Is Sunlight a Factor in the Geographical Distribution of Human Skin Color?

Abstract

r | HE idea that the geographical distribution of human skin color is related causally to the distribution of sunlight is widespread in both scientific and lay circles. Concentration of Negroes in the tropics and of fair-skinned people in northern latitudes is sometimes explained on this basis, though groups with intermediate complexions-for example, the Eskimo-may not fit well into the picture. Natural selection is often invoked. Charles Darwin suggested in 1871 in The Descent of Man that the pigment of Negro skin offered a basis for selection through protection against harmful of sunlight.' Darwin was cautious about his suggestion, but later Darwinians have not always been so, and the notion of an adaptive role for skin color has sometimes hardened into dogma. That Negro skin is less susceptible to some effects of sunlight than most lighter skins are is clear enough, but the extent of the protection afforded by the melanin pigment, which is responsible for the dark color, is less certain, and under some climatic conditions this pigment could be disadvantageous. To determine the relative importance of the various factors concerned in the supposed adaptation to environment would require measurements that have not been made, and some of them would tax our present technical capacities. In Darwin's day little was known about the spectral character of sunlight, and even less about its physiological and pathological actions. Dire effects were at one time attributed to tropic sunlight-a special kind of z rays was even invented by way of explanation2-and red spine pads and pith helmets were de rigueur for Europeans who ventured into equatorial lati* This paper has benefited from criticisms kindly given me by Professors Paul Bener, Larry G.