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. 2010 May 11;107 Suppl 2(Suppl 2):8962-8.
doi: 10.1073/pnas.0914628107. Epub 2010 May 5.

Colloquium paper: human skin pigmentation as an adaptation to UV radiation

Nina G Jablonski  1 , George Chaplin
Affiliations

Colloquium paper: human skin pigmentation as an adaptation to UV radiation

Nina G Jablonski et al. Proc Natl Acad Sci U S A. .

Abstract

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 degrees and 46 degrees , 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.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
(A) Annual mean UVB (305 nm). Intensity is indicated by gradations from dark to light varying from 1 to 135 Jm−2 in 10 steps with oceans partially grayed-out. (B) Annual CoV for UVB (305 nm). Gradations of dark to light varying from 10 to 300 in 10 steps, with oceans area partially grayed-out.
Fig. 2.
Fig. 2.
(A) Annual mean UVA (380 nm). Intensity is indicated by gradations from dark to light varying from 65 to 930 Jm−2 in 10 steps with oceans partially grayed-out. (B) Annual CoV for UVA (380 nm). Gradations of dark to light varying from 1 to 13 in 10 steps, with oceans area partially grayed-out.
See this image and copyright information in PMC

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