sex at Dawn, are among the most explicit about this claim, and the most eager to debunk it. They refer to Darwin's "anti-erotic bias" and, more directly, claim tha t "Darwin says your mother 's a whore."9
Evolutionary psychologists, who extend some of the principles of evolutionary biology to the mental and behavioral adaptations of our
own species, also come in for criticism by Ryan and Jetha, who note that many works coming from that field also assume that the female libido is sluggish, at least in comparison to that of the male. My own
opinion is that while it's certainly t rue that biologists, like everyone else, have brought their own biases to the study of human sexual- ity, whether in early evolution or not, the differential investments of the sexes into offspring do not automatically lead to a reduction in female lust. Jus t because the consequences of sex are different for men and women does not mean that either one enjoys it less.
Who, really, is your daddy?
Another reason males are said to be more likely to roam is central to the difference in how males and females reproduce. With rare excep- tions, any offspring produced by a female is guaranteed to carry
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half of her genes, which makes any investment into tha t offspring a sound one in evolutionary terms. The same is no t necessarily t rue for a male; he cannot be sure, anthropomorphically speaking, that
a given offspring is his, because the mating happened days, weeks, or months before the young are hatched or born, and the female could have mated with another male in the meantime.
This dispari ty be tween the sexes means t ha t what is called con-
fidence of pa tern i ty can be ra ther low in some species, especially
those in which females mate more t h a n once before thei r eggs are ferti l ized. If a male is not the genetic fa ther of the offspring a
female produces, he is be t te r off, evolutionarily speaking, f inding another female to mate wi th t h a n sticking around to help rear the young of his first mate . Females, on the other hand, would
benefi t f rom having a male's help in taking care of thei r babies, regardless of whether he is thei r genetic father . Hence, the s tory
goes, females are more inclined to monogamy and males to play- ing the field. And indeed, in many species t he males pe r fo rm a
variety of activities t ha t seem to have evolved to increase t he likelihood tha t they are indeed the only one to have ma ted wi th a female; in bluebirds, for example, males stick close to the female dur ing the critical few days sur rounding ovulation, chasing any
other suitors away. We humans seem to be no less concerned than the bluebirds. So-
called paternity fraud, with women fooling men about their rela- tionship to the women's children, has become a staple of talk shows and TV crime series. Billboards provocatively ask, "Paternity ques- tions?" and advise that the answers are for sale at your local phar-
macy in the form of at-home DNA paterni ty tests. Some fathers ' rights groups in Australia have called for mandatory paternity test- ing of all children at birth, with or without the mother 's consent or even her knowledge.
And people seem quite convinced tha t all this vigilance is neces- sary. When asked to est imate the frequency of such misassigned
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paternity in the general population, most people hazard a guess of 10, 20, or even 30 percent. The last number came from a class of biology undergraduates in a South Carolina university tha t I polled a few years back. I pointed out that this would mean tha t nearly
twenty people in the class of sixty-some students had lived their
lives calling the wrong man Dad, at least biologically. They just nodded knowingly, undaunted. Even scientists will of ten respond with the 10 percent figure, as a geneticist colleague of mine who studies the male sex chromosome—and knew the real answer— found when he queried fellow biologists at conferences.
The t ruth, however—insofar as we can tell—is much less sensa- tional. The most unbiased research suggests tha t the real incidence
of misassigned paternity in Western countries hovers around 1 per- cent, with a few studies pushing tha t number to 3 percent or nearly 4 percent. Even at the high end, that 's only one- tenth as common as conventional wisdom would have it. Obtaining a truly unbiased estimate is difficult because mos t people undergo paterni ty test ing
only if they have a reason to suspect a discrepancy between the purported father and the genetic one. As a result, using data f rom the companies that sell the at-home tests, for example, is certain to yield an overestimate of misassigned paternity.
A handful of studies that get around this problem do exist, mainly in the medical literature. Most of these studies gathered information on the parents of children with genetic disorders like Tay-Sachs disease or cystic fibrosis, in which the child mus t inherit a copy of the defective gene f rom bo th parents to show the disease.
When large numbers of families are surveyed for such research, a certain proportion of fa thers t u r n out n o t to have the gene tha t their purported child inherited, thus yielding the figures of 1-3.7
percent. Higher numbers, particularly the often-cited 10 percent, apparently come f rom more biased populations, or, more likely, simply turn out to be urban legend, akin to cell phones being able to pop popcorn.
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Interestingly, no one seems to question the apparent contradic- t ion between the stereotype of fa i thful females and all this suspi-
cion. The problem is that if men are going to have urges to stray, they have to stray to, or with, a par tner—it takes two to cheat, of
course. As Hester and her scarlet letter would at test , however, we have a double s tandard about philandering. Regardless of who is
to blame, why we are so ready to believe an inflated figure of our own infidelity? Perhaps our cynicism feeds into already-held beliefs
about the na ture of male and female sexuality, as evidenced in t he online comments quoted earlier in this chapter. Of course, we really don' t know how low, or high, confidence of paterni ty might have been in our ancestors, but these modern results suggest that men might not be such philanderers, or women such deceivers, as the
more pessimistic among us would have us believe.
