In his recent commentary, Robins (2009) disputed the role played by ultraviolet radiation (UVR), namely, the vitamin-D-producing wavelengths of ultraviolet B (UVB), in the evolution of human skin. He questioned the theory that reduced levels of pigmentation in human skin were selected to facilitate absorption of UVB. He provided evidence to support his idea that people can produce enough vitamin D in their skin, regardless of pigmentation, if they are not pursuing a modern lifestyle. He asserted that, within his framework, rickets was the only selective force that could have influenced the evolution of light pigmentation because other detrimental effects of vitamin D deficiency are unproven. As rickets is increased by industrialization, Robins concluded that ''. . . vitamin D status could not have constituted the fitness differential between lightly pigmented and darkly pigmented individuals at high latitudes that favored the evolutionary selection of the former'' (Robins, 2009).
In this article, we examine the current evidence for what has been termed the ''vitamin D theory,'' and highlight the importance of UVB penetration in the evolution of human skin. We begin with an overview of the solar processes involved in cutaneous vitamin D synthesis, followed by a discussion of causal arguments and causation in the context of the vitamin D theory, and conclude with a review of physiological mechanisms and their evolutionary significance.
Grant sponsor: The Fletcher Foundation (Alphonse Fletcher, Sr. Fellowship).