The evolution of the different skin colors is thought to have occurred as follows: the haired ancestor of humans, like modern great apes, had light skin under their hair. Once the hair was lost, they evolved dark skin, needed to prevent low folate levels since they lived in sun-rich Africa. (The skin cancer connection is probably of secondary importance, since skin cancer usually kills only after the reproductive age and therefore doesn't exert much evolutionary pressure.) When humans migrated to sun-poorer regions in the north, low vitamin D3 levels became a problem and light skin color evolved.
Dark-skinned people who live in sun-poor regions often lack vitamin D3, one reason for the fortification of milk with vitamin D in some countries.
The Inuit are a special case: even though they live in an extremely sun-poor environment, they have retained their relatively dark skin. This can be explained by the fact that their traditional animal-based diet provides plenty of vitamin D.
Albinism is a condition characterized by the absence of melanin, resulting in white skin and hair; it is caused by a genetic mutation.
Research on skin color variability:
The color of human skin varies from dark brown to pale pink. In attempting to discover the mechanisms that have generated such a wide variation in human skin color, Nina Jablonski and George Chaplin (2000) discovered that there is a high correlation between the coloration of the human skin of indigenous peoples and the average annual ultraviolet (UV) radiation available for skin exposure where the indigenous peoples live.
Accordingly, Jablonski and Chaplin plotted the whiteness (W) of skin coloration of indigenous peoples who have stayed in the same geographical area for the last 500 years versus the annual UV available for skin exposure (AUV) for over 200 indigenous persons and found that whiteness W of skin coloration is related to to the annual UV available for skin exposure AUV according to
W = 70 - [(AUV)/(10)]
(Jablonski and Chaplin (2000), p. 67, formula coefficients have been rounded to one-figure accuracy) where the whiteness W of skin coloration is measured as the percentage of light reflected from the upper inner arm at which location on humans there should be minimal tanning of human skin due to personal exposure to the sun; a lighter skinned human would reflect more light and would have a higher W number. Judging from the above linear fit to the empirical data, the theoretical maximum whitest human skin would reflect only 70 per cent of incident light for a hypothetical indigenous human-like population that lived where there was zero annual UV available for skin exposure (AUV = 0 in the above formula). Jablonski and Chaplin evaluated average annual UV available for skin exposure AUV from satellite measurements that took into consideration the measured daily variation in the thickness of the ozone layer that blocked UV hitting the earth, measured daily variation in opacity of cloud cover, and daily change in angle at which the sunlight containing UV radiation strikes the earth and passes through different thicknesses of earth's atmosphere at different latitudes for each of the different human indigenous peoples' home areas from 1979 to 1992.
Jablonski and Chaplin proposed an explanation for the observed variation of untanned human skin with annual UV exposure. By Jablonski and Chaplin's explanation, there are two competing forces affecting human skin color:
- the melanin that produces the darker tones of human skin serves as a light filter to protect against too much UV light getting under the human skin where too much UV causes sunburn and disrupts the synthesis of precursors necessary to make human DNA; versus
- humans need at least a minimum threshold of UV light to get deep under human skin to produce vitamin D, which is essential for building and maintaining the bones of the human skeleton.