Not-so-modern love
How does the extraordinary fatherly capacity of seahorses or the burden of the elaborately ornamented peacock apply to humans? People looking to unders tand what our ancestral mat ing behavior was like use the same sources as those examining other aspects of human evolution: other primates, especially chimpanzees and bonobos; modern hunter-gatherer societies, along with wri t ten records of marriage pa t te rns in ancient civilizations; and the evi-
dence contained in our own bodies, which bear witness to the selec- tion pressures that acted on men and women in our evolutionary history. All of these reference points have their shortcomings, bu t each is also valuable, so it is wor th examining them in turn .
First our pr imate relations. The great apes—gorillas, chimpan- zees, bonobos, and orangutans—vary quite a bit in their mat ing
pat terns . Gorillas live in groups with a single dominant male (the silverback, so named because age has grayed his fur) and multi-
1/4 Paleofantasy
pie females, while orangutans are solitary for mos t of their lives
and both chimps and bonobos have complex societies with many individuals of both sexes. Chimpanzee society is marked by pro- nounced male aggression, even violence, while bonobos are more likely to share food with other group members. Males and females in both species will mate with multiple partners , and none of our closest relatives show long-term pair bonds.
Because the chimps and bonobos are the animals with whom we
mos t recently shared a common ancestor, people have long been interested in whether some of our sexual proclivities, such as fidel- ity or t he lack of it, or t he preference for a particular kind of sexual
partner, might have its origins in their behavior. Whether finding
such common ground means tha t t he aforementioned proclivities are "natural" (whatever that means), much less genetically deter- mined and unable to be altered, is a separate question. For now, let's just see how our sexuality could be mirrored in that of our kin.
Virtually everyone who has studied bonobos is struck by the pri- mary role that sexual behavior plays in their lives; they tend to settle conflicts with sexual activity, bo th between males and females and between members of the same sex. Like the chimpanzees, bono- bos live in large and somewhat fluid groups containing males and
females, but unlike chimpanzees, bonobos tend to be less violent in their day-to-day behavior, and female bonobos can dominate males and chase them away f rom food at least some of the time.
Scientists have used both chimps and bonobos as models of what early hominin sexuality might have been like for many years, although the latter have been well studied since only the late 1970s, while researchers began documenting chimpanzee social behavior several decades earlier. The emphasis on aggression in the chimps
and open sexuality in the bonobos has not gone unremarked, with the f requent claim tha t "chimpanzees are f rom Mars, bonobos are from Venus." Eminent primatologist and author Frans de Waal has written extensively on how we can—and can't—use the bonobo
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sex life to unders tand our own, pointing out tha t while they "pro-
vide a concrete alternative to 'macho' evolutionary models derived
f rom . . . chimpanzees," the bonobo's society is very different f rom our own, and we do not know how closely it resembles that of our
early ancestors.10 De Waal goes on to speculate, "Had bonobos been known earlier,
reconstructions of human evolution might have emphasized sexual relations, equality between males and females, and the origin of the family, instead of war, hunt ing, tool technology, and o ther mas- culine fortes."11 This is one possibility; the other is that if we had known about bonobos earlier, we would have characterized them as more violent and warlike than we do now, simply because anthro-
pologists and primatologists in the 1960s and '70s were disposed to emphasize male aggression, which bonobos do exhibit, albeit to a
lesser extent than chimps do. In thei r book, Ryan and J e t h a are very pro-bonobo, appreciat-
ing the face-to-face kissing t ha t the species exhibits, along with t h e slow development of its infants , like t ha t of h u m a n s but dif- ferent f rom chimpanzee development ." They also note t ha t b o t h people and bonobos have sex under circumstances other t han procreation, such as when resolving conflict or cementing bonds between individuals, and they cite this similarity as grounds to believe tha t the bonobos are a more accurate representat ion of our ancestral s tate t h a n are those no-nonsense chimps. They then
go on to promulgate a ra ther orgiastic view of h u m a n sexual- ity, saying t ha t our monogamous woes arise f r o m an uphill, and ult imately doomed, bat t le with our more bonobo-like desires for multiple sexual par tners . An abundance of casual sex, or as Ryan
and Je tha put it, "Socio-Erotic Exchanges," could then func t ion to make the wheels of society run more smoothly, though I have
trouble imagining the kind of genital-genital rubbing t ha t is com- monplace among female bonobos ever catching on at book clubs.
