Human skin color: Origin, variation and significance

Abstract

Human evolution has been characterized by a marked reduction in body hair and the consequent increased importance of the naked pigmented epidermis as a screen against the harmful effects of solar radiation. Within the epidermis and hair follicles are melanin-containing melanosomes synthesized by melanocytes and transferred to surrounding keratinocytes. Human skin and hair color chiefly depend on the quantities as well as the qualities of melanosomes which are distributed throughout the keratinocytes of the epidermis and hair. Melanin synthesis requires the copper-containing enzyme tyrosinase which acts on tyrosine as its initial substrate. Eumelanin (black-brown) synthesis may be found at all pigmented sites in the body whereas pheomelanin (yellow-red) apparently is only produced within hair follicles. Albinism results when melanocytes of hair, skin and eyes either fail to synthesize melanin owing to a tyrosinase defect, or produce markedly reduced amounts of it for as yet little understood reasons. In general, the human melanin pigmentary system follows the general mammalian plan and shows few, if any, striking differences from those possessed by other primates. Accordingly, although many genes may influence skin and hair color in man as they do in other mammals such as the mouse, it is likely that ethnic variation in human skin pigmentation may involve as few as three to six gene pairs which interact during melanogenesis. Multiple alleles at each of the loci of the polygenic series permit the generation of a gradient of skin color which closely models the variation found on a worldwide basis among humans today. Solar radiation-induced somatic gene mutations within melanocytes may account for the origin of human freckles and spontaneous somatic mutations of some type for the documented reduction of functional epidermal melanocytes with advancing age. Although human cutaneous melanin pigmentation may have functions beyond photoprotection, many of the proposed adaptive functions of melanin are probably fanciful, for they result from dubious attempts to create too tight a link between natural selection as a perfecting force in human evolution and current human physiology.