The Hirsute, the Hairless, and the Human

Homo sapiens has no shortage of distinguishing features: We alone among primates are furless (that most mammalian of characteristics); we are the only species clever enough to ponder why that might be; and we can ponder such matters while pacing back and forth on two legs. Our cleverness, however, hasn't helped us determine why we have these qualities, but as recent developments in the hirsute-less debate reveal, they make for interesting pondering. A mammal needs a good reason to discard its woolly coat. Fur shields the sun, barricades the cold, and protects the skin from abrasion. Of the few mammals that have dispensed with it--whale, elephant, walrus, pig, dugong, manatee, rhinoceros, hippopotamus, naked mole rat, and humans--most live in water, where fur is useless. Unable to see a furless survival advantage for terrestrial humankind, Darwin ascribed it rather vaguely to sexual selection, whereby competition for mates drives the evolution of eye-catching peculiarities. The theory remained unchallenged until the latter half of the last century, when another theory came under discussion: Hairlessness and bipedality evolved to prevent overheating on the savannah. Maybe that's true for bipedality, but increased nocturnal heat loss likely cancels out any daytime advantages of naked skin. Faring slightly better is the aquatic ape theory: Similarities between humans and aquatic mammals, notably hairlessness and subcutaneous fat, are due to a recent aquatic phase in human evolution. Even our bipedalism, some say, is an adaptation to wading in deep water. According to theory proponent Elaine Morgan, its strength is that it "offers one possible explanation of a whole range of physical anomalies [that] distinguish Homo." Palaeontological evidence does not support this theory. Moreover, British scientists Mark Pagel, University of Reading, and Walter Bodmer, John Radcliffe Hospital, Oxford, wonder why modern humans would have retained adaptations to a past aquatic life. Their alternative explanation for human hair loss,1 says Pagel, "is rooted in selective forces that held throughout the evolution of the genus Homo." Pagel and Bodmer argue that parasites exert those selective forces. Mammalian fur is an ideal habitat for fleas, ticks and lice, which drain host resources and spread disease. But, for a mammal to rid itself of its protective covering and its unwanted passengers, it must devise a better way of regulating body temperature. And that, say the two scientists, is just what humans did when they harnessed fire and started wearing clothes, which, unlike fur, can be changed and laundered, making them far less parasite-friendly. One of the most attractive features of the parasite-reduction hypothesis is its testability. Racial differences in body hair, for example, are expected to reflect geographical variations in parasite prevalence. But parasite reduction via technological innovation is only half the story, say Pagel and Bodmer. They point to the plethora of products, such as razors/waxes/lotions/electrodes that are used for shaving/plucking/bleaching/dissolving/electrolyzing hair from our eyebrows/legs/chests/bikini lines/backs as evidence that Darwin's sexual selection theory was on target. "We humans don't seem to like hair very much," says Pagel. If baldness means fewer parasites, it pays to choose hairless mates: Bald becomes beautiful. Pagel and Bodmer point out that women having less body hair than men could be due to the greater sexual selection that is conventionally exerted by men on women. Meanwhile, humans only have retained proper hair on the head as a shield from the midday sun perhaps, and the darker, danker body recesses, where it might function as a pheromone trap, enhancing sexual chemical communication. The theory is not problem-free. Our sexual preferences are influenced by culture and biology; for example, men who previously flaunted their hairy chests would now be hesitant to do so. Pagel accepts that fashions come and go, but he observes that no cosmetics have ever been designed to emphasize body hair (excluding the head). Nonetheless, recent research found that women prefer men with chest and abdominal hair.2 Timing might also be a problem, especially in light of new evidence that clothing is a more recent innovation than was believed.3 If Pagel and Bodmer are correct, then we didn't so much lose our fur as reinvent it, which implies that our aptitude to solve problems has had a significant bearing on the way we look today. Of course, this sheds little light on the evolution of our peculiar bipedal gait. But, then again, walking on only two legs does let us pace the room, pondering our baldness for twice as long before our carpets wear out. Stuart Blackman (stuart.Blackman@talk21.com) is a freelance science journalist in the UK. References 1. M. Pagel, W. Bodmer, "A naked ape would have fewer parasites," Proc R Soc Lond B Biol Sci, 270[suppl 1]:S117-9, Aug. 7, 2003. 2. A. F. Dixson et al., "Masculine somatotype and hirsuteness as determinants of sexual attractiveness to women," Arch Sex Behav, 32:29-39, 2003. 3. R. Kittler et al., "Molecular evolution of Pediculus humanus and the origin of clothing," Curr Biol, 13:1414-7, August 2003. function sendData() { document.frm.pathName.value = location.pathname; result = false if (document.frm.score[0].checked) result = true; if (document.frm.score[1].checked) result = true; if (document.frm.score[2].checked) result = true; if (document.frm.score[3].checked) result = true; if (document.frm.score[4].checked) result = true; if (!result) alert("Please select") return result; } .myradio{background-color : #94bee5} Please indicate on a 1 - 5 scale how strongly you would recommend this article to your colleagues? Not recommended 1 2 3 4 5 Highly recommended Please register your vote

The Hirsute, the Hairless, and the Human

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