The Privatization of Sex
Ape societies are driven by intrasexual competition, the rivalry between males for access to females. Although male chimpanzees form coalitions to seize power within the male hierarchy, these are shifting, ad hoc arrangements. A male chimp probably sees most other adult males as potential rivals, an attitude that limits the degree of cohesiveness in chimp society.
The human line of descent probably inherited the ape system of separate male and female hierarchies. But around 1.7 million years ago, the size difference between males and females started to diminish, according to the paleontological record. This shift in size is almost certainly a sign that competition between males had diminished because of the transition to the pair bond system.
The novel arrangement of pairing off males and females creates a whole new set of social calculations. Most males in the society now have a chance to reproduce since they possess socially endorsed mating access to at least one female. So each male has a much greater incentive to invest in cooperative activities, such as hunting or defense, that may benefit the society as a whole.
The pair bond takes much of the edge out of male-to-male aggression. It also requires that men trust one another more, and can have some confidence that those who go hunting won’t be cuckolded by those who stay to defend the women.
For the females there is a trade-off. They must give up mating with all the most desirable males in the community and limit their reproductive potential to the genes of just one male. On the other hand they gain an implied guarantee of physical protection for themselves and their children, as well as some provisioning. In some foraging societies the men bring back meat but the staple foods are plants and small animals that are mostly gathered by the women. But a man’s food gathering efforts were probably particularly helpful during the frequent periods that a woman was nursing and could find less food for herself.202
Like most of evolution’s behavioral arrangements, pair-bonding was not a rigid prescription, one that dictated a one man, one woman nuclear family. Many human societies are polygamous, allowing men to have more than one wife if they can support more. Societies in special ecological conditions allow women to have more than one husband, such as in the high altitude agriculture of Tibet where a set of brothers may marry one wife and raise the children as a single family.
With the institution of pair bonds, sex became something conducted within families. It was presumably at this time that human societies developed a taboo against public sex, a custom that would bring chimp or bonobo societies to an almost complete standstill. The privatization of sex would help considerably in removing sex as a provocation of male rivalry.
But though the pair bond system alleviated discord between males, it raised new tensions between men and women. The asymmetry between the male and female roles in the family unit—a woman looks after the children, a man protects and supports her and her children—sets up an inevitable difference of reproductive interests and strategies. Men have evolved traits like sexual jealousy, for the sound reason that complaisant husbands are likely to pass on fewer of their genes.
Sexual infidelity poses very different kinds of risk for men and for women. For a woman, the threat posed by her husband having a mistress is not so much the sexual dalliance in itself but rather the possibility that he may switch his support to his paramour and away from his family. The withdrawal of support would reduce her reproductive fitness, as measured by the chance of raising her children successfully to maturity.
Serious as that danger may be to a woman’s interests, the risk to a man of his wife’s infidelity is considerably graver. For him, the threat is that he may not be the father of his wife’s children. In evolutionary terms, a man who devotes his life to raising another man’s children has seen his Darwinian fitness reduced to zero.
Men’s fear of being deceived is not without basis, in a general sense, since a woman has a heavy incentive to seek another partner should her husband prove infertile, a not uncommon occurrence. Even if her husband is fertile, a woman might improve her reproductive success by having children with more than one partner. Ideally a woman will seek both support and good genes from her husband, but as long as that support is guaranteed her reproductive interests could in principle be improved by seeking better genes elsewhere. Many men may be willing to offer her this service, since they can greatly improve their reproductive success by having children with as many women as possible, especially if another man bears the cost of raising them.
It is no surprise, therefore, that men have gone to great lengths to secure exclusive access to women whose children they have undertaken to raise, with methods that range from foot-binding and genital mutilation to purdah, veils, chadors and an array of laws and customs restricting women’s activities. However abhorrent the means, the motivation stems from the inherent vulnerability of male reproductive strategy: mother’s baby, father’s maybe.
How often do women conceive children with men who are not their husbands? Ornithologists used to rhapsodize about the marital fidelity of bird species that stayed pair-bonded for life. That was until the advent of protein-based and later DNA tests for assessing paternity. Despite the appearance of fidelity, extra-pair liaisons in the bird world turned out to be routine. The preeminent adulteress is an Australian bird, the Superb Fairy-wren, 76% of whose offspring are fathered by extra-pair copulations.203
Human geneticists testing people for heritable diseases quite frequently stumble across cases where the father of record cannot be the biological parent. Genetic counselors have a rule of thumb that these discrepancies, known delicately as nonpaternity cases, will range from 5 to 10% in an average American or British population. For the U.S. population as a whole, “The generic number used by us is 10 percent,” said Bradley Popovich, vice president of the American College of Medical Genetics.204
The degree of nonpaternity that has come to light in the United States and Europe is particularly surprising in light of the control that women now exert over their reproductive behavior. Presumably many of the children involved in nonpaternity cases are not conceived by accident. The evident implication is that the woman’s conception with a man other than her husband is in some cases deliberate.
