Variation 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 (Darwin, 1871a). 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” (1871a, 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 (Desmond and Moore, 2009). It is also well known that early in his career, Darwin collected copious notes on human origins and descent (van Wyhe, 2007), but “without any intention of publishing on the subject, but rather with a determination not to publish, as I thought that I should thus only add to the prejudices against my views” (Darwin, 1871a, p. 1). Darwin thus deflected potential criticism of natural selection in the first decade after publication of The Origin by avoiding almost entirely discussion of humans in an evolutionary context.

The causes of variation in human skin pigmentation were much discussed long before Darwin’s time. Observers beginning with Hippocrates in the fifth century associated human traits and temperament with the environment and recognized that skin color was part of this package (Isaac, 2004). The association of dark skin pigmentation with intense sunshine and heat was further developed by Aristotle and his followers as part of a comprehensive “climatic theory,” which related human features, dispositions, and cultures to the environment. By the mid-18th century, naturalists such as John Mitchell and, later, Samuel Stanhope Smith recognized a pronounced latitudinal gradient of skin pigmentation among the world’s peoples—from dark near the equator to light toward the poles—and related it mainly to differences in sunshine heat experienced by people at different latitudes (Mitchell and Collinson, 1744; Smith and