Sunday, April 1, 2007

Parental Selection, Human Hairlessness, and Skin Color

Judith Rich Harris, known for her writings on child psychology (No Two Alike: Human Nature and Human Individuality), has just published an article on the evolution of hairlessness and lighter skin color in humans through "parental selection." She begins with the words of a !Kung woman who decides against killing her newborn daughter: “I don't want to kill her. This little girl is too beautiful. See how lovely and fair her skin is?”

Infanticide is common among the !Kung, especially if the youngest child has not been weaned yet, but the decision is never made lightly and arbitrary factors—such as a cute physical appearance—can make the difference between life and death. This is what Harris calls "parental selection." The decision to kill or not to kill often lies at a tipping point where extraneous factors can push the balance one way or the other. Over time, especially given the prevalence of infanticide in most human cultures, such factors may have eventually altered the appearance of children and even adults, making our skin less hairy, smoother, and fairer in color.

There remains, of course, the question as to why less hairy, smoother, and fairer skin is cute. Harris attributes the reason to culture: "Once the notion that “hairlessness is us, hairiness is them” became part of the culture of a group of hominids, then sexual selection plus parental selection could have eliminated hairiness very quickly. Any infant born too hairy would have elicited an “ugh” response; in all likelihood, it would have been killed or abandoned at birth."

Fine. But is the "ugh" response something we learn from our culture? Doesn't it exist in other animals? Animals that don't have culture? And why would culture predispose us to reject hairy, rough, and dark skin? Because such skin is "ugh"?

This sounds like a circular argument. Yes, culture may strengthen the "ugh" response by telling us whether others feel the same way, either directly through conversation or indirectly through songs, tales, and the very words we use—all of which continually remind us what is "ugh" and what isn't. But the response itself needs neither words nor a human mind. It exists in other animals. Birds do it, bees do it.

This differentiation between "ugh!" and "cute!" has been shown in non-human primates, particularly when they judge the physical appearance of infants. Jay (1962:472-473) notes:

The coat color of the newborn infant of all species of Old World monkeys for which information is available is different from that of an adult of the same species. Often this difference is extremely striking, as in the dark-brown fur of the newborn langur. Skin color of the infant langur, baboon, and macaque is pink, in contrast to the almost black skin of the older infant or adult. The infant’s pink face, hands, and feet and its large pink ears are in sharp contrast to its dark brown fur. The natal coat color is present during the first two or three months of life, when the infant most needs protection and nourishment from its mother and older monkeys. … As the infant’s coat changes to white and it becomes more and more independent of its mother, the interest of adult females decreases and they no longer seek out the infant to hold and groom it as often as they did when it was smaller, younger, and brown.

Infant fur is the primary releaser of these positive adult responses, apparently because it covers most of the body surface. Infant skin (in particular its pink color) seems to be a secondary releaser (Jay 1962:472-473; Alley 1980:418-419). As juvenile dependence progressively lengthened in ancestral humans, infant skin might have proved more effective than infant fur in stimulating adult care after weaning. For whatever reason, if skin had edged out fur in the child's struggle for adult attention, there would have been selection to project this visual cue more effectively, i.e., by increasing the hair-free portion of the body surface. This in turn would have increased its visual importance. Result: runaway selection for more and more naked skin.

Parental and sexual selection might then have extended the denuding process to adults. Or maybe not. Any selection for infants and juveniles with hair-free skin would inevitably have spilled over onto older age groups … unless there was selection for adults with hairy skin. Perhaps no such balancing selection existed. Or perhaps it wasn't strong enough.

On this subject, Judith Harris raises an interesting point: human hairlessness appeared very late in human evolution. Neanderthals seem to have adapted very well to subzero climates without the benefit of tailored clothing. Both needles and the human body louse (which lives in clothing) seem to date back no earlier than 50,000 years ago, i.e. the transition from Neanderthals to modern humans. Conclusion: body fur must have persisted until this transition, as probably did many other ape-like traits. Even the Neanderthal brain was much more ape than human. It was big, but only because it had to stockpile so many problem-specific and situation-specific algorithms. The quantum leap to general intelligence had not been made. Not yet.

If you could meet a Neanderthal face to face, you wouldn't think it was human. It would look like a big ape. Nor would its behavior change your initial impression.

This conclusion may seem startling. As Mrs. Harris points out: "Paintings and exhibits on human evolution often feature a line-up of increasingly humanlike creatures, starting from some apish ancestor, progressing through Homo habilis and Homo erectus, and ending up triumphantly at Homo sapiens." In fact, human evolution was not a straight line running from early hominids to modern humans. It was more like a logarithmic curve, with most of the big changes taking place recently … in the past 200,000 years. Our hominid ancestors were not man-apes. They were simply apes who stood a bit more erect and handled tools a bit better than do today's apes. Only hindsight makes us see their evolutionary potential.

This leads to an even more startling conclusion: the appearance of Homo sapiens was not a culmination of human evolution. It was a beginning. Such a view has recently gained support from a presentation by Gregory Cochran and John Hawks to the American Association of Physical Anthropologists. The two authors argue that "the origin of modern humans was a minor event compared to more recent evolutionary changes." The pace of evolution actually seems to have accelerated after the transition to modern humans.


Alley, T.R. (1980). “Infantile colouration as an elicitor of caretaking behaviour in Old World primates,” Primates 21:416-429.

Harris, J. R. (2006). Parental selection: A third selection process in the evolution of human hairlessness and skin color. Medical Hypotheses, 66: 1053-1059.

Harris, J. R. (2006). No Two Alike: Human Nature and Human Individuality. W.W. Norton, ISBN 978-0-393-05948-9

Jay, P.C. (1962). “Aspects of maternal behavior among langurs,” Annals of the New York Academy of Sciences 102:468-476.

1 comment:

Anonymous said...

The hair that appears at puberty would be all the more significant. Body hair removal often goes along with tanning.

Tanning could be a reversal of hairless lightening of the skin, especialy for females.

Darkening the skin says "I am all grown up now age twenty one, you know babyee we can have a lot of fun".

Tanning is growing an adult coat colour as a sexual signal. Now I get it.

I wish I had read this post more carefuly the first time, it would have saved me from cluttering up the comments at What About Tanning.