Skin Pigmentation Variation

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Skin pigmentation in modern humans is an adaptation to ultraviolet radiation. The most darkly pigmented skin is found among people living at or near the equator; the most lightly pigmented skin is found among people living above 45N latitude. The primary pigment in human skin is melanin, mostly in the form of the very dark brown eumelanin. Yellow-red pheomelanin is present in human skin to varying degrees, and is most obvious as the pigment causing freckles in people with very lightly pigmented skin. Eumelanin is a superior natural sunscreen with abilities to absorb wavelengths of visible and ultraviolet radiation. Darkly pigmented skin with high concentrations of eumelanin confers protection against high levels of both ultraviolet A (UVA=315-345 nm) and ultraviolet B (UVB=280-315 nm) radiation. Ultraviolet radiation is mostly harmful to humans because it damages DNA and breaks down folate, a B vitamin necessary for DNA synthesis and cell division.

The earliest members of Homo sapiens living in eastern and northeastern Africa were darkly pigmented, and genetic evidence indicates that variation in one of the primary genes controlling eumelanin production, the melanocortin 1 receptor (MC1R), was lost as the result of a selective sweep about 1.2 myr. Human dispersal into higher latitudes with lower levels of UVR was associated with positive selection for depigmentation. Although UVR is mostly harmful, UVB (optimally at 297 nm) initiates the production of vitamin D in the skin from a cholesterol-like precursor. Vitamin D is required for absorption of calcium and the growth and maintenance of a healthy skeleton; it is also important for normal development and functioning of the immune system and brain. Positive selection for depigmentation was driven by the importance of maintaining vitamin D photosynthesis in the skin under conditions of reduced UVB at higher latitudes. Evolution of depigmented skin occurred at least twice in Homo sapiens, in both the ancestors of modern northern Europeans and eastern Asians.

References

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