I believe, though I cannot prove it, that three—not two—selection processes were involved in human evolution.
The first two are familiar: natural selection, which selects for fitness, and sexual selection, which selects for sexiness.
The third process selects for beauty, but not sexual beauty—not adult beauty. The ones doing the selecting weren't potential mates: they were parents. Parental selection, I call it.
What gave me the idea was a passage from a book titled Nisa: The Life and Words of a !Kung Woman, by the anthropologist Marjorie Shostak. Nisa was about fifty years old when she recounted to Shostak, in remarkable detail, the story of her life as a member of a hunter-gatherer group.
One of the incidents described by Nisa occurred when she was a child. She had a brother named Kumsa, about four years younger than herself. When Kumsa was around three, and still nursing, their mother realized she was pregnant again. She explained to Nisa that she was planning to "kill"—that is, abandon at birth—the new baby, so that Kumsa could continue to nurse. But when the baby was born, Nisa's mother had a change of heart. "I don't want to kill her," she told Nisa. "This little girl is too beautiful. See how lovely and fair her skin is?"
Standards of beauty differ in some respects among human societies; the !Kung are lighter-skinned than most Africans and perhaps they pride themselves on this feature. But Nisa's story provides a insight into two practices that used to be widespread and that I believe played an important role in human evolution: the abandonment of newborns that arrived at inopportune times (this practice has been documented in many human societies by anthropologists), and the use of aesthetic criteria to tip the scales in doubtful cases.
Coupled with sexual selection, parental selection could have produced certain kinds of evolutionary changes very quickly, even if the heartbreaking decision of whether to rear or abandon a newborn was made in only a small percentage of births. The characteristics that could be affected by parental selection would have to be apparent even in a newborn baby. Two such characteristics are skin color and hairiness.
Parental selection can help to explain how the Europeans, who are descended from Africans, developed white skin over such a short period of time. In Africa, a cultural preference for light skin (such as Nisa's mother expressed) would have been counteracted by other factors that made light skin impractical. But in less sunny Europe, light skin may actually have increased fitness, which means that all three selection processes might have worked together to produce the rapid change in skin color.
Parental selection coupled with sexual selection can also account for our hairlessness. In this case, I very much doubt that fitness played a role; other mammals of similar size—leopards, lions, zebras, gazelle, baboons, chimpanzees, and gorillas—get along fine with fur in Africa, where the change to hairlessness presumably took place. I believe (though I cannot prove it) that the transition to hairlessness took place quickly, over a short evolutionary time period, and involved only Homo sapiens or its immediate precursor.
It was a cultural thing. Our ancestors thought of themselves as "people" and thought of fur-bearing creatures as "animals," just as we do. A baby born too hairy would have been distinctly less appealing to its parents.
If I am right that the transition to hairlessness occurred very late in the sequence of evolutionary changes that led to us, then this can explain two of the mysteries of paleoanthropology: the survival of the Neanderthals in Ice Age Europe, and their disappearance about 30,000 years ago.
I believe, though I cannot prove it, that Neanderthals were covered with a heavy coat of fur, and that Homo erectus, their ancestor, was as hairy as the modern chimpanzee. A naked Neanderthal could never have made it through the Ice Age. Sure, he had fire, but a blazing hearth couldn't keep him from freezing when he was out on a hunt. Nor could a deerskin slung over his shoulders, and there is no evidence that Neanderthals could sew. They lived mostly on game, so they had to go out to hunt often, no matter how rotten the weather. And the game didn't hang around conveniently close to the entrance to their cozy cave.
The Neanderthals disappeared when Homo sapiens, who by then had learned the art of sewing, took over Europe and Asia. This new species, descended from a southern branch of Homo erectus, was unique among primates in being hairless. In their view, anything with fur on it could be classified as "animal"—or, to put it more bluntly, game. Neanderthal disappeared in Europe for the same reason the woolly mammoth disappeared there: the ancestors of the modern Europeans ate them. In Africa today, hungry humans eat the meat of chimpanzees and gorillas.
At present, I admit, there is insufficient evidence either to confirm or disconfirm these suppositions. However, evidence to support my belief in the furriness of Neanderthals may someday be found. Everything we currently know about this species comes from hard stuff like rocks and bones. But softer things, such as fur, can be preserved in glaciers, and the glaciers are melting. Someday a hiker may come across the well-preserved corpse of a furry Neanderthal.