That women in modern societies sometimes choose to conceive with alternative partners is a matter that bears on an issue of considerable debate among primatologists, that of whether the phenomenon of sperm competition occurs to any significant degree in people. In many species the female is inseminated by more than one male at the same time, and direct competition takes place in the female’s reproductive tract between the sperm of rival males to fertilize the eggs. The female reaps the significant genetic benefit of having her eggs fertilized by the best of the competing sperm. Has evolution dispensed with this useful grading method in humans or does it apply in our species too? “Sperm competition is possible in Homo sapiens, though whether it has played a significant role during human evolution remains highly debatable,” says Alan Dixson, an authority on primate reproduction. 205
Geneticists have recently studied the DNA sequence of several genes involved in sperm production in three primate species, chimpanzees, gorillas and people. In chimpanzees, among whom sperm competition is fierce, the genes show signs of being under strong selective pressure. The pressure is much less fierce in the gorilla version of these genes, as would be expected given the silverback’s exclusive access to his harem. In humans the genes have evolved at a rapid clip, faster than that of gorillas and equal to that of chimps. The human sperm genes are clearly under some kind of fierce evolutionary pressure, and sperm competition may be the reason.206
Sperm competition requires not just that a woman has more than one lover but that she has two within a rather short time of each other.207 Some 4% of people in Britain are conceived under such competitive conditions, according to Robin Baker, a University of Manchester biologist.208 This estimate receives some support from data on heteropaternity, a phenomenon
that occurs in fraternal twins. Unlike identical twins, who arise from splitting of the same egg, fraternal twins result when the mother releases more than one egg in an ovulatory cycle. Heteropaternity refers to the circumstance in which each of two eggs is fertilized by different fathers. In Greek mythology the twins Castor and Pollux were the sons respectively of Tyndareus, a king of Sparta, and Zeus, who in the guise of a swan seduced Tyndareus’s wife Leda. This may have been the first case of heteropaternity but it was by no means the last. Of fraternal twins born to white women in the United States, 1 in 400 pairs is estimated to have two fathers.209 Among cases where paternity is disputed, 2.4% of cases have been found to be heteropaternal.210
For a woman to have a child extramaritally carries a serious risk—that the infant of an extramarital liaison may look suspiciously unlike the father of record, putting both its own life and its mother’s at risk. This hazard would have created a strong selective pressure in favor of genes that prevented infants from looking too much like their parents. And indeed babies tend to have chubby faces with indistinct features that give them a rather generic appearance, sharply mitigating the risk that they will look like the wrong father.
To the extent they resemble anyone, babies would be expected to look as much like their mother as their father. But researchers have found that grandparents and others comment far more frequently on a baby’s similarities to its father. Mothers tend to state that a baby resembles its father, and do so more often when the father is present, as if trying to assure him of his paternity. “Whether mothers do this consciously, knowing full well that the baby looks nothing like its dad, or whether they deceive themselves into thinking that the baby really does look like the father is unclear,” say the authors of a textbook on evolutionary psychology.211
Because of the central significance of reproductive success, evolutionary psychologists have paid particular attention to human mating habits, exploring the signals that govern male and female choice of a mate, and the strategies that each pursues to accomplish its reproductive goals.
In studying the mating signals that the human psyche is genetically primed to assess, evolutionary psychologists have found that men in many different cultures of the world prefer women with a waist to hip ratio of 7 to 10. The male eye is probably attuned to these proportions because they signal a woman’s fertility. Young women tend to put fat on their hips, breasts and buttocks whereas older women, and those who are pregnant, get thicker at the waist. “A relatively narrow waist means ‘I’m female, I’m young, and I’m not pregnant,’” writes the evolutionary biologist Bobbi Low.212 Symmetry of features, especially of the face, is another indicator of good genes; it requires a normal development in the womb and is thus a marker for general health. There are of course variations on the general theme. Among the !Kung, men are driven wild by a sizable protuberance of fat on a woman’s buttocks, presumably a signal of being able to nourish a child in difficult environments.
Because reproduction is a greater risk and investment for women than for men, according to the biologist Robert Trivers, it follows that women will be more choosy about their partners than men are; and because women are more selective, men will find themselves being more competitive with each other for women’s favors. A woman looks for indicators not just of good health in a man but also of commitment to look after her and her family. This is a matter partly of emotional commitment, which women assess with care, and also of wealth or the ability to acquire it, as may be indicated by a man’s social status.
