Humans became hairless to avoid parasites and to enhance sexual attractiveness. Story Tools

LONDON, England -- A pair of UK scientists have come up with a new theory as to why humans are not as hairy as our ancestors, the ape.

Humans -- one of the only mammals without a lot of hair -- began showing more skin to ward off parasites and biting flies that live in fur, which had the added benefit of enhancing sexual attractiveness, according to the scientists.

The new theory -- by Professor Mark Pagel from the University of Reading and Professor Sir Walter Bodmer from Oxford University -- challenges the widely held view that hairlessness evolved to control body temperatures in hot climates.

The scientists claim that because humans can respond effectively to their environment -- by producing fire, shelter and clothing -- hairlessness was possible and desirable, given that clothes and shelter could be changed or cleaned to get rid of parasites.

The theory also attempts to explain the difference in body hair between sexes.

"Hairlessness would have allowed humans to convincingly 'advertise' their reduced susceptibility to parasitic infection and this trait therefore became desirable in a mate and the greater loss of hair in women follow the stronger sexual selection from men to women," Pagel said.

"Facial and head hair can be explained by their continued importance in sexual attraction and selection, although pubic hair does pose a challenge for our theory. There is some evidence however, that pubic hair enhances pheromone signals involved in mate choice."

Pagel added: "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."

The traditional temperature theory was flawed when scientists looked at hot or cold climates. The naked skin increases the rates of energy gain and loss when it is too hot or cold, suggesting the naked skin is a worse solution.

The findings will be published in the journal Biology Letters published by the Royal Society of Biologists.