Evidence of Sexual Selection in Humans

By primate standards, humans look strange, even after we step out of our sport utility vehicles. Compared with other apes, we have less hair on our bodies, more on our heads, whiter eyes, longer noses, fuller lips, more expressive faces, and more dextrous hands. In most species, sexual ornaments like long head hair, hairless skin, and full lips would have evolved only in males, because females would have been the choosy sex. Males have few incentives to reject any female mates. The fact that both human sexes evolved distinctive sexual ornaments shows that both female choice and male choice was important in human evolution. If both sexes were choosy about bodies, they might also have been choosy about minds.

Not only do we look different from other apes, but each human sex also has distinctive body traits shaped by sexual selection. Men are taller and heavier on average than women, with more upper body strength, higher metabolic rates, more hair, deeper voices, and slightly larger brains. Some of these traits may have evolved for sexual competition against other males. But male bodies are also living evidence of the sexual choices made by ancestral females. Men grow beards, and possess penises that are much longer, thicker, and more flexible than those of other primates. These are more likely to reflect female choice than male com- petition. Women also evolved to incarnate male sexual preferences. Women have enlarged breasts and buttocks, narrower waists, and a greater orgasmic capacity than other apes.

Sexual selection has also made male bodies grow according to a higher-risk, higher-stakes strategy. For males there is a higher incidence of birth defects, more death in infancy, higher mortality at every age, earlier senescence, and greater variation in health, strength, body size, brain size, and intelligence. This risky, go-for- broke strategy suggests that sexual competition among males was often a winner-takes-all contest. It was better to take a big gamble on producing the most attractive image during a short peak, rather than aiming to create a mediocre impression over a long period of time.

Our bodies are rich sources of evidence about sexual selection pressures because they are visible, measurable, easily comparable with those of other species, and relatively undistorted by human culture. In recent years much nonsense has been written by post-modern theorists such as Michel Foucault about the "social construction of the body," as if human bodies were the incarnation of cultural norms rather than ancestral sexual preferences. These theorists should go to the zoo more often. What they consider a "radical reshaping" of the human body through social pressure is trivial compared to evolution's power. Evolution can transform a dinosaur into an albatross, a four-legged mammal into a sperm whale, and a tiny, bulgy-eyed, tree-hugging, insect-crunching proto-primate into Julia Roberts—or Arnold Schwarzenegger. Selection is vastly more powerful than any cosmetic surgeon or cultural norm. Minds may be sponges for soaking up culture, but bodies are not.

Many characteristics of our bodies which differentiate us from other primates, are probably the result of mating preferences of our ancestors.