Human Skin Pigmentation as an Example of the Action of Natural Selection

Skin pigmentation is one of the most visible ways in which humans vary. The clinal distribution of skin pigmentation – from darkest hues nearer the equator to lightest hues nearer the poles – is related primarily to the intensity of ultraviolet radiation (UVR) at the earth’s surface. UVR is a highly energetic and invisible form of solar radiation, which is capable of damaging DNA and breaking down other important biological molecules, such as forms of the B vitamin, folate. Folate is needed to produce DNA and support cell metabolism and, without adequate folate, DNA production and cell division cannot proceed normally. These processes are particularly important in the early embryo and in the production of sperm and so are critical for successful reproduction. In humans, protection against damage to DNA and folate by UVR was achieved by evolution of high concentrations of highly efficacious natural sunscreen, eumelanin, in the skin. Permanent eumelanin pigmentation evolved by natural selection in the ancestors of all modern people who lived in the high-UVR environments of equatorial Africa. As modern humans dispersed away from the most intensely sunny parts of Africa into southern Africa, Asia, and Europe, they began to live under lower levels of UVR. This had important biological consequences because medium-wavelength UVR or UVB is necessary for initiating the process of making vitamin D in the skin. Vitamin D is required for maintenance of normal health and reproduction because it is required for calcium absorption and for the development and maintenance of strong skeletal and immune systems. High concentrations of eumelanin retard the formation of vitamin D in the skin, and so the active loss of pigmentation by natural selection was an important biological adaptation for humans living outside of the tropics. Light skin is depigmented skin. The ability to develop temporary eumelanin pigmentation in the skin in response to UVR – tanning – has evolved numerous times in peoples living under highly seasonal patterns of sunshine.  The genetic bases of these important phenotypic changes are now well understood as the result of studies of comparative and functional genomics. Skin pigmentation in humans is the product of an evolutionary compromise between the demands of UVR protection and vitamin D production, both of which are essential for survival and successful reproduction. Skin pigmentation thus provides one of the best examples of evolution by natural selection acting on the human body.