I will re turn to the idea of monogamy and its role in our evolution
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later in the chapter, but for now I want to comment on whether the bonobos, or any other living pr imate species, are a realistic model for our earlier behavior. It's t rue tha t the bonobos and the chim- panzees are our closest relatives on Earth, but at the same time,
we have not shared a common ancestor for at least 5 million years,
so more than enough t ime has elapsed for selection to act sepa- rately on each of the three species. The bonobos may share non- reproductive sexual activity with humans, bu t vervet monkeys, a species much less closely related to us than are any of the apes, also have sex outside the t ime when a female can conceive. There is no a priori reason to expect that any particular aspect of our sexuality has been preserved f rom one ancestor and no t another.
Why do the gender relations of our primate relatives mat te r at
all? As anthropologist Craig Stanford points out, "The behaviors at the heart of the chimpanzee-bonobo interspecific variation—sex-
uality, power and dominance, aggression—are those tha t also he at the center of the debate about human gender issues and what
molds our own behavior."13 He also extended feminist anthropolo- gist Sherri Ortner 's contention that "men are to women as culture is to nature"14 by wondering if "chimpanzees are to bonobos as men are to women."15 In other words, even now that we know about bonobos, males and females are still seen as different, and the gen- ders are still stereotyped, bu t we get to pick whether we like the old male version with the war toys or the new female one with lesbian
sex and food sharing. Either way, however, we are simply impos- ing our preexisting biases ("free love is natural," "males are violent brutes") on species tha t are complex in their own way, not as cari-
catures of people. A bet ter idea is to figure out what the animals are like without using them as role models.
What's more, new information on the role of evolutionary history in primate social life suggests that an impor tant gap lies between us and our ape relatives. A 2011 study by Susanne Shultz, Christopher Opie, and Quentin D. Atkinson traced the evolutionary history of
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social behavior and found tha t genetic relatedness was more impor- tan t than environmental conditions in determining whether a given
species lived in pairs, small groups, or large groups.16 History is impor tan t because if one is interested in unders tand-
ing the evolution of any trait, whether social-group composition or a tendency to eat leaves, and tha t t rai t occurs in a number of species, the first question is whether each species inheri ted tha t trait f rom the same common ancestor, or whether i t arose inde- pendently multiple t imes because of the same selection pressures. In the case of pr imate social-group structure, the conventional wisdom had it tha t our more dis tant ancestors lived in simpler soci-
eties with just a pair of animals or perhaps a family group. Then,
larger, more complicated groupings evolved. The details of group s t ructure—whether solitary or in single- or multi-male societies, for example—were thought to depend on the location of food in
the environment and other ecological variables. Shultz and colleagues, however, were able to place published
information about the social organization of 217 living pr imate species in the context of the genetic relationships, and hence the evolutionary history, of those species. They discovered that social organization tended to be more similar among closely related spe- cies t han would be expected by chance, which means that genes, not ecology, play a big role in the kind of society a species exhibits. Perhaps even more interesting, they found tha t , about 52 million years ago, primates went f rom a solitary life to fairly unst ructured groups, and then to more stable ones. Then, both pair-bonded soci- eties and single-male harems, like those seen in gorillas, emerged
roughly 16 million years ago, rather t han having a more linear evo- lution with groups always becoming more complex.
The age of social systems is significant in the context of the evolu-
tion of our sex lives because we know that the last common ances- tor of humans and chimpanzees lived about 5-7 million years ago. Since modern chimpanzees and bonobos live in stable communities
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with multiple males and females, pair bonds—the early version of monogamy—must have emerged sometime af ter we all diverged from this more recent ancestor. When that happened is still a mys- tery, but the new data suggest that human mating pat terns evolved on their own for some considerable time after that divergence.
Genetic and linguistic signatures of mating history
The second source of information about our sexual natures is the life of humans in contemporary cultures. Human beings are unusual among animals because of our frequent, though by no means universal, monogamy; a number of species pair up for the duration of a breeding season, and a small handful mate for life, but by and large, animals tend to display a variant on the multiple sexual par tner theme. And indeed, sexual selection theory suggests
that because males so frequently gain by at tempting to mate with multiple females, we expect "polygyny" (the scientific term for a single male mating with more than one female) to be the most common mating system. A few species do show the counterpart to this system—polyandry—in which one female mates with several males at the same time, but it is limited to situations in which a female can mate and produce offspring with one male, leave those young with him, and then proceed to the next. Polyandry has been observed in human societies as well, but again only under very lim- ited circumstances.
