Paleofantasy: What Evolution Really Tells Us about Sex, Diet, and How We Live (22 page)

And people seem quite convinced that all this vigilance is necessary. When asked to estimate the frequency of such misassigned 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 that I polled a few years back. I pointed out that this would mean that 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 often 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 truth, however—insofar as we can tell—is much less sensational. The most unbiased research suggests that the real incidence of misassigned paternity in Western countries hovers around 1 percent, with a few studies pushing that 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 most people undergo paternity testing only if they have a reason to suspect a discrepancy between the purported father and the genetic one. As a result, using data from 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 must inherit a copy of the defective gene from both parents to show the disease. When large numbers of families are surveyed for such research, a certain proportion of fathers turn out not to have the gene that their purported child inherited, thus yielding the figures of 1–3.7 percent. Higher numbers, particularly the often-cited 10 percent, apparently come from more biased populations, or, more likely, simply turn out to be urban legend, akin to cell phones being able to pop popcorn.

Interestingly, no one seems to question the apparent contradiction between the stereotype of faithful females and all this suspicion. The problem is that if men are going to have urges to stray, they have to stray to, or with, a partner—it takes two to cheat, of course. As Hester and her scarlet letter would attest, however, we have a double standard 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 nature of male and female sexuality, as evidenced in the online comments quoted earlier in this chapter. Of course, we really don’t know how low, or high, confidence of paternity 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 understand what our ancestral mating 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 written records of marriage patterns in ancient civilizations; and the evidence contained in our own bodies, which bear witness to the selection pressures that acted on men and women in our evolutionary history. All of these reference points have their shortcomings, but each is also valuable, so it is worth examining them in turn.

First our primate relations. The great apes—gorillas, chimpanzees, bonobos, and orangutans—vary quite a bit in their mating patterns. Gorillas live in groups with a single dominant male (the silverback, so named because age has grayed his fur) and multiple females, while orangutans are solitary for most of their lives and both chimps and bonobos have complex societies with many individuals of both sexes. Chimpanzee society is marked by pronounced 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 most recently shared a common ancestor, people have long been interested in whether some of our sexual proclivities, such as fidelity or the lack of it, or the preference for a particular kind of sexual partner, might have its origins in their behavior. Whether finding such common ground means that the aforementioned proclivities are “natural” (whatever that means), much less genetically determined 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 primary role that sexual behavior plays in their lives; they tend to settle conflicts with sexual activity, both between males and females and between members of the same sex. Like the chimpanzees, bonobos 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 from 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 frequent claim that “chimpanzees are from 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 sex life to understand our own, pointing out that while they “provide a concrete alternative to ‘macho’ evolutionary models derived from . . . chimpanzees,” the bonobo’s society is very different from our own, and we do not know how closely it resembles that of our early ancestors.
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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, hunting, tool technology, and other masculine fortes.”
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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 anthropologists 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 their book, Ryan and Jethá are very pro-bonobo, appreciating the face-to-face kissing that the species exhibits, along with the slow development of its infants, like that of humans but different from chimpanzee development.
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They also note that both people and bonobos have sex under circumstances other than procreation, such as when resolving conflict or cementing bonds between individuals, and they cite this similarity as grounds to believe that the bonobos are a more accurate representation of our ancestral state than are those no-nonsense chimps. They then go on to promulgate a rather orgiastic view of human sexuality, saying that our monogamous woes arise from an uphill, and ultimately doomed, battle with our more bonobo-like desires for multiple sexual partners. An abundance of casual sex, or as Ryan and Jethá put it, “Socio-Erotic Exchanges,” could then function to make the wheels of society run more smoothly, though I have trouble imagining the kind of genital-genital rubbing that is commonplace among female bonobos ever catching on at book clubs.

I will return to the idea of monogamy and its role in our evolution later in the chapter, but for now I want to comment on whether the bonobos, or any other living primate species, are a realistic model for our earlier behavior. It’s true that the bonobos and the chimpanzees 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 time has elapsed for selection to act separately on each of the three species. The bonobos may share nonreproductive sexual activity with humans, but vervet monkeys, a species much less closely related to us than are any of the apes, also have sex outside the time when a female can conceive. There is no
a priori
reason to expect that any particular aspect of our sexuality has been preserved from one ancestor and not another.

Why do the gender relations of our primate relatives matter at all? As anthropologist Craig Stanford points out, “The behaviors at the heart of the chimpanzee-bonobo interspecific variation—sexuality, power and dominance, aggression—are those that also lie at the center of the debate about human gender issues and what molds our own behavior.”
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He also extended feminist anthropologist Sherri Ortner’s contention that “men are to women as culture is to nature”
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by wondering if “chimpanzees are to bonobos as men are to women.”
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In other words, even now that we know about bonobos, males and females are still seen as different, and the genders are still stereotyped, but 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 imposing our preexisting biases (“free love is natural,” “males are violent brutes”) on species that are complex in their own way, not as caricatures of people. A better 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 important 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 social behavior and found that genetic relatedness was more important than environmental conditions in determining whether a given species lived in pairs, small groups, or large groups.
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History is important because if one is interested in understanding the evolution of any trait, whether social-group composition or a tendency to eat leaves, and that trait occurs in a number of species, the first question is whether each species inherited that trait from the same common ancestor, or whether it arose independently multiple times because of the same selection pressures. In the case of primate social-group structure, the conventional wisdom had it that our more distant ancestors lived in simpler societies with just a pair of animals or perhaps a family group. Then, larger, more complicated groupings evolved. The details of group structure—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 primate 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 species than 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 that, about 52 million years ago, primates went from a solitary life to fairly unstructured groups, and then to more stable ones. Then, both pair-bonded societies and single-male harems, like those seen in gorillas, emerged roughly 16 million years ago, rather than having a more linear evolution with groups always becoming more complex.

The age of social systems is significant in the context of the evolution of our sex lives because we know that the last common ancestor of humans and chimpanzees lived about 5–7 million years ago. Since modern chimpanzees and bonobos live in stable communities with multiple males and females, pair bonds—the early version of monogamy—must have emerged sometime after we all diverged from this more recent ancestor. When that happened is still a mystery, but the new data suggest that human mating patterns 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 partner theme. And indeed, sexual selection theory suggests that because males so frequently gain by attempting 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 limited circumstances.

Where, then, did our monogamy come from? Is it a cultural artifice that denies our basic nature, as Ryan and Jethá would have it, dooming countless men and women to lives of guilt and secret philandering? Or is it an adaptive part of human society? In a volume of multicultural perspectives on the family, Bron Ingoldsby claims, “Monogamy is certainly the most common of the marital types; the generally equal sex ratio throughout the world sees to that.”
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But this is specious reasoning; after all, males and females occur in roughly equal numbers in many animal species, but they exhibit a variety of mating systems, with many individuals, usually males, simply not mating at all. The real question is, what happened in human evolution? Or put another way, from a blog entry by historian of science Eric Michael Johnson, “Were our ancestors polygamists, monogamists, or happy sluts?”
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People have argued the question from all sides, but we are only now gathering the data that allow us to test it.