Pleistocene Passions

For well over a million years, our hominid ancestors survived on the African savannas, scavenging or hunting meat and gathering plants. It was during that vast preamble to modern life that our ancestors first came to depend on tools for their existence and, according to Dunbar, the first time that they lived in complex societies, understanding their fellow hominids with a theory of mind.

In that world, natural selection would have favored certain behaviors and abilities. Some of them were basic survival skills–the ability to make a stone tool, perhaps, or powerful eyesight for spotting game at great distances. Other behaviors helped them find mates. The same powerful evolutionary forces that created the peacock’s tail and infanticidal lions were presumably at work on our Pleistocene ancestors.

If hominid behavior was sculpted by the demands of sex and family, are we still ruled by those Pleistocene passions? Few questions in evolution have triggered so much debate, so much anger and rancor. A number of scientists argue that we are ruled by these passions; they even go so far as to claim that we can dissect them and discover their original adaptive value. Opponents say that human behavior has been unmoored from its evolutionary pier: any attempt to pin an emotion or custom in living humans to some adaptation on the African savanna a million years ago is pure scientific hubris. This debate is about more than nature versus nurture: it gets to the very heart of how we should go about trying to understand the evolutionary past.

Edward O. Wilson of Harvard opened up this psychological can of worms in his landmark 1975 book Sociobiology. Most of his book surveyed the great success that scientists were having in using evolutionary theory to understand the social lives of animals. What at first seemed like evolutionary paradoxes turned out, after some careful study, to make sense. Sterile worker ants can help pass on their own genes because they are closely related. When a male lion takes over a pride and kills the cubs, he brings their mothers into estrus so that they can bear his own offspring.

“Let us now consider man,” Wilson wrote in the opening of the last chapter of Sociobiology, “as though we were zoologists from another planet completing a catalog of social species on Earth.” Human beings are primates living in big societies. They descend from hominids who probably evolved reciprocal altruism and food sharing. Barter, exchange, and favors became a crucial part of early human society, just as did deception and subterfuge. Males and females had specific roles in these early societies, the males killing game and the females raising children and gathering plants. Sexual selection, Wilson speculated, must have helped drive human evolution. “Aggressiveness was constrained and the old forms of primate dominance replaced by complex social skills,” he wrote. “Young males found it profitable to fit into the group, controlling their sexuality and aggression and awaiting their turn at leadership.”

By trying to turn psychology into evolutionary biology, Wilson created a sensation. Sociobiology became a best‑seller and inspired a front‑page article in The New York Times. Human behavior, the newspaper declared, “may be as much a product of evolution as is the structure of the hand or the size of the brain.” But Wilson inspired a lot of antipathy as well. Much of it came from the academic left, which accused Wilson of using science to justify the status quo, to apologize for all the inequalities of modern life. Protestors stormed scientific conferences where Wilson spoke, chanted their anti‑sociobiology slogans; once they even dumped water on his head.

More restrained critics argued that humans didn’t fit into sociobiology’s straitjacket. Inclusive fitness might account for how a sterile ant cares for a queen’s offspring but not for the many forms that human families can take. Consider the Nuer people of the Sudan. They treat barren women as men and allow them to marry other women who are impregnated by other men. The children that these wives give birth to are treated by the Nuer as the barren woman’s. Such a family comes into being out of cultural traditions, not out of a genetic imperative.

Today many anthropologists still raise this sort of objection to sociobiology. But beginning in the 1980s, some dissenters found that their own data was consistent with it. Kristen Hawkes was one of them. She launched her anthropological career among the Binumarien people of the New Guinea highlands, studying the ways in which kinship influenced the way they behaved. She studied the categories that the Binumarien used to classify relatives, for example, and the ways in which relatives helped one another. Only when she returned to the United States did Hawkes start to think seriously about the possibility that evolution might influence human culture, and she decided to use her data on the Binumarien to test sociobiology.

If Wilson was right, the Binumarien should make clear distinctions between people of different levels of genetic relatedness. After all, to boost their inclusive fitness, they should help a brother before they help a cousin. But Hawkes found that the Binumarien language did not make sociobiological distinctions possible. Two men that Westerners would call cousins, for example, are called brothers among Binumarien. (Western society has its own slippery vocabulary: an uncle may be your parent’s brother–and share on average a quarter of your genes–or the husband of your aunt, sharing no genes at all.)

It might seem as if Hawkes had found a case against sociobiology–that the relative closeness of relatives didn’t make much difference to how a Binumarien thought of them. But underneath the surface of their family terms, it appears that inclusive fitness is at work. For food, the Binumarien raise pigs and grow crops such as sweet potatoes in their gardens. All adults in the community have gardens of their own, so any gardening help they might give to someone else means that they have less time to tend their own garden. Regardless of the terms they may use for their kin, Hawkes found that they spent more time working in the gardens of people who are genetically closer to them than those who were more distantly related.

As some anthropologists warmed to sociobiology, sociobiology itself grew more nuanced in the 1980s. Its advocates no longer argued that genes created behavior in a deterministic way; instead, they showed how genes regulate the way in which animals make unconscious decisions about mating and raising their offspring. These adaptive strategies–“decision rules,” as Stephen Emlen has called them–let animals behave in different ways in different circumstances.

Emlen himself has shown just how complex decision rules can be among the bee‑eater birds he studies in Kenya. A young female bee‑eater can choose to breed at the start of her first breeding season, help a breeding pair at her nest burrow, or sit out the season altogether. If an unpaired dominant older male courts her, she almost always leaves her family and home territory to join him in nesting in a different part of the colony, particularly if he has a group of helpers to assist in feeding the offspring they will produce. But if young subordinate males are the only mates around for the choosing, the young female bee‑eater will refuse to join them, because young males come with few helpers and will be harassed by their father to come back to their nest to raise their younger siblings.