Where, then, did our monogamy come from? Is it a cultural arti- fice that denies our basic nature, as Ryan and Je tha would have it, dooming countless men and women to lives of guilt and secret philandering? Or is it an adaptive par t of human society? In a
volume of multicultural perspectives on the family, Bron Ingoldsby claims, "Monogamy is certainly the most common of the marital
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types; the generally equal sex ratio throughout the world sees to that."17 But this is specious reasoning; a f ter all, males and females
occur in roughly equal numbers in many animal species, but they exhibit a variety of mat ing systems, with many individuals, usu- ally males, simply no t mat ing at all. The real question is, what hap- pened in human evolution? Or put another way, f rom a blog entry by historian of science Eric Michael Johnson, "Were our ancestors
polygamists, monogamists, or happy sluts?"18 People have argued the question from all sides, but we are only now gathering the data
that allow us to test it. Polygyny is still found in many historical and modern h u m a n
groups, with wealthier men able to have multiple wives, while
poorer ones can afford to have only one or stay single. Monoga- mous marriage as we now know it was thought to have become more prevalent af ter societies became more complex and agricul- tural and less nomadic. Laura Fortunato, an anthropologist at Uni- versity College London, analyzed marriage pa t te rns f rom Eurasia, using a tree of cultures that mimicked an actual evolutionary tree such as tha t showing, say, the relationships among various mem-
bers of the cat family, with lions and leopards more closely related to each other than either one is to lynx or pumas. In Fortunato's case, she incorporated the group's historical relationships based on language, so that , for example, Portuguese- and Spanish-speaking peoples were seen as more closely related to each other than either was to Lithuanians.19
The results of the tree construction were surprising. Of the twenty-seven societies sampled, eighteen, or two-thirds, were
monogamous, while the remaining one-third were polygynous. This finding in itself was not unexpected, bu t more interesting,
monogamy appeared to have arisen well before modern record keeping. In other words, at least some of the first settlers emerging af ter our hunter-gatherer nomadic ancestors appear to have been
monogamous. This pu ts to rest the argument that monogamy is a
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recent invention that required the development of a more industri- alized way of life.
A different line of evidence, however, suggests that the pa t tern of multiple partners, or at least multiple females mating with a single male, was also a par t of our history, just as Ryan and Je tha would have it, albeit with a twist. New information about the human
genome allows scientists to use our genes to look into the past .
Specifically, to estimate ancient mat ing pat terns, researchers can study the way genes vary on the sex chromosomes and then com- pare that variation to the way genes vary on other chromosomes.
Women have two X chromosomes, while men have only one,
which means that mothers always pass an X on to their children but men pass one on to only their daughters. This disparity means that women contribute disproportionately to the genetic diversity
on the X chromosome, compared to men. And what that means is that if a society is polygynous, so tha t a relatively small number
of men have multiple wives, those few men will reduce the over- all genetic diversity in their offspring—except for the X chromo- some. The men contribute an X to their daughters, but the women also contribute an X to the daughters, as well as one to the sons. Thus, although the fathers reduce the diversity overall, the mothers should maintain the X chromosome diversity. Therefore, a compar- ison of the genetic variability on the X chromosome compared to the rest of the chromosomes should reveal higher genetic diversity
on the former—which is exactly what Michael Hammer and his col- leagues found in samples f rom populations around the world. They
concluded that polygyny was a par t of our history, and the evidence is written in our genes.20
What about that twist? The idea that more women than men left
genes in succeeding generations is supported by these new data. But genetic variation in a population is influenced by more than just how many males mated with the available females. Another important contributor is how much each sex moves f rom its birth-
place: new immigrants contribute new genes, while those who stay
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home and marry neighbors or especially relatives (even dis tant ones) will reduce the overall genetic variability. In some societies, men stay and women go to the areas where their husbands were living; in others the reverse occurs.
Hammer's study and others similar to i t could track the genetic signature of polygyny back to the s tar t of agriculture, about 10,000
years ago. Anthropologists believe that around tha t time, along
with eating grain, people began to have more patrilocal residence pat terns, with women leaving the places where they were born. Such societies are more likely to be polygynous, because multiple women can all go to the same man's residence, and they would also reinforce the pa t tern of X chromosome variation that Hammer
and his coworkers found. But such an increase in polygyny would have been a fairly recent phenomenon, rather than one established deep in our history, which means that monogamy could have been
ancestral to humans . Other studies have found results that differ f rom Hammer's, with the discrepancy now being at t r ibuted to dif- ferences in the timescale at which the data are analyzed.
The paleofantasy of a caveman past in which a few dominant
males held sway and women meekly served them is as unrealistic as one in which we all paired up and never strayed. Humans have suc- cessfully reproduced under a variety of mat ing systems, depending on where on the planet and when in our history one looks. As with diet, as with exercise, as with all the other features of our biology
tha t people want to make into a single "natural" way—we don' t
have just one natural pa t tern of the sexes.
Hunting, gathering, and sex
Whether our ancestors were monogamous or polygamous, another hallmark of humani ty is a sexual division of labor, with men and women of ten performing different tasks in a society. That division
is frequently linked to other differences between the sexes, like the
182 Paleofantasy
purported lack of mathematical ability in women, and to the evo-
lution of the nuclear family and (once again) the knot ty issue of infidelity. So, do we really come f rom a i950s-style family struc- ture, where men went out and brought home the mastodon meat while women raised children and dug for roots in the soil? Or—and you can probably guess what's coming here—is the real story more complicated?
