My recent post on Sexual Selection mentioned the theory that the 'hairless' skin of humans is due to sexual selection.

After writing this I thought I would check out what is known about the evolution of human body hair. One interesting result is this Royal Society paper by Pagel and Bodmer. Their theory is that hairless skin makes it easier to remove ectoparasites like fleas and ticks, and that humans' loss of body hair was first favoured by natural selection for this reason, then reinforced by sexual selection. [Added: this was a free pdf when I got it a week or two ago, but may now require subscription. For those who want to track it down in a library, the reference is 'A naked ape would have fewer parasites', by Mark Pagel and Walter Bodmer, Proceedings of the Royal Society of London (B) (Suppl.), 270, 2003, pp.S117-119. Or you may still be able to Google up a freebie. Try searching Google Scholar for key words 'Pagel Bodmer naked ape'. When I tried this just now it still gave free access to the full text.]

As P & B recognise, it isn't clear whether the advantage of removing crawling parasites like ticks would be offset by an increase in attacks by mosquitoes and other flying pests. Another obvious question is why, if hairless skin makes it easier to remove parasites, man is the only primate to have lost his body hair. Other primates certainly spend a lot of time grooming themselves and each other to remove fleas and ticks. P & B suggest that the loss of body hair was related to the invention of fire and clothing, which would have enabled humans (unlike other primates) to regulate the surrounding temperature. The selective advantage of removing parasites more easily would then not be constrained by the need to retain body hair for warmth at night.

Like many hypotheses in human evolution, this one seems vulnerable to the criticism of being a 'Just So Story'. It is attractive, but difficult to prove one way or the other. And the link between the loss of body hair and the use of clothing becomes less plausible when one remembers that most modern hunter-gatherers in tropical countries (Australian aborigines, Bushmen, various pygmies), who are presumably the best model for our out-of-Africa ancestors, traditionally wore little or no clothing. The aborigines are noted for their ability to sleep naked on cold nights.

I think that any successful theory for the loss of body hair will need to look more closely at the physiology and genetics of human hair development. Humans are not really hairless at all, but have dense hair everywhere except the palms of the hands and soles of the feet. The appearance of hairlessness depends on the length and fineness of the hair, which is under complex hormonal control in different parts of the body and at different stages of the life cycle. And then there is the puzzle of lanugo, the long hair which grows all over the human fetus but disappears before or shortly after birth. Explain that by sexual selection! [Added: Wikipedia has this to say about lanugo: "Lanugo are hairs that grow on the body to attempt to insulate it because of lack of fat. It is a type of pelage. It occurs on fetuses and it is normal for the unborn baby to consume the hair, which then contributes to the newborn baby's first faeces. Lanugo hair is usually shed and replaced by vellus hair at 36-40 weeks gestation. The presence of lanugo in newborns is a sign of premature birth. It is also a common symptom of serious anorexia nervosa, as the body attempts to insulate itself as body fat is lost." The 'insulation' theory sounds to me like pure guesswork: a fetus in the womb doesn't need insulation! I suppose the anorexia point might appear to support it, until one reflects that there is unlikely to be an evolved response to anorexia: in pre-modern conditions, anyone who got that thin would be doomed.]