Young Brunette Girl Shows Off Her Body And Long Hair

Evolutionary variation
Evolutionary biologists suggest that the genus Homo arose in East Africa approximately 2.5 million years ago. They devised new hunting techniques. The higher protein diet led to the evolution of larger body and brain sizes. Jablonski postulates that increasing body size, in conjunction with intensified hunting during the day at the equator, gave rise to a greater need to rapidly expel heat. As a result, humans developed the ability to sweat and thus lost body hair to facilitate this process. A major problem with this theory, however, is that it does not explain why males are larger, hairier, and were more active in hunting than females. The female-male size differential among other closely associated primates is much greater than among humans, however, so it might have been reduced during evolution.
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.