Thus far in this chapter I have been ignoring an important , per- haps the most important , component of the story about sex and evolution: the a f te rmath of the act—namely, the children. Among
animals, monogamy usually evolves when the offspring require so much care that one parent cannot rear them alone, and humans
fulfill this requirement in spades. As the noted anthropologist Sarah Blaffer Hrdy painstakingly recounts in her books Mother Nature and Mothers and Others, and as I will discuss in Chapter 8, human babies are almost breathtakingly demanding. They need food provided to them well past the time of weaning, and they need to be protected f rom the elements and predator attacks. From a
cold-blooded, practical perspective, it is many years before they can start returning on the investment made in their care by contribut- ing resources to the family
How do these time- and energy-sucking little creatures manage to survive? Many biologists have reasoned that the answer lies in family structure, and particularly in the fathers of the children.
Starting with Darwin, scientists have theorized that prehistoric men hunted and brought the food home to their mates and off- spring. In return, each woman stayed by the fire and remained faithful to her man, guaranteeing him confidence of paternity. The evolution of our big brains was also par t of this scenario, because selection would have favored smarter men to be bet ter hunters,
who were then more likely to successfully provide meat for their families. According to this hypothesis, sexual division of labor is thus essential to our human uniqueness because it drove the evolu-
Paleofantasy Love 183
tion of intelligence, creating a feedback loop in which ever-smarter individuals reinforced the social system.
Hrdy calls this exchange of mea t for fidelity the "sex contract," and versions of it have remained a par t of the s tory of our evo- lution for the last several decades. Hrdy's interest in it per ta ins mainly to how it affects child-rearing and the h u m a n family, which I will discuss in more depth in Chapter 8, b u t the sex contract is also relevant for unders tanding gender differences and our sexual nature itself.
For many reasons, hunt ing is seen as a lot more glamorous, and hence important , an occupation than gathering, and for much of the twentieth century, bringing home the meat was viewed as more
central to human evolution than picking the berries. Putt ing hunt - ing, and males, f ront and center in ideas about the evolution of
gender roles also meant tha t women's work was no t seen as par- ticularly valuable to the group. Anthropologist Lori Hager sug- gests tha t the early models of Man the Hunter were popular in par t because they validated the way Western families were s tructured in the 1 9 4 0 S - 6 0 S . 2 1 Even today, mos t museum dioramas and other illustrations of prehistoric family life depict a man set t ing out with a spear, or holding a captured rabbit, while a woman sits by the
campfire, an infant at her bosom. The implication is tha t women stay home and care for the children, while men go out and bring home the bacon, or m a m m o t h meat .
Starting in the 1 9 7 0 s , a number of mainly women anthropolo- gists, most notably Adrienne Zihlman f rom UC Santa Cruz, began to question this macho perspective. Zihlman and several others dis- covered tha t in contemporary foraging peoples, women's gathering of ten provided the bulk of the nutr ients consumed by the group, and fur thermore , tha t in some cultures, such as some Australian
aboriginal tribes, women hunted as well. But the "Man the Hunter" model has persisted; Zihlman suggests, "It came to s tand for a way of life tha t placed males center-stage, gave an evolutionary basis
184 Paleofantasy
for aggressive male behavior and justified gun use, political aggres- sion, and a circumscribed relationship between women and men as
a 'natural' outcome of human evolutionary history."22 Zihlman and others since the 1970s have promoted a different
version of human evolution, dubbed "Woman the Gatherer," focus- ing on female contributions and women's lives beyond child rear-
ing. Other ideas have been proposed over the last few decades; for
example, food sharing, particularly among non-kin, is sometimes viewed as an impor tant component of early human evolution, because it sets the stage for complicated social exchanges among groups beyond the family, as I will explain. In addition, the rela-
tive roles of hunt ing and gathering in our ancestors are still being
debated by anthropologists. Rebecca Bliege Bird of Stanford University noted that among
modern foraging peoples, although the food items acquired by men and women differ, women tend to bring back abundant , small, and low-risk foods, like shellfish or berries, while men obtain the rarer
and harder-to-catch items, like sea turt les or deer.23 Is this division of labor simply a mat ter of everyone doing what he or she does
best to support the household? Maybe not; by concentrating on the more risky items, the men may simply fail to hold up their end of the bargain. Even when men procure plant foods, as with the yams that Melanesians use as a staple, according to Bird, men "com- pete to grow outsized roots, sometimes spending days preparing a single hole, while women plant dozens of 'table' yams in smaller holes."24 The resulting men's contributions, though impressive, are
apparently used in the Pacific equivalent of county fairs, and end up
being divided to use for fu ture planting rather than eaten. Bird examined the possibility that women mus t trade off the
need to take care of their children with the demands of foraging,
since hunt ing big game is not compatible with caring for infants and toddlers, but while the need for work-life balance seems as real for the Ache of South America as it is for Manhattanites, tha t
Paleofantasy Love 185
trade-off is no t the whole answer. Women who are past reproduc- tive age do not seem to spend more t ime hunt ing than do younger women, at least in some foraging cultures, so there mus t be more to the story than the demands of child care.
