Biological significance of human pigmentation 339 Thermoregulation The results of the present analysis suggest also thal thermoregulation may also be in- voked as a selective mechanism for skin pigmentation, though in a subordinate but measurable role. Although melanin may bestow on deeply melanized skin the potential selective advantage of protection against the carcinogenic effects of ultraviolet rays, at the same time it imposes an additional heat load, because of the greater absorption and relatively less reflection of solar energy. Black skin absorbs 80•o or more of incident visible light, while light skin absorbs only about 605/0 or less. Black and white skin reflectance curves differ less at the shorter infrared wavelengths (700-1000 •), though again the black absorbs slightly more. About 955/0 of the infrared part of the radi- ation responsible for thermal effects is absorbed by a depth of 2 mm of skin surface, and 995/0 of it by a depth of 3 ram, thus suggesting that the differential response of heat load from solar energy resides in the pigmented parts of the epidermis (18). It is therefore to be expected that black skin would be heated more than white skin by the absorption of sunlight. The expected energy absorption has been calculated for white and for black skin exposed to radiant energy of similar spectral distribution to sunlight at the surface of the atmosphere relatively untanned black skin absorbed 34•o more energy than white skin. Since the passage of sunlight through the atmosphere •'lters out more energy in the infrared region than in the visible, the contrast in heat load from the visible spectrum may well be greater by the time sunlight has reached the earth's surface. In the American desert the heat absorption of unclothed white men ex- posed to sunlight amounted to 140 calories per hour on the average, a considerable heat load when it is remembered that the resting heat production is usually taken as 90 calories per hour. Under hot desert conditions, therefore, the additional heat absorption of a Negro due to skin colour may be 40 calories per hