The Mating Mind: How Sexual Choice Shaped the Evolution of Human Nature (38 page)

their allocation of resources to different indicators. One individual may grow very tall and muscular; another may grow very symmetric breasts; yet another may grow very intelligent. Each may advertise the same general level of fitness, but may advertise it in a very different way. If height, breast symmetry, and intelligence are all fitness indicators, then—by definition—they must all correlate with fitness, so they must also be positively correlated with one another to some extent. However, such correlations might be quite modest. This implies that even if individuals select mates for their overall fitness, sexual selection may not have the power to drive every fitness indicator to its maximum value. Instead, sexual selection may produce a great diversity of strategies for allocating scarce bodily resources among different indicators. Variation in overall fitness level, combined with variation in these allocation strategies, may account for the rich human variation that we observe. It also explains why not all women have very large breasts—many women may be genetically programmed to prioritize other indicators of physical and mental fitness.

Like penises, breasts have given us some practical information about mate choice in the Pleistocene. The amplification of female human breast size beyond what was useful for milk production reveals the importance of male mate choice in human evolution. If males had not been picky about their sexual partners, female humans would be as flat-chested as chimpanzees. The clitoris does not yield evidence of male mate choice, but breasts do. This opens the door to the possibility of male mate choice influencing the evolution of female brains as well as bodies. Breasts seem to act simultaneously as indicators of youth, indicators of developmental stability, and indicators of foraging ability. We shall see that many of the human mind's most distinctive abilities seem to serve the same range of functions.

Buttocks and Waists

Other great apes such as chimpanzees have small, hairy, flat rumps with tough skin patches on which they sit. But once our hominid ancestors started walking upright around 4.2 million years ago, the legs and buttocks were re-engineered. Much larger, stronger muscles evolved for powering the leg backwards so that it could propel the body forwards. These muscles are what give the human buttocks their basic rounded shape. Beyond this increased muscularity in both sexes, females evolved larger deposits of fat on the buttocks, hips, and upper thighs. Like breasts, these probably evolved through male mate choice as indicators of youth, adequate fat, and perhaps developmental stability.

We are the only species of primate with permanently protruding hemispherical buttocks, and the only species where this protrusion is permanently amplified in adult females by the addition of fat deposits. Buttock size and shape is a unique human feature and shows substantial sex differences. Buttocks are also age-specific, with almost no differences between the sexes before puberty, followed by a rapid accumulation of fat in female buttocks, hips, and thighs over a few years. Buttock size and protuberance normally peaks in young adulthood, around the time of peak female fertility, and then gradually diminishes relative to the rest of the body's fat reserves. Buttocks also show differences between human populations. In southern African Koi-San populations, female buttocks evolved through male mate choice to be especially prominent.

The sex difference in buttock size and shape is hard to explain through natural selection. Because female breasts and buttocks are composed primarily of fatty tissue, it has been suggested that they evolved to provide adequate fat reserves, to protect against the unpredictability and seasonality of food. However, almost all female vertebrates have evolved to store fat reserves, and only female humans have such an unusual distribution of fat. Gorilla females store plenty of fat inside their abdomens, as do human males. Why did human females deviate from this normal primate pattern to store fat below their hips?

Evolutionary psychologist Dev Singh has suggested that the female human distribution of fat evolved as an indicator of youth, health, and fertility. He found that men around the world generally prefer women who have a low "waist-to-hip ratio": a relatively narrow waist and relatively broad hips. Young, fertile women who are not pregnant have waist-to-hip ratios of around 0.7. This ratio would result from a waist circumference of 24 inches and a hip and buttock circumference of 36 inches, for example. Men almost always have a waist-to-hip ratio of at least 0.9, as do prepubescent girls and women past menopause. Obviously, pregnant women have even higher waist-to-hip ratios. Women with various health problems that impair fertility also tend to have higher than average waist-to-hip ratios. Indian temple sculptors have traditionally depicted Hindu goddesses with waist-to-hip ratios as low as 0.3, to symbolize their supernatural fertility and sexuality. In European fashion, corsets and bustle skirts have been used to lower waist-to-hip ratios deceptively. If male hominids have preferred low waist-to-hip ratios for many generations, this may explain why human females have such narrow waists, such broad hips, and such fleshy buttocks.