Furthermore, humans f rom many societies worldwide share food, not only with their children and mates, but with other mem- bers of society. Such generosity can play an impor tan t role in oiling
the wheels of social interaction, but it needs to be balanced with
what each sex is capable of providing; if women cannot bring down large prey, for instance, they are unlikely to have episodic food bonanzas to distribute. To look at how food is provided to family members compared to the group at large, Rebecca Bird, along with Brian Codding and Douglas Bird, reviewed foraging by men and women reported in three foraging peoples: the Ache, the Martu of nor thwestern Australia, and the Meriam f rom the eastern Torres
Strait Islands.25 These societies differ dramatically in their sexual division of labor, with men in the Ache contributing more than 85
percent of the food, while Martu women bring mos t of the calories
to their groups. The researchers reasoned that at t imes when the available food sources were high-risk sources, women's contribu- tions should be more important , since hungry children can't just wait for daddy to t ry again next week to bring down a kangaroo, and providing for children takes precedence over sharing with the group. Alternatively, when gett ing food is more reliable, men are expected to contribute, more and emphasize sharing.
The researchers' predictions were upheld across the three very different societies, with the emphasis on both hunt ing and gather-
ing changing depending on what was available to eat. For example, among the Meriam, turt le hunt ing is a chancy business much of the year, but during the nesting season it becomes more reliable. At
tha t time, but not otherwise, the women participate by helping to plan the h u n t and butcher the meat; unmarr ied men are the ones to take on the more risky hunt ing outside the nesting season. In all
186 Paleofantasy
three societies, bo th men and women alter what they do depending
on the likely yield of the catch. The discovery that hunted food items, particularly big-game
animals, are of ten shared among the group members, rather than being consumed only by the hunter 's family, as well as the unreli- able nature of hunted meat as a staple in the diet, has caused some anthropologists to question whether the main function of hunt ing is even subsistence at all. Kristen Hawkes, an anthropologist at the University of Utah, has suggested, instead, tha t hunt ing is a way for men to show off to prospective mates, and tha t good hunters
gain high status in their social groups.26 It is no t tha t hunted meat isn't eaten and appreciated by the group, but tha t a given man's share does not necessarily increase the survival of his own children
more than other sources of food do. The unreliability of big-game hunt ing therefore may make it a
poor choice as the sole source of support for a family; saying you want to maintain your wife and children on it is the ancestral equiv- alent of claiming tha t you will be able to fulfill your familial respon- sibility on the proceeds of playing lead guitar in a band. But hunted meat is not simply food—it is a signal. As Hawkes and Bird pu t it, "More than its value as a source of nutri t ion, meat is a medium of communication through which the hunte r t ransmits information
to potential mates, allies, and competitors."27 By contributing to the group, good hunters gain the respect of their peers. The anthro- pologists point out tha t "showing off" in this context does not have to involve actual bragging; in fact, among the Ache, meat is often brought into camp quietly and without much fanfare. But everyone
knows who can, quite literally, deliver the goods. This "status signaling" hypothesis has been critiqued by other
anthropologists, who continue to discuss how best to calculate
hunters ' contributions to their families, whether shared meat is repaid in kind, and which other factors play into the sexual division of labor. But the present-day perspective on hunt ing and gather-
Paleofantasy Love 187
ing in modern—and presumably ancestral—societies still leads to three conclusions. First, sexual divisions of labor are widespread
today, and very likely occurred in ancient humans, with men proba-
bly doing the more high-risk, potentially low-yield par t of foraging. But second, and maybe more important , the tasks tha t men and women do are remarkably flexible across societies and over time. Anthropologist Jane Lancaster has noted that in various human cultures, women have been known to do virtually everything tha t men do, with a notable exception of metalwork, which she specu- lates requires too much single-minded concentration to be compat-
ible with the presence of small children.28 Finally, nothing about these sex-specific tasks, whether gathering limpets, digging yams, or snaring monitor lizards, suggests a justification for all manner of modern gender stereotypes, f rom women liking to shop while men watch football to an ability (or lack thereof) to ask for directions. That paleofantasy of the cavewoman staying home with the kids while the caveman went out for meat would have ended up with no one getting enough to eat.
