Mark Pagel, head of the evolutionary biology group at the University of Reading in England, replies:
Scientists have suggested three main explanations for why humans lack fur. Al l revolve around the idea that it may have been advantageous for our evolving lineage to become less and less hairy during the six million years since we diverged from the common ancestor we shared with our closest living relative, the chimpanzee.
The “aquatic ape” hypothesis suggests that six million to eight million years ago, our apelike ancestors had a semiaquatic lifestyle, foraging for food in shallow waters. Fur is not an effective insulator in water, and so the theory asserts that we evolved to replace it, as other aquatic mammals have, with relatively high levels of body fat.
Imaginative as this explanation is—and helpful in excusing our girth— paleontological evidence for an aquatic phase of human existence has proved elusive.
A second theory is that a loss of fur allowed for better control of our body temperature when we adapted to life on the hot savanna. Our ape ancestors spent most of their time in cool forests, but a furry, upright hominid walking around in the sun would have overheated. This idea seems sensible, but even though hairlessness might have made it easier to stay cool during the day, our ancestors also would have lost heat at night when they needed to retain it.
Recently my colleague Walter Bodmer of the University of Oxford and I suggested that natural selection favored nakedness because it reduced the prevalence of external parasites.
A furry coat provides an attractive safe haven for ectoparasites such as ticks, lice and biting flies.
These creatures not only bring irritation and annoyance but also carry an assortment of diseases, some of which can be fatal. Humans, being capable of building fires, constructing shelters and producing clothes, would have been able to lose their fur, and thereby most of their parasites, without suffering from the cold.