A new theory proposes that humans lost their thick fur as an adaptation to cut down on parasites and that smooth skin became a sexy symbol of health. Other scientists are intrigued, but they say the authors need to disprove the old hypothesis that humans lost their hair to keep cool.
"The nakedness of humans is a glaring difference between humans and other mammals," says evolutionary biologist Mark Pagel of the University of Reading, U.K. Although humans have about as many follicles as other primates, the hairs are fine, exposing the skin and creating an evolutionary enigma. In 1984, scientists first proposed what has become the most accepted theory, positing that hair loss allowed our ancestors to better cool off by sweating.
Pagel and co-author Walter Bodmer, a geneticist at Oxford University, thought that there could be another explanation for nakedness: protection against parasites. At some point, early humans began to don clothing and build shelters, which would protect hairless bodies but also provide fertile breeding grounds for parasites. They may have lost their hair because hairless skin is easier to keep clean, the researchers say, citing evidence that more parasitic infections occur in still-hairy regions such as the head and pubic area. Sexual selection might have given the evolution of hairlessness a boost, as smooth, healthy skin signaled that a prospective mate was parasite-free, Pagel and Bodmer argue in a paper published online 9 June by Biology Letters.
Bodmer and Pagel point to the naked mole rat as another example of a creature that may have taken this road to hairlessness. It is one of the few other hairless, land-dwelling mammals. The animals' underground burrows protect them from temperature swings and serve the same purpose as clothing and shelter in human communities.
"It's an interesting idea," says evolutionary biologist Robin Dunbar of the University of Liverpool. The next step is to test it against evaporative cooling, he says. Scientists might look to see if populations in areas teeming with parasites have less hair, for example. Also, the timing of human hair loss must be addressed, Dunbar says. According to the earlier theory, hairlessness would have followed quickly on the heels of upright walking, probably more than 2 million years ago. In the new theory, hairlessness likely evolved much later, when humans began sharing shelter, Dunbar says, probably no more than a half-million years ago.