The Physical Characteristics of Humans

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At some point in the past, then, as our large brain evolved, our ancestors lost the characteristic hairiness of the apes and acquired our typically naked skin. The hot climate of Africa, where our ancestors evolved, must have been a key factor in that development. A relatively hairless skin with many sweat glands would make heat diffusion of the chemical energy generated by physical effort much more efficient, thus allowing our ancestors to run further and faster without harming their brains from over-heating.

If naked skin developed as an efficient way to diffuse body heat under an equatorial sun, the skin itself would have been protected by melanin, the dark pigment in human skin that forms a barrier to ultra-violet rays.  That is the point of the painting of dark-skinned, relatively hairless australopithecines on the left. 

Most mammals have hair or fur for a good reason, however, and we ought to consider how losing our protective coat of hair could have been a disadvantage as well.  Hair insulates mammal bodies from both heat and cold.  Until the development of fire and clothing, then, the loss of body hair would have prevented expansion of Hominid or human populations into cooler environments. The ability to survive sudden temperature drops, such as those that occur at night at moderate elevations or away from the equator, would have been a problem for humans. It seems reasonable, in fact, to suppose that the need for warmth at night may have driven the discovery of the uses of fire. Using fire for cooking may, in fact, have been a later development.