Long Haired Girl

Other primates have sweat gland in their armpits that function as those of humans, and thus it is probable that human sweat glands evolved from a similar distribution, spreading to more areas of the body, rather than occurring through evolution of a new trait. It is not known whether the increased distribution of sweat glands occurred before, during, or after, the change in body hair, or even whether the two are related developments. Horses also sweat, and they are larger, hairier, and expend more energy running than human males, so there may not be any connection between the ability to sweat and the apparent hairlessness of humans.
Another factor in human evolution that also occurred in the prehistoric past was a preferential selection for neoteny, particularly in females. The idea that adult humans exhibit certain neotenous (juvenile) features, not evinced in the great apes, is about a century old. Louis Bolk made a long list of such traits, and Stephen Jay Gould published a short list in Ontogeny and Phylogeny. In addition, paedomorphic characteristics in women are widely acknowledged as desirable by men. For instance, vellus hair is a juvenile characteristic. However, while men develop longer, coarser, thicker, and darker terminal hair through sexual differentiation, women do not, leaving their vellus hair visible.