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Thursday, June 12, 2003

NATURAL SELECTIONS

Hair today, gone tomorrow


A friend of mine once observed a Japanese high-school student waxing his chest at the back of her class. And everyone has seen girls depilating their eyebrows on the train, skillfully matching their plucking to the shunting from station to station. Apparently, some women even shave their forearms (though not on the train).

But the desire to be hairless and to show off smooth, healthy, blemish-free skin is hardly restricted to Japan. It is a worldwide desire, fed by a vast, multibillion dollar cosmetics industry. Where does this desire come from? Sure, we all want to look young and fresh, and hairlessness apparently enhances this, but how, and why? Before we can answer that question, we must address another: Why are humans so different from their relatives with respect to their hairiness?

"There are 193 living species of monkeys and apes," wrote English anthropologist Desmond Morris in 1967. "192 of them are covered with hair. The exception is a naked ape self-named Homo sapiens."

And not just most primates, but almost all mammals are covered with hair or fur: It's one of the distinguishing features of the class. There are a few exceptions: Naked mole rats don't have fur, but they live entirely underground and presumably have evolved to be rid of it. Dolphins and whales don't have fur (though baby dolphins are born with hairs on their beak), but they are aquatic and have evolved a streamlined skin.

The usual explanation for why humans don't have thick fur is that when we evolved to stand on two legs, on the baking African savannah, we lost our body hair to cope with the high temperature. Going naked, the theory went, allowed us to be active and go hunting during more of the daytime.

But Mark Pagel, an evolutionary biologist at the University of Reading in England, disagrees.

"This theory has a major flaw," he says. "Hairlessness would keep us cool during the day, but it would also mean we would get disastrously cold at night. The equations don't add up."

Another explanation is the aquatic ape hypothesis. This suggests that hominid ancestors went through an aquatic phase in their evolution, and so developed traits such as high body fat and hairlessness, similar to other marine mammals. But this theory doesn't explain why such traits have been retained for millions of years after our ancestors left the water. Nor is there any fossil evidence of an aquatic stage.

So Pagel and colleague Walter Bodmer of Oxford University have this week published a new theory explaining the evolution of human hairlessness.

The scientists propose that humans became hairless to reduce the effects of the many biting flies and other disease-carrying parasites that live in fur.

"Ectoparasites exact a large toll on the fitness of furry or feathered animals," the authors write.

Moreover (and bear in mind the mighty cosmetics industry here) hairlessness enhances sexual attractiveness. We'll see how in a moment.

As we have seen, humans are nearly alone among mammals in lacking a dense layer of protective fur or hair. If hairlessness is so advantageous, why haven't other primates evolved it?

Pagel and Bodmer's theory suggests that it is because humans developed culture. By producing fire, shelter and clothing, humans were able to respond flexibly and effectively to their environment and hairlessness became possible. Indeed, it became desirable, because clothes and shelter can be changed or cleaned if infested with parasites, which is better than a permanent layer of fur.

But what about the marked sex difference in body hair in humans?

As long ago as 1888, Darwin wrote: "In all parts of the world women are less hairy than men and we may reasonably suspect that this character has been gained through sexual selection."

As in so many other cases, Darwin was way ahead of his time. He even considered that human hairlessness evolved to deal with "the multitude of ticks and other parasites."

Pagel and Bodmer have updated Darwin's comments and produced a framework for testing their ideas. The new theory is published by the new Royal Society journal Biology Letters.

"Hairlessness would have allowed humans to convincingly 'advertise' their reduced susceptibility to parasitic infection and this trait therefore became desirable in a mate," write Pagel and Bodmer, "and the greater loss of hair in women follows the stronger sexual selection from men to women. Facial and head hair can be explained by their continued importance in sexual attraction and selection."

In other words, natural selection favored the loss of body hair because it reduced the parasite burden, and sexual selection reinforced the loss because it advertised the reduced parasite load to potential mates.

We all know how important head hair is when selecting a mate (otherwise how do you explain the amount of hair salons in Harajuku?). Facial hair presumably separates the men from the boys, so is also important in mate selection. But what about pubic hair?

"Pubic hair does pose a challenge for our theory," admit Pagel and Bodmer. "There is some evidence, however, that pubic hair enhances pheromonal signals involved in mate choice."

The scientists say that their theory can be tested. "One expects to find that humans whose evolutionary history has been in regions with higher concentrations of disease-carrying parasites, such as in the tropics, will have less body hair than others. We also know that fur is not an effective protection against biting flies and we expect to find that the flies have simply evolved in ways that circumvent it," they write.

"Common use of depilatory agents testifies to the continuing attractions of hairlessness, especially in human females," the authors add.

Girls (and boys) plucking their eyebrows on the train demonstrate not just the whims of fashion, but a legacy of evolution.

Rowan Hooper welcomes comments at rowan.hooper@tcd.ie


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