Our bodies, our genitals, ourselves
Our behavior is slippery stuff, with men and women acting differ- ently in different societies and under different circumstances. And
without preserved footage of Neandertal dating sites, we are lef t to speculate about the sexual proclivities of our ancestors. Or are we? Behavior may not fossilize, but bodies do, and we can infer a sur-
prising amount about behavior f rom those remains. For instance, we are reasonably sure tha t some dinosaurs cared for their young, because of the discoveries of eggs or young dinosaurs associated with an adult.
We can also draw conclusions about how natural and sexual selec- t ion have acted on the sexes in the past, simply by looking at the
188 Paleofantasy
kind and magnitude of differences between male and female bodies today, as well as by comparing those differences across species.
Along with the rest of his focus on sex, Darwin was extremely inter- ested in these differences, and he drew a distinction between what
he called primary and secondary sexual characteristics. The pri- mary sexual characteristics are what define us as male or female— the plumbing, so to speak, with human (and other mammal) males
having testes and females having ovaries, for example. All animals have primary sexual characteristics, and they can be very obvious or quite subtle; in many rodents, for instance, males and females are difficult to tell apart even af ter inspection of the nether regions,
at least by a nonexpert . The secondary sexual characteristics are even more variable,
and they were also of much interest to Darwin. These are all the other differences between the sexes—the ones that aren' t directly
required for reproduction but are still sex-specific, such as the pea- cock tail I ment ioned earlier or the songs of male frogs, crickets,
or birds. Human secondary sexual characteristics include enlarged breasts in women, facial hair in men, and—most germane to our quest for the ancestral mat ing system—differences in body size, with men being on average 15 centimeters (about 6 inches) taller
than women. This body size difference is mirrored to greater or lesser
degrees in many other species, though in some, including whales and many insects, it is the female t ha t is the larger sex. (Larger females are believed to be favored when they can lay more eggs or otherwise provide more for the offspring.) Larger males are thought to be the result of sexual selection for bet ter fighters, and the fighting is generally over access to mates. In elephant seals, for example, the 2-ton males are more t h a n twice the size of the
females, and the bulls spend hours bat t l ing for supremacy on the coastal breeding grounds. The champions are able to seques- ter and mate with a majori ty of the females tha t arrive on the
Paleofantasy Love 189
beach, making the largest males big winners f rom an evolution- ary perspective. So the general idea is t ha t species wi th more male competit ion, and more polygyny, are likely to show a greater difference in body size between the sexes.
Among our pr imate relatives, t he gorillas, which live in groups
with a single male t ha t mates wi th several females, have the mos t pronounced sexual size difference, wi th males about twice t he
size of females; s imilar sex differences are seen in orangutans . Males and females in the monogamous gibbons, by contrast , are roughly equal in size, as are the two sexes in t he muriqui monkeys of Brazil, which have a polygamous ma t ing system in which both males and females have several sexual pa r tne r s dur ing a season. Many pr imates also show sexual differences in
thei r tee th , wi th male baboons, for example, having elongated,
sharp canines t ha t females lack. This difference, too, is though t to result f rom sexual selection, wi th the tee th used as weapons dur ing fights between males.
Humans, along with chimps and bonobos, have a much more
modest difference, which has led many researchers to conclude that we were only moderately to slightly polygynous in our evolution- ary history. Fossil evidence also indicates that human sex differ- ences in size have decreased over the last several hundred thousand years, though it can be difficult to draw conclusions, because skel- etal remains are sometimes classified as male or female in the first place by comparing the size of the bones. Owen Lovejoy, an
anthropologist at Kent State University in Ohio, extended this idea to suggest that human monogamy arose at least 4.5 million years
ago, when the bipedal human ancestor Ardipithecus ramidus lived, and fur thermore that it was facilitated by bipedalism, with men making use of the freeing up of their arms to hold tools, going off to h u n t and bringing back meat for the women. Women, in turn, would reward their mates for the food with fidelity, a la Hrdy's "sex contract," tying things up in a nice tidy package.39
190 Paleofantasy
Of course, as I have already pointed out, this sharp division of labor is not upheld in modern hunter-gatherer societies, and fur- thermore, the similarity in relative sexual size between us and
the chimps and bonobos suggests that we need more information before we can explain why we would end up monogamous and they would mate with many partners. Lovejoy also suggests that Ardipi- thecus was relatively peaceful, like the bonobos, bu t as de Waal notes, "unless the diggers come up with a male and female fossil holding hands and having wedding rings, the idea that these ances-
tors avoided conflict through pair-bonding remains pure specu- lation."30 (One could nitpick tha t even this finding would not be
conclusive, but de Waal's point is well taken.) Furthermore, although very highly sexually dimorphic species
are thought to be polygynous ("dimorphic" is the technical term indicating size and shape differences), it's not so clear what it means when the sexes are quite similar. Sometimes being bigger isn't all tha t useful, as in hummingbirds or other species with aerial
displays of aggression, where a more agile opponent wins the day. Other variables, such as the likelihood of mating within one's group or outside it, can also influence the s trength of selection on male
competitiveness. Even when we find evidence of sexual size differences, the degree
of those differences is not necessarily the result of mating competi- tion. Modern cultures vary in how dimorphic they are in height, with the Maya Indians of South America differing by nearly 10 per- cent, while the Taiwanese differ by only 5.5 percent. Claire Holden and Ruth Mace f rom University College London looked at seventy- six populations around the world for which they could find infor- mation on height, degree of polygyny, type of subsistence (hunting or agriculture), and the sexual division of labor during a preindus- trial period.31 If polygyny has made us dimorphic as a species, then
it seems reasonable, if we look within human cultures, tha t the more polygynous cultures should also be more sexually dimorphic.
