. "9 Human Skin Pigmentation as an Adaptation to UV Radiation--Nina G. Jablonski and George Chaplin ." In the Light of Evolution IV: The Human Condition. Washington, DC: The National Academies Press, 2010.
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In the Light of Evolution Volume IV: The Human Condition
conditions of reduced sunlight (Murray, 1934; Loomis, 1967). Vitamin D3 is made in the skin when UVR penetrates the skin and is absorbed by 7-dehydrocholesterol (7-DHC) in the epidermis and dermis to form previtamin D3. This reaction only occurs in the presence of wavelengths of 290–310 nm in the UVB range, with peak conversion occurring at 295–297 nm. Photosynthesis of vitamin D3 in the skin depends upon the solar zenith angle, which changes with season, latitude, and time of day, and is further controlled by the amount of pigment and thickness of the skin (Mawer and Davies, 2001; Lips, 2006). The importance of vitamin D3 as a selective force in the evolution of skin pigmentation is related to the manifold effects of the vitamin on fitness as reviewed in earlier papers (Jablonski and Chaplin, 2000; Chaplin and Jablonski, 2009). The vitamin D endocrine system is involved in the regulation of many independent biological processes including bone metabolism, the innate immune response, cell proliferation, and differentiation (Norman, 2008; Köstner et al., 2009). The roles of vitamin D3 in the regulation of intestinal calcium absorption, and in bone formation and remodeling, have been known for decades, but only recently has the importance of vitamin D3 in the establishment and maintenance of innate immunity, and in the normal functioning of the pancreas, brain, and heart, been recognized (Holick, 2004; Norman, 2008). Reduction of fertility due to vitamin D3 deficiencies is greatest in cases of nutritional rickets, but is also significant because of increased prevalence of bacterial and viral infections and increased risk of autoimmune diseases such as multiple sclerosis and type 1 diabetes (Yuen and Jablonski, 2010). Natural selection to promote continued vitamin D production through loss of constitutive pigmentation under conditions of reduced UVR was strong, and its independent action on hominin populations dispersing to low-UVR habitats was inferred before genetic evidence demonstrating positive selection for depigmentation became known (Jablonski and Chaplin, 2000). Generally low and highly seasonally variable levels for UVB created a selective environment favoring the capture of UVB photons required for vitamin D3 photosynthesis through loss of melanin pigmentation. Genetic verification of three independent occurrences of evolution of depigmented skin in hominin populations has been documented in the lineages leading to modern northern Europeans and modern east Asians (Lamason et al., 2005; Norton et al., 2007) and in Homo neanderthalensis (Lalueza-Fox et al., 2007). It is significant that the genetic and physiological mechanisms causing light-skinned phenotypes in each group were different from one another. Regulatory mechanisms involve the control of the formation of melanosomes (the organelles in which melanins are produced and stored) (Lamason et al., 2005; Norton et al., 2007), and the production of the different types and mixtures of melanins. The mechanisms whereby similar phenotypic ends have been reached by different