Surveys conducted over many years have consistently indicated that American women care more about a partner’s wealth than men do. The evolutionary psychologist David Buss expanded this survey to 10,000 individuals in 37 world cultures and found the same pattern—that women placed more value than did men on a partner’s financial prospects.213 Women in almost all cultures prefer men of high status, presumably because this is likely to be correlated with wealth. Women consistently prefer men who are slightly older, for reasons that are not obvious. The preference could be a holdover, Buss suggests, from hunter-gatherer days when older men, at least through their twenties and thirties, were stronger and better able to offer physical protection to their family.214 Perhaps for the same reason, women consistently prefer tall men to short.
If fitness indicators for health and fertility are useful, wouldn’t indicators for mental ability be even more useful in choosing a partner for a long-term relationship? The evolutionary psychologist Geoffrey Miller has advanced the striking theory that such indicators do exist, but they are familiar under names that give no clue to their biological function. The indicators of mental fitness, in his view, include both cultural activities such as art, music, dance and literature and moral qualities such as kindness.215
Evolutionary biologists have gained considerable insight into what makes fitness indicators true signals, and why they must be qualities that vary in a population. Fitness indicators, and the behavioral preferences for them, are brought about by sexual selection, a form of natural selection but one that works through mating success rather than physical survival. The mechanism of sexual selection was first recognized by Darwin, who had long been puzzled why the males of many species are heavily ornamented, with conspicuous horns or antlers or feathers. These baroque decorations seemed to contribute nothing to survival, posing an apparent challenge to Darwin’s idea that the fittest survive. The solution he proposed in The Descent of Man was female choice: peahens for some reason preferred peacocks with gaudy tails, who got to sire more offspring, including sons with gaudy tails and daughters with a taste for them. These male adornments were therefore a worthwhile handicap to their owners because they assisted toward evolution’s bottom line of getting more genes into the next generation.
That still left the question of how these male embellishments evolved in the first place. Darwin’s theory of sexual selection was largely ignored for a century—his contemporaries placed no credence in the idea that female choice could be a major evolutionary force—and it was not until the 1970s that biologists started to develop the theory. One insight was that male ornaments like long plumage were hard to grow and therefore served as an overall indicator of good genes. But if long red tail feathers, say, were the key to male reproductive success, soon every male would be wearing them and they would lose their utility in helping females choose between males of different quality.
The evolutionary biologist Amotz Zahavi realized that sexual signals of one’s health and fitness, if they were to be true and reliable, had to be so costly as to constitute a serious handicap for the displayer. Weak peacocks grow unappetizing tails and only the strongest can grow really beautiful ones. It’s that spectrum of ability that provides peahens with a basis for choice. Biologists call such a trait heritable—the quality of the tail varies from one individual to another and part of the variation is caused by the genes.
The physical features that have evolved as fitness indicators in the human mating dance—symmetry of features, fine skin, a shapely body—are known by another name: beauty. People find these features attractive not through some arbitrary criterion or dictate of fashion but because the male and female minds have evolved to look for and appreciate such qualities in a potential mate.
Zahavi’s costly signaling theory explains, sad to say, why it is impossible for everyone to be equally beautiful or handsome. Since beauty serves as a fitness indicator, it needs to vary from one individual to another. If everyone were equally beautiful, beauty would have no value as a criterion for sexual selection.
The privatization of sex that began 1.7 million years ago did not bring an end to all competition between males for females. But it was a major step in reducing human aggressiveness within societies. And it was followed, many thousands of years later, by a serious evolutionary reduction in the level of aggression between societies.
The Domestication of People
The evidence that human tribes have become less passionately host
ile to each other lies in a worldwide thinning, or gracilization, of the human skull that took place during the Upper Paleolithic era. The fossils of early modern humans are both large and very robust, or thick boned. But these generic early modern skulls started to change around 40,000 years ago. In each region of the world they follow an independent, but largely parallel course, as if similar genetic changes are occurring independently in each population. “Cranial size reduction and gracilization may have been homoplasic [arising by independent evolution] in most populations,” writes the physical anthropologist Marta Mirazón Lahr.216
The gracilization occurred at different rates in different regions but all followed a common trend, except for two populations at the extremities of the human diaspora, Australian aborigines and Fuegians at the tip of South America. The Australian skulls became smaller like the rest, but retained their robusticity, presumably as a result of independent evolution. The Fuegians seem to be a case of genetic drift—a small isolated population developing its own special characteristics.217 Gracilization is farthest advanced in sub-Saharan Africans and Asians, with Europeans still in some instances showing large size and robusticity.218
What caused the gracilization of human skulls and the shrinking of human skulls and teeth all over the world? This is a large and complex issue, not least because a very similar downsizing affected the sheep, goats and other animals that were domesticated in the Neolithic era after the advent of agriculture. Researchers have attributed the smaller size of domesticated animals (compared with their wild forebears) to such facts as different diet, less physical activity, and a relaxation of the selective pressure favoring larger males under the conditions of captive breeding.219
Before the Dawn: Recovering the Lost History of Our Ancestors Page 21