for National Geographic News
Humans are unique among primates for our near-total bodily hairlessness. In fact, only a handful of the 5,000 or so mammalsmostly semi-aquatic species such as whales, walruses, and hippopotamusesare not covered in dense fur.
Now, a controversial new theory suggests that human hairlessness evolved as a strategy to shed the ticks, lice, fleas, and other parasites that nestle deep in fur.
Unique human cultural adaptations such as the use of fire, shelter, and clothing allowed ancient humans the luxury of ditching their insulating hairy layer, says the study soon to appear in print in the journal Biology Letters.
"One of the most unusual things about humans is that we don't have fur," said study co-author Mark Pagel, evolutionary biologist at Reading University in England. Though humans are not literally hairless, much of our hair has become so small and fine as to render it virtually invisible.
The new theory may also explain the difference in hairiness between men and women. If having less hair leads to a reduction in parasites, then it would offer a big advantage in the race of life and might become a sexually attractive feature in a mate, said Pagel. Hair might have been retained on the head, and on the face in men, as a feature of sexual display, he said, like peacocks' tails or stags' antlers.
Hot and Hairy
The most accepted current theory argues that hairlessness was a strategy for cooling the body on the sun-baked African savanna. Researchers have suggested that hair loss occurred when hominids (bipedal primates that include recent humans, their ancestors, and related species) first left the forests. A combination of an upright posture and lack of hair might have made it easier to radiate heat back into the environment.
"Physiological equations have shown that hairlessness would have doubled the distance hominids could travel off a liter of water," said Robin Dunbar, behavioral ecologist at the University of Liverpool in England.
However, hairlessness may also be a disadvantage in terms of heat regulation, argue Pagel and his co-author Walter Bodmer of Oxford University's Weatherall Institute of Molecular Medicine in England. Hairless animals without shelter or clothing would become too cold at night, said Pagel.
The next best contender is the "aquatic ape hypothesis," which suggests that at around 8 million years ago, human ancestors underwent a semi-aquatic phasealso explaining our improved swimming abilities compared to the other great apes. The majority of hairless mammals are semi-aquatic, probably because hair offers no insulating benefits under water. However, "very little solid evidence has been presented to back up this [aquatic ape] theory," said Pagel. It's also perplexing why features acquired for life in water so many millions of years ago should have been retained, he added.
Despite its other attributes, "fur is rich repository for parasites," said Pagel. Parasites, and the diseases they carry, are a hugely important guiding force in evolutionfleas are carriers of plague, for example. "For most of the [world's animals], most of the time, the majority of death is from parasites," said Pagel.
The scientists propose that pressure imposed by parasites, in combination with man's unique intelligence, and cultural adaptations, allowed humans to shed fur along with its resident fleas, ticks, and lice. Though parasites also infect clothing, clothes can more easily be cleaned, or changed, to remove the problem.
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