Chimpanzees are the closest living relatives to humans, and since they have light skin covered by hair, it is likely that our shared common ancestor would also have lacked pigmentation and been covered by hair. As human brain size increased the increase in its energy requirements would have required finer thermoregulation to avoid overheating. This may be one reason why humans developed sweat glands, an evolution we share with only a small number of creatures (including swine, many species of which are also hairless). The additional loss of body hair would have increased the effectiveness of evaporation of sweat, and produced better cooling. Though naked skin is advantageous for thermoregulation, it exposes the epidermis to destructive levels of UV radiation that can cause sunburn, skin cancer and birth defects resulting from the destruction of the essential vitamin B folate. Consequently strong natural selection in Africa favored increased levels of melanin in the skin, and the hairless Hominina ancestors of modern humans lost their light skin.
Light skin color would have been a severe disadvantage to those living under the bright African sun. However, when humans left Africa for less sun-intense regions of the world, the selective pressure against lighter skin would have relaxed. This probably explains the greater variety of skin color found outside sub-Saharan Africa. Lighter skin colors may have been advantageous at higher latitudes since they allow greater penetration of the sun's UV radiation, a requirement for vitamin D synthesis. This may have led to selection for lightly pigmented skin. Scientists have identified at least 100 genes associated with pigment processing. Though African populations are relatively dark, according to a recent study[citation needed] they possess a greater diversity in skin complexion than all other populations. It is therefore likely that many of the alleles associated with light pigmentation were already present in an ancestral population in Africa prior to their dispersal. When humans migrated out of Africa, the lighter skin causing alleles may have accumulated in one population, either by genetic drift, natural selection, sexual selection or a combination of these effects. Since their effects are additive it is possible light skin could arise over several generations without any new mutations taking place.
According to Luigi Luca Cavalli-Sforza, light skin probably arose in North Africa or both in the north and east.
A 2006 study provides evidence that the light skin pigmentation observed in Europeans and East Asians arose independently. They concluded that light pigmentation in Europeans is at least partially due to the effects of positive directional and/or sexual selection.
Good times.