Women's breasts and buttocks did not evolve because hominid men happened to develop some arbitrary fixation on hemispheres as Platonic ideals of beauty They evolved as reliable indicators of youth, health, fertility, symmetry, and adequate fat reserves. Starving, sickly women cannot maintain large breasts and buttocks. They need to burn up their fat reserves to stay alive, not keep them hanging around in the hope of attracting a mate. Because starving women tend to turn off ovulation, women without fleshy breasts and buttocks are usually women without fertility. Female longdistance runners, ballerinas, and anorexics who lose most of their body fat tend to have much smaller breasts and buttocks, and often stop menstruating and ovulating. Buttocks, like breasts, reveal the importance of male mate choice in human evolution.

not exhaust the body's complement of sexual ornaments. Because they are sexually differentiated, they are especially informative about male mate choice and female mate choice. However, they are relatively minor contributors to physical attractiveness compared with the face, and with overall body height, proportions, and condition. Our lack of body hair, our long head hair, and our sex differences in musculature are also important signs of sexual choice. Nancy Etcoff's
Survival of the Prettiest
and Desmond Morris's
Bodywatching
have discussed these charms in great detail.

However, I would like to note a few features of the human head that put the human brain in its bodily context. The head is a major target of sexual choice in both sexes. It is rich in fitness information because it is such a complicated piece of the body to grow, and so many things can go wrong. The front of the head has evolved a convoluted shape because evolution tends to pile sense organs up at the front of the body, where they are best placed to sample that part of the environment toward which we are headed, and from which signals can reach the brain quickly. This is why we have eyes, ears, noses, and tongues all huddled together, rather man spread around the body more evenly. The orifice for ingesting food also evolved to be near the brain so that we could efficiently control what we eat and how we chew. The result of evolution assembling the mourn and sense organs so close to the brain is called the face.

An alien biologist might consider such an unseemly concentration of organs on one tiny area of the body rather disgusting, so it is striking that we consider faces so crucial to physical beauty. If the alien did not understand fitness indicators, he or she (or it) might be puzzled that we pay so much attention to the one part of the body that is too complicated for anyone to grow in a perfect form. Wouldn't we find it easier to focus on thighs or backs, which are so easy to get right? Yes, it would be easier, but it would not give us the fitness information we want. Instead of averting our eyes from the unsightly front of another person's head, where harmful mutations show themselves most readily as unusual proportions and asymmetries, we are sometimes so rude as to

stare at it, instead of their penis or their breasts. Have we no courtesy? Indeed, we pick the one part of the body where fitness differences are most manifest, and regard that as the seat of personhood. Where mutations show their effects most readily is where we direct our sexual judgment and social attention. A portrait of a human implies a representation of the face.

Much of this book applies the same fitness-indicator argument to the brain as well. Whereas we can perceive facial form visually, we can perceive a brain efficiency only indirectly, through a person's courtship behavior. Beauty is no longer skin-deep in our species. Sexual choice reached behind our faces to tinker with our minds. Mostly, it did so by connecting our brains in a unique way to our mouths, so that we could talk instead of just chewing and grunting. The attention we pay to faces and brains in sexual choice, our obsession with just those body parts that are most difficult to grow perfectly, is powerful evidence for the fitness-indicator view of sexual selection.

Weak Bodies, Strong Minds?

Now that we have seen a few examples of how sexual selection has shaped our bodies, we can step back and consider how the human body's evolution relates to the human mind's evolution. In the mid-20th century, many evolutionary theorists suggested that human bodies represent a degeneration from the wild, robust strength of other apes. They speculated that our supposed bodily weakness somehow forced our brains to become strong, so we could hold our own in the competitive ecology of prehistoric Africa. Reflecting this view, a persistent theme in Robert Heinlein's "Waldo" science fiction stories of the 1950s was that, as humans were allegedly ten times weaker and ten times smarter than chimpanzees, our space-faring, zero-gravity descendants will be ten times weaker and ten times smarter than us. Anthropologist Ashley Montagu influenced a whole generation of anthropologists with his view of neoteny: that the human body is weaker and more childlike than ape bodies, giving it a generality and flexibility uniquely suited for culture.

