Sunday, October 16, 2011

A Lousy Theory of Hair

As far as I know, my bathroom mirror isn’t magic. I never actually tested the hypothesis by asking it who is the fairest of all, which is just as well either way; if it had an answer, no doubt it would break itself laughing before saying it. The glass needed no magic, though, to reveal a trip to the barber is in my immediate future. In a way, this is one small thing for which to be grateful. A modest majority of my head hair is still there, and from a distance still looks dark brown. On my last barber visit, however, I couldn’t help noticing that the falling trimmings on balance were lighter than they once were.

“Am I going blond by any chance?” I asked.

“No,” she answered simply.

The reflected shagginess this morning raised a couple questions in my mind, however, for which I can find no definitive answers in the books on my shelves or in internet searches. There are plenty of speculative answers though. It is currently fashionable among anthropologists to classify humans (and all extinct hominids/hominins) among the Great Apes. Why are we the only members of that family with a mane that, uncut, will grow to the waist or longer? Why, at the same time, did body hair all but vanish?

In truth we don’t know. The most common answer for the mane is “sexual selection” – rather like the peacock’s tail. Any peacock that can keep its life-threatening encumbrance of a tail looking good must be healthy and a good catch, so the argument goes, and so the flashy-tailed bird attracts more mates and has more offspring; good lustrous hair says much the same about a human. Maybe. When did the trait develop? Did Neanderthals have long tresses? Homo erectus? We have no idea, but it is rather fun to picture them that way.

There are competing (perhaps complementary) ideas about body hair. Two lead the pack. One, once again, is sexual selection, though it is hard to see what (originally) would have been so attractive about hairlessness, either in a positive way or in the peacock tail encumbrance way. Another possible reason is thermal regulation. In combination with our exceptionally abundant sweat glands, hairlessness lets us shed heat far more efficiently, which would have been a daytime advantage on the hot African savanna. On the other hand, it would have been a nighttime disadvantage. Besides, our baboon cousins live in the identical environment, and are both hairy and maneless.

Didn’t hairlessness just put us to the trouble of making clothes? Apparently not for quite a while: Doctors Rogers, Iltis, and Wooding make a good argument in Current Anthropology that hairlessness dates back at least 1.2 million years. Clothing, on the other hand, may be no more than 70,000 years old. How do we know anything about the timing of clothing? Our friends the lice tell us. The various species of lice are very particular about what species they infect and where. They are specialists. In the case of humans, there are three separate species of lice who enjoy our company: head, pubic, and body. They are related to each other, and using a DNA clock (the mutation rate) we can make a good estimate about when they diverged from each other. Body lice are not really body lice: they infest clothes. They snack on their hosts, but live in the clothes. They diverged from head lice only 70,000 years ago, which strongly implies there weren’t any clothes before then; lice would have adapted to the niche soon after it became available. This is also close to the time modern humans left Africa for chillier climes where the clothes would have been handy, despite the little bugs.

The million years of nakedness and the lice suggest another possible advantage to hairlessness: a million years without body lice. It would have reduced the spread of disease significantly, and disease always is a bigger killer than all the predators, accidents, and wars combined. Is that the real message when people flirtatiously show skin? “Hey look! I’ve got no bugs on me!”

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