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 competition. 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.
The most sexually selected parts of our bodies have been neglected in theories of human evolution because they don't fossilize. Sexual choice sculpts body ornaments out of muscle, fat, skin, and nerves, often without leaving many clues in the bones. This makes it hard to know when and where these traits evolved. We don't know how hairy our ancestors were a million years ago, whether Homo erectus males had huge penises, or whether Neanderthal females had large breasts. But we do know that our body's sexual ornaments are universal across human groups, so they must have evolved at least 60,000 years ago or so, when human groups colonized different areas of the world. In these respects our bodily ornaments are like many of our mental adaptations for courtship: we don't have much fossil evidence about their antiquity, but we can infer a lot from their modern human form and their absence in closely related ape species.
If sexual selection drove our bodily divergence from other apes, it may have driven our mental divergence as well. Rene Descartes saw a dichotomy between body and mind, but sexual choice judges them as a package. As Walt Whitman put it in his 1855 poem "One's-Self I Sing:"
Of physiology from top to toe I sing,
Not physiognomy alone nor brain alone is worthy for the Muse,
I say the Form complete is worthier far,
Penises, clitorises, breasts, and beards are fascinating not only in their own right, but also for what they reveal about sexual selection among our ancestors.
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