However, this compensatory view that our brains made up for our lack of brawn does not fit the fossil evidence. Since the rise of
Homo erectus
1.7 million years ago, our ancestors were among the largest and strongest primates ever to have evolved.
Homo erectus
males seem to have averaged almost six feet tall, with robust skeletons suggestive of powerful muscles. When modern
Homo sapiens
lived as a hunter-gatherer in reasonably food-rich environments, they also grew tall and massive. While brain size was tripling in our ancestors, body size was increasing as well. We are two feet taller and twice as heavy as our earliest bipedal ancestors of 4.2 million years ago. They would be more immediately impressed by our astounding size and strength than by the little puffs of air we call language.

For the last 2 million years, our ancestors have been larger than any insect or amphibian, and larger and stronger than about 90 percent of birds, reptiles, and mammals (to a first approximation, most mammals are rodents and rabbits). Among more than 300 species of modern primates, only male gorillas (averaging around 350 pounds) are significantly larger than humans (around 150 pounds); female gorillas and male orangutans are slightly heavier than male humans, while male chimpanzees weigh up to 130 pounds, and bonobos up to 90 pounds, for both sexes. Our ancestors were the most powerful omnivores in Africa. There were some larger hoofed herbivores, a handful of larger carnivores, and the odd elephant, mastodon, hippopotamus, or rhinoceros. But once our ancestors evolved the ability to throw stones, to wave torches around, to attack in groups, and to run for long distances under the midday sun, they were probably the most terrifying animals in Africa. It is a wonder they bothered to evolve more intelligence at all.

Good Condition as the Evolutionary Norm

It is a mistake to envision our hominid ancestors as bedraggled, dirty, shuffling, sniffling, unhealthy cave-dwellers. They lived outside on a sort of perpetual camping trip, and got a lot of exercise. They had an excellent diet by modern standards, probably

consuming about four pounds of fresh fruit and vegetables a day, and perhaps one pound of lean meat on good days (undomesticated game animals have very low body fat). They consumed hardly any salt or sugar, no chocolate, and no beer. They had no dairy products other than their mother's milk. They could not even eat pasta, bread, noodles, or oatmeal until cereal grains were domesticated around 10,000 years ago. The females would have been used to walking miles every day carrying infants and plant foods, and perhaps firewood and water. The males would have been used to chasing down wounded game, running for very long distances. Even our middle-aged ancestors would have remained in very good condition because they would still have made their livings as foragers.

Were we to be transported back 100,000 years in a time machine, we should not expect ancient humans of the opposite sex to fall on their knees and worship our god-like forms. If they were living in a reasonably food-rich habitat, they would probably have been as tall and healthy as us, and in considerably better shape. A week of living in the bush would have obliterated our initial cleanness and reduced our fine clothes to tatters. Any initial sexual interest we provoked would probably evaporate entirely after our total incompetence at hunting and gathering was revealed, and our cowardice in the face of wild baboons, leopards, snakes, elephants, and lions became the subject of jokes. Our bodies would, however, have provoked greater respect in any of the more recent pre-modern agricultural civilizations, in which nutrient-poor diets and communicable disease shrank average human stature by a foot and shortened human lifespans by decades.

Our ancestors would have considered most modern humans to be ridiculously fat, weak, breathless, unfit, and clumsy. They could not drive to the convenience store for a six-pack or a half-gallon of ice cream. They would not have been burdened by excess fat or by the excess muscle attained by modern bodybuilders by using weight machines, protein shakes, and steroids. Conan the Barbarian would have been too musclebound to run after and catch injured gazelles. Like modern human hunter-