The Evolution of Tanning Needs Its Day in the Sun

Variation in human pigmentation has long been an area of interest in biological anthropology, with the advent of genetic technologies allowing deeper plumbing of its evolutionary history. Genome-wide scans of selection show that pigmentation genes have undergone some of the strongest selection in many geographically distant populations. A variety of hypotheses for the photoprotective effects of melanin have been developed, but these hypotheses, as well as genetic studies, focus nearly exclusively on constitutive (basal) pigmentation levels. Failing to consider the contribution of the ultraviolet radiation (UVR) environment neglects the true interface between humans and our environment. Data drawn largely from dermatology demonstrate that constitutive pigmentation and tanning response are weakly coupled in populations from East Asia and the Americas. This suggests a possible role for persistent, UVR-induced pigmentation as a convergent adaptation akin to the protective effect of constitutive pigmentation. The adaptive potential of tanned skin, particularly in the Americas, where constitutive pigmentation is lower than expected, may fill in an important gap in our understanding of the evolution of skin color.