Human skin pigmentation is the product of two clines produced by natural selection to adjust levels of constitutive pigmentation to levels of UV radiation (UVR). One cline was generated by high UVR near the equator and led to the evolution of dark, photoprotective, eumelanin-rich pigmentation. The other was produced by the requirement for UVB photons to sustain cutaneous photosynthesis of vitamin D 3 in low-UVB environments, and resulted in the evolution of depigmented skin. As hominins dispersed outside of the tropics, they experienced different intensities and seasonal mixtures of UVA and UVB. Extreme UVA throughout the year and two equinoctial peaks of UVB prevail within the tropics. Under these conditions, the primary selective pressure was to protect folate by maintaining dark pigmentation. Photolysis of folate and its main serum form of 5-methylhydrofolate is caused by UVR and by reactive oxygen species generated by UVA. Competition for folate between the needs for cell division, DNA repair, and melanogenesis is severe under stressful, high-UVR conditions and is exacerbated by dietary insufficiency. Outside of tropical latitudes, UVB levels are generally low and peak only once during the year. The populations exhibiting maximally depigmented skin are those inhabiting environments with the lowest annual and summer peak levels of UVB. Development of facultative pigmentation (tanning) was important to populations settling between roughly 23°and 46°, where levels of UVB varied strongly according to season. Depigmented and tannable skin evolved numerous times in hominin evolution via independent genetic pathways under positive selection.ariation in skin color is the most noticeable of human polymorphisms. As visually dominant mammals, we readily notice differences in skin color in each other. As primates who uniquely use language to create categories, we readily give names to these differences. Since the mid-18th century, skin color has been the single most important physical trait used to define human groups, including variously named varieties, races, subspecies, and species. Charles Darwin observed variation in human skin color while abroad during the voyage of the H.M.S. Beagle (1831-1836), but he soundly rejected the notion that physical differences such as skin color constituted the basis for distinguishing separate human species (1). Darwin's rejection of the existence of distinct human species was based upon his observation that human groups "graduate into each other, and that it is hardly possible to discover clear distinctive character between them" (1, p. 226). His aversion to the separation of humans into discrete species was also motivated by his vehement aversion to slavery, which in his lifetime was defended and promoted on the basis of the superiority and inferiority of allegedly distinct human species (2). It is also well known that early in his career, Darwin collected copious notes on human origins and descent (3), but "without any intention of publishing on the subject, but rather with a det...