Paleofantasy: What Evolution Really Tells Us about Sex, Diet, and How We Live (23 page)

Polygyny is still found in many historical and modern human groups, with wealthier men able to have multiple wives, while poorer ones can afford to have only one or stay single. Monogamous marriage as we now know it was thought to have become more prevalent after societies became more complex and agricultural and less nomadic. Laura Fortunato, an anthropologist at University College London, analyzed marriage patterns from Eurasia, using a tree of cultures that mimicked an actual evolutionary tree such as that showing, say, the relationships among various members 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.
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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, but more interesting, monogamy appeared to have arisen well before modern record keeping. In other words, at least some of the first settlers emerging after our hunter-gatherer nomadic ancestors appear to have been monogamous. This puts to rest the argument that monogamy is a recent invention that required the development of a more industrialized way of life.

A different line of evidence, however, suggests that the pattern of multiple partners, or at least multiple females mating with a single male, was also a part of our history, just as Ryan and Jethá 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 mating patterns, researchers can study the way genes vary on the sex chromosomes and then compare 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 that a relatively small number of men have multiple wives, those few men will reduce the overall genetic diversity in their offspring—except for the X chromosome. 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 comparison 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 colleagues found in samples from populations around the world. They concluded that polygyny was a part of our history, and the evidence is written in our genes.
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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 from its birthplace: new immigrants contribute new genes, while those who stay home and marry neighbors or especially relatives (even distant 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 it could track the genetic signature of polygyny back to the start of agriculture, about 10,000 years ago. Anthropologists believe that around that time, along with eating grain, people began to have more patrilocal residence patterns, 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 pattern 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 from Hammer’s, with the discrepancy now being attributed to differences 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 successfully reproduced under a variety of mating 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 that people want to make into a single “natural” way—we don’t have just one natural pattern of the sexes.

Hunting, gathering, and sex

Whether our ancestors were monogamous or polygamous, another hallmark of humanity is a sexual division of labor, with men and women often performing different tasks in a society. That division is frequently linked to other differences between the sexes, like the purported lack of mathematical ability in women, and to the evolution of the nuclear family and (once again) the knotty issue of infidelity. So, do we really come from a 1950s-style family structure, 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, perhaps the most important, component of the story about sex and evolution: the aftermath 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 from 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 contributing 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 offspring. 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 part of this scenario, because selection would have favored smarter men to be better 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 evolution of intelligence, creating a feedback loop in which ever-smarter individuals reinforced the social system.

Hrdy calls this exchange of meat for fidelity the “sex contract,” and versions of it have remained a part of the story of our evolution for the last several decades. Hrdy’s interest in it pertains mainly to how it affects child-rearing and the human family, which I will discuss in more depth in Chapter 8, but the sex contract is also relevant for understanding gender differences and our sexual nature itself.

For many reasons, hunting 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. Putting hunting, and males, front and center in ideas about the evolution of gender roles also meant that women’s work was not seen as particularly valuable to the group. Anthropologist Lori Hager suggests that the early models of Man the Hunter were popular in part because they validated the way Western families were structured in the 1940s–60s.
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Even today, most museum dioramas and other illustrations of prehistoric family life depict a man setting out with a spear, or holding a captured rabbit, while a woman sits by the campfire, an infant at her bosom. The implication is that women stay home and care for the children, while men go out and bring home the bacon, or mammoth meat.

Starting in the 1970s, a number of mainly women anthropologists, most notably Adrienne Zihlman from UC Santa Cruz, began to question this macho perspective. Zihlman and several others discovered that in contemporary foraging peoples, women’s gathering often provided the bulk of the nutrients consumed by the group, and furthermore, that 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 stand for a way of life that placed males center-stage, gave an evolutionary basis for aggressive male behavior and justified gun use, political aggression, and a circumscribed relationship between women and men as a ‘natural’ outcome of human evolutionary history.”
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Zihlman and others since the 1970s have promoted a different version of human evolution, dubbed “Woman the Gatherer,” focusing on female contributions and women’s lives beyond child rearing. Other ideas have been proposed over the last few decades; for example, food sharing, particularly among non-kin, is sometimes viewed as an important 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 relative roles of hunting 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 turtles or deer.
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Is this division of labor simply a matter 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 “compete to grow outsized roots, sometimes spending days preparing a single hole, while women plant dozens of ‘table’ yams in smaller holes.”
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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 future planting rather than eaten.

Bird examined the possibility that women must trade off the need to take care of their children with the demands of foraging, since hunting big game is not compatible with caring for infants and toddlers, but while the need for work-life balance seems as real for the Aché of South America as it is for Manhattanites, that trade-off is not the whole answer. Women who are past reproductive age do not seem to spend more time hunting than do younger women, at least in some foraging cultures, so there must be more to the story than the demands of child care.

Furthermore, humans from many societies worldwide share food, not only with their children and mates, but with other members of society. Such generosity can play an important 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 Aché, the Martu of northwestern Australia, and the Meriam from the eastern Torres Strait Islands.
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These societies differ dramatically in their sexual division of labor, with men in the Aché contributing more than 85 percent of the food, while Martu women bring most of the calories to their groups. The researchers reasoned that at times when the available food sources were high-risk sources, women’s contributions should be more important, since hungry children can’t just wait for daddy to try again next week to bring down a kangaroo, and providing for children takes precedence over sharing with the group. Alternatively, when getting 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 hunting and gathering changing depending on what was available to eat. For example, among the Meriam, turtle hunting is a chancy business much of the year, but during the nesting season it becomes more reliable. At that time, but not otherwise, the women participate by helping to plan the hunt and butcher the meat; unmarried men are the ones to take on the more risky hunting outside the nesting season. In all three societies, both 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 often shared among the group members, rather than being consumed only by the hunter’s family, as well as the unreliable nature of hunted meat as a staple in the diet, has caused some anthropologists to question whether the main function of hunting is even subsistence at all. Kristen Hawkes, an anthropologist at the University of Utah, has suggested, instead, that hunting is a way for men to show off to prospective mates, and that good hunters gain high status in their social groups.
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It is not that hunted meat isn’t eaten and appreciated by the group, but that a given man’s share does not necessarily increase the survival of his own children more than other sources of food do.