New theory on human hairlessness

Humans became hairless to reduce the effects of the many biting flies and other disease-carrying parasites that live in fur and to enhance sexual attractiveness and selection, according to scientists from the Universities of Reading and Oxford. The new theory, which challenges the accepted view that human hairlessness evolved to control body temperature in hot climates, is to be published by the Royal Society in Biology Letters, a new companion to Proceedings: Biological Sciences.

Humans are nearly alone among mammals in lacking a dense layer of protective fur or hair. This new theory claims that because humans could respond flexibly and effectively to their environment by producing fire, shelter and clothing, hairlessness was both possible and desirable because clothes and shelter can be changed or cleaned if infested with parasites, better than a permanent layer of fur.

Other features of human hairlessness – such as the marked sex difference in body hair and its retention on the face, hair and pubic areas – are also more easily explained by this new theory.

“Hairlessness would have allowed humans to convincingly ’’advertise’’ their reduced susceptibility to parasitic infection and this trait therefore became desirable in a mate,” say Professors 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, although pubic hair does pose a challenge for our theory. There is some evidence however, that pubic hair enhances pheromonal signals involved in mate choice.”

There are acknowledged flaws in the traditional theory of a lack of hair controlling body temperature. The most obvious is that naked skin increases the rates of energy gain and loss during periods of too much or too little heat, suggesting that naked skin is a worse solution because at night more heat is lost and during the day, more heat has to be dissipated.

The professors say that the 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 say.