Paleofantasy Love 191
The latter two variables were of interest because the research- ers hypothesized tha t polygyny might no t be the only thing affect-
ing sexual size differences; perhaps the way that men and women
live is also a factor. And indeed, the more women contributed to
food production in a culture, the smaller the difference in height was between the sexes, perhaps because such contributions meant that the women had more control over food distribution. Whether or not a society was polygynous made no difference, al though the
authors caution tha t their sample might no t have allowed detec- t ion of a contribution by the mat ing system. They had to account for the historical relationships among the societies, as Fortunato
did in her marriage analysis, described earlier; and once they did, they were lef t with a rather small sample.
The last piece of evidence about our mat ing history tha t we can glean f rom our bodies is a bit more personal than height. In many mammals, the size of the male testis is correlated with the number of females a male might potentially mate with over a shor t period of t ime. The testes, of course, produce sperm, and generally speak- ing, the larger the testis, the more sperm a male can manufacture. Usually, male animals produce enormous numbers of sperm cells
(human ejaculates are 1.5-5.0 milliliters and contain anywhere
from 20-150 million sperm per milliliter), but ejaculates need to be replenished, and might not be sufficient to fertilize the available females if a male is mat ing very frequently. In addition, if a female mates with more than one male in a short period of time, the sperm in her reproductive tract can compete with each other, in which case the male supplying the most competitors is at an advantage.
Several studies of animals, including primates, have found tha t species with more male competit ion for mates have larger testes than do species in which monogamy prevails. Interestingly, however, a 2010 study by Carl Soulsbury showed no relationship
between the amoun t of mat ing outside a group—the "extra-pair paternity" that many paired-up animals show—and testes size
192 Paleofantasy
among mammal species, suggesting tha t other factors besides sperm competition may be at play.32 Soulsbury also found that spe- cies with larger litters had larger relative testes, again suggesting
that male competition is no t the whole story behind the evolution of our genitalia.
Where do humans fit into this picture? Human testes are smaller relative to body size than those of chimpanzees and bonobos, but
larger than those of either the monogamous gibbons or the goril- las. Although male gorillas are substantially larger than females, because they live in groups where only one male routinely mates with the females, the silverbacks have relatively little need for competition with other males. Most researchers have concluded that this finding supports our decreasingly polygynous history, although Ryan and Je tha t ry to make the case tha t it points to a life of polyamory, with simultaneous multiple par tners for both
sexes.33 They also note tha t men f rom different ethnic backgrounds differ in relative testis size, though since we do not know how each culture varied in the degree of multiple mating, it is hard to draw any conclusions f rom such information.
More recently, detailed examination of the stretches of DNA that are present in our ape relatives but absent in modern humans revealed a loss tha t women, at least, have cause to celebrate: the
genes coding for "genital tubercles," or more graphically, penis spines. In many other mammals, including chimpanzees, the penis has hardened growths tha t may serve to sweep away the sperm of
previous mates. These structures are absent in humans because we lack the genes responsible for the hormone signals tha t would cause
them to develop. The relative smoothness of the human penis is thought to be linked to a reduced frequency of sperm competition.
So, is monogamy swimming against an evolutionarily promiscu- ous tide? I doubt it. Lifetime fidelity to a single par tner may be rare among animals, and even among humans (whether it should be lik- ened to corn syrup, as the quote at the beginning of the chapter
Paleofantasy Love 193
says, I leave up to the reader), but the sheer variation in mat ing systems among human societies in bo th space and t ime makes i t unlikely that we have all been ignoring our t rue natures. If evolu-
t ion favored a single marriage or sexual system, why would we no t all have converged on tha t pat tern?
The piece of the puzzle tha t still needs addressing is the real evo- lutionary payoff: the "F" word. No, no t tha t "F" word; several of the
musings on human sexuality are as explicit as anyone could want.
I am referring instead to "fitness," the biological t e rm for success at passing on one's genes. A mating system will persist if its fol- lowers are bet ter than others at having offspring that survive and
reproduce themselves. How humans do that is the topic of the next chapter.