Human beings are unusual creatures in many ways — one distinction that often gets overlooked is that we are land-dwelling mammal that isn’t furry. How did this come about? Scientific American says there are three possible explanations:
We used to be semi-aquatic. This theory imagines our human ancestors foraging for food in shallow water. I thought only nut-cases believed this, but apparently it has been put forth as a serious hypothesis. The idea is that we lost our fur the way dolphins and seals did. There’s not much evidence backing this up, however.
Hairless bodies are a heat adaptation. After moving out of the cool shady trees to the hot savannas, our ancestors quickly lost the fur as a means of adapting to the extreme heat. This would have been a drawback at night, however. Also, you have to wonder why we don’t see other examples of land-based mammals ever making a similar adaptation?
We lost the fur in order to get rid of the accompanying parasites. This one makes sense. Imagine living naked in a world with no showers and the possibility of being infested by tics, chiggers, lice — not just in a few areas of your body, but all over. I would definitely do anything I could to evolve away from that.
The other possibility, listed as likely contributing factor, but not a major cause, of human hairlessness is sexual selection. Since we don’t have examples of other species losing fur to avoid parasites or keep cool — keeping in mind that human beings can build fires and make blankets, cold-weather options not available to other creatures who might have gone in a non-fur direction — I tend to think that sexual selection may have been a pretty significant factor. Early human populations may have decided that lighter coats of fur were more attractive and desirable.
And come to think of it, isn’t that still pretty much the case today? Sure, there are people out there with naturally hairy chests and backs, but an outright preference for such bodies (on either aesthetic or sexual terms) would be — I think — in just about any corner of the world, more in the nature of an exception than the norm. Our hairless bodies might well be our oldest cultural artifact!