Biological significance of human pigmentation 341 New roles Recently new concepts of the protective role of melanin have emerged. Melanin acts as an efficient photosensitizer in the presence of incident radiation. It has been suggested that melanin effectively eliminates those cells genetically damaged on exposure to ultraviolet. Ebling & Rook (27) propose a phototoxic role of melanin in addition to its photo- protective role, Riley (28) speculates on the phototoxic role of melanin as a substitute for photoprotection, suggesting that by their lighter skin the Caucasoids in the tropics suffer more from skin cancer more through failure to remove the genetically damaged cells than through the absence of the melanin buffer. Also on account of its stable free radical nature, which gives a free electron receptor site on its molecule, melanin can combine with many other substances. It is able to form lipofuscin, a lipomelanin important in ageing (29). It forms complexes with chloro- promazine, a drug which not only promotes melanin transport (30), but is also effective in treatments of some mental disorders and leaves symptoms of Parkinsonism and hyper- pigmentation as side effects. Melanin moreover, is phagocytized by leucoytes, can there- f6re circulate in the body (31, 32), and indeed melanin granules have been reported in 75•o of skin-draining lymph nodes of Bantu but in only 20•o of Caucasoid lymph nodes. Wasserman (33) therefore suggests a possible role for melanin in intracellular metabolism. That some of the new concepts of the role of melanin may be true finds support from Ebling & Rook (27), who point out that it occurs in many organs besides the epidermis and this may have a variety of roles other than environmental adaptation. Conclusion Whatever the mechanisms, the dose correlation with geographical factors suggests a very strong adaptive role. It seems that protection against ultraviolet radiation in areas where this is intense, and increased synthesis of vitamin D where there is minimal ultraviolet radiation, provide the two most important selective roles for heavy and light melanin pigmentation. References 1 Gates, R. R. (1949) Pedigrees of negro families. Blakiston. Philadelphia. 2 Fleure, H. J. (1945) The distribution of the types of skin colour. Geogr. Rev. 35 580-595. 3 Weiner, J. S. (1951) A spectrophotometer for measurement of skin colour. Man 51 152-153. 4 Roberts, D. F. and Kahlon, D. P.S. (1972) Skin pigmentation and assortative mating in Sikhs. J. Biosoc. Sci. 4 91-100. 5 Roberts, D. F. and Kahlon, D. P.S. (1977) Human Skin Pigmentation (in press). 6 Stern, C. (1953) Model estimates of the frequency of white and near white segregants in the American Negro. Acta Genet. Stat. Med. 4 281-298. 7 Harrison, G. A., Owen, J. J. T., da Rocha, F. J. and Salzano, F. M. (1967) Skin colour in Southern Brazilian populations. Human Biol. 39 21-31. 8 Harrison, G. A. and Owen, J. J. T. (1964) Studies on the inheritance of human skin colour. Ann. Hum. Genet. 28 27-37. 9 Livingstone, F. B. (1969) Polygenic models for the evolution of human skin colour differences. Human Biol. 41 480-493. 10 Livingstone, F. B. (1972) Genetic drift and polygenic inheritance. Amer. J. Phys. Anthr. 37 117-126. 11 Roberts, D. F. and Kahlon, D.P.S. (1976) Environmental correlations of skin colour. Annals Human Biol. 3 11-22. 12 Darwin, C. (1871) The Descent of Man and Selection in Relation to Sex. Murray, London.

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