Emlen has shown that the forces of evolution can create a subtle, flexible strategy in a humble bird, an animal with a tiny brain hardly capable of much thought. Why couldn’t equally complex–and unconscious–decision rules have evolved in hominids as well?

The new sociobiologists also began to focus on the evolutionary pressures our ancestors experienced on the African savanna. For more than a million years our ancestors were living in the same place–African grasslands–and survived there in the same way–as small bands of hunter‑gatherers. They killed or scavenged game with stone tools, and the rest of their food they got by digging up tubers and collecting other plants. They had to find mates and raise children under the same conditions. Over time both their bodies and their minds adapted to this way of life. As part of that adaptation, our ancestors might have evolved mental modules well‑suited to life on the savanna. They used the modules like the blades on a Swiss army knife, each well‑adapted for its own job in a world of hunter‑gatherers.

For many contemporary sociobiologists, those days on the savanna are gone, but in terms of evolution they’re not forgotten. Industrialized civilization has existed only for a couple of centuries, and it has been a matter of only a few thousand years since humans shifted from hunting and gathering to agriculture. That’s a fraction of percent of the entire span of hominid evolution. Although our lives may now be very different, there hasn’t been enough time for natural selection to change our psychology very much.

Looking at ourselves this way may help us understand why we are better at some mental tasks than at others. This approach to the mind, called evolutionary psychology, is championed by the husband‑and‑wife team of Leda Cosmides and John Tooby, a psychologist and an anthropologist, respectively, at the University of California at Santa Barbara. Cosmides and Tooby have used this approach to interpret some peculiar results from psychological experiments. In one case, Cosmides updated a classic psychological experiment in logic called the Wason Test. Imagine that someone lays four cards in front of you. They read Z, 3, E, and 4. You are told that there are also symbols on the other sides of the cards, and that there’s a general rule that a vowel card always has an even number on the other side. Which card or cards do you need to turn over to see if this rule holds?

The answer is that you need to check E and 3. (You don’t need to check the 4 card, because even if it has a consonant on the other side, the rule isn’t broken.) People generally do very badly with the Wason Test. But Cosmides showed that their success rises dramatically if it is translated into social terms. Say someone sets down another four cards in front of you that read 18, Coke, 25, and Rheingold. The cards have the ages of people in a bar on one side and the drinks they have ordered on the other. Which cards would you have to turn over to find out if any of them were breaking the law by drinking alcohol under the age of 21?

The correct answer is 18 and Rheingold. Almost everyone who takes this form of the test gets it right, even though it is based on precisely the same underlying logic as the original Wason Test. Cosmides and Tooby argue that we are so good at this form of the test because we are skilled at keeping track of social complexities. Our ancestors evolved a module for sensing cheaters, because in a band of hunter‑gatherers who had to share meat, tools, and other valuable items, people would benefit from being able to figure out if any individual was trying to take advantage of the rest.

Evolutionary psychologists argue that our ancestors would have also needed particularly powerful modules for our behavior with the opposite sex and with our children. It’s with them, after all, that our ultimate reproductive success rests. In this way, we are no different from other animals. A peahen depends on evolved decision rules to choose a mate, but she doesn’t actually work out an equation of the costs and benefits of her choice in her head. Instead, the things she sees, smells, and experiences trigger her to do certain things. People, likewise, don’t fall in love according to cold, rational calculations of genetic benefits. But according to evolutionary psychologists, feelings like love and lust and jealousy are triggered by adaptations in our brain.

What, for example, do people find attractive in others? That’s the first step, after all, in choosing a mate. Many animals show a strong liking for symmetry, possibly because it’s a reliable clue to good health. It may also be a good clue in humans as well. In one study, David Waynforth of the University of Mexico measured the symmetry of men’s faces in Belize and found that the men with more asymmetrical faces were more likely to suffer from serious diseases.

The symmetry or asymmetry of a face is often too subtle for people to consciously recognize, but it does influence how we judge people’s attractiveness. David Perrett of St. Andrews University used a computer to manipulate photographs of faces, creating ones that were more or less symmetrical. He then showed his subjects a group of original pictures and their doctored versions, asking them to rate how attractive they were. Perrett’s subjects tended to pick the symmetrical over the asymmetrical faces.

Faces may not have been the only clues that our Pleistocene ancestors used to choose potential mates. When girls reach puberty, they take on certain features that signal that they are becoming fertile. Their hips widen because they are putting on fat that can act as a reserve of energy during pregnancy. Fertile women have waist measurements that are between 67 percent and 80 percent of their hips. Men, children, and women who are postmenopausal have more closely proportioned waists that are 80 percent to 95 percent the size of their hips. A low hip‑to‑waist ratio correlates with youth, health, and fertility.

We appear to have a finely tuned awareness of these proportions. Devendra Singh, a psychologist at the University of Texas, has surveyed men and women of different ages and cultures, showing them pictures of women with different hip‑to‑waist ratios. As a rule, a ratio of 60 to 70 percent is considered attractive. Even as tastes in women’s appearance change, this ratio holds up. Singh has found that while Playboy models and pageant queens have gotten thinner over the years, their hip‑to‑waist ratio has remained the same. This enduring taste may have emerged more than a million years ago as a way for men to choose mates who were more likely to bear children.