The evolution of human skin coloration

Skin color is one of the most conspicuous ways in which humans vary and has been widely used to define human races. Here we present new evidence indic ating that variations in skin color are adaptive, and are related to the re gulation of ultraviolet (UV) radiation penetration in the integument and it s direct and indirect effects on fitness. Using remotely sensed data on UV radiation levels, hypotheses concerning the distribution of the skin colors of indigenous peoples relative to UV levels were tested quantitatively in this study for the first time. The major results of this study are: (1) skin reflectance is strongly corre lated with absolute latitude and UV radiation levels. The highest correlati on between skin reflectance and UV levels was observed at 545 nm, near the absorption maximum for oxyhemoglobin, suggesting that the main role of mela nin pigmentation in humans is regulation of the effects of UV radiation on the contents of cutaneous blood vessels located in the dermis. (2) Predicte d skin reflectances deviated little from observed values. (3) In all popula tions for which skin reflectance data were available for males and females, females were found to be lighter skinned than males. (4) The clinal gradat ion of skin coloration observed among indigenous peoples is correlated with UV radiation levels and represents a compromise solution to the conflictin g physiological requirements of photoprotection and vitamin D synthesis. The earliest members of the hominid lineage probably had a mostly unpigment ed or lightly pigmented integument covered with dark black hair, similar to that of the modern chimpanzee. The evolution of a naked, darkly pigmented integument occurred early in the evolution of the genus Homo. A dark epider mis protected swear glands from W-induced injury, thus insuring the integri ty of somatic thermoregulation. Of greater significance to individual repro ductive success was that highly melanized skin protected against W-induced photolysis of folate (Branda & Eaten, 1978, Science 201, 625-626; Jablonski , 1992, Proc. Australas. Sec. Hum. Biol. 5, 455-462, 1999, Med. Hypotheses 52, 581-582), a metabolite essential for normal development of the embryoni c neural tube (Bower & Stanley, 1989, The Medical Journal of Australia 150, 613-619; Medical Research Council Vitamin Research Group, 1991, The Lancer 338, 31-37) and spermatogenesis (Cosentino er al., 1990, Proc. Natn. Acad. Sci. U.S.A. 87, 1431-1435; Mathur et al., 1977, Fertility Sterility 28, 13 56-1360). As hominids migrated outside of the tropics, varying degrees of depigmentat ion evolved in order to permit WE-induced synthesis of previtamin D-3. The lighter color of female skin may be required to permit synthesis of the rel atively higher amounts of vitamin D-3 necessary during pregnancy and lactat ion. Skin coloration in humans is adaptive and labile. Skin pigmentation levels have changed more than once in human evolution. Be cause of this, skin coloration is of no value in determining phylogenetic r elationships among modern human groups. (C) 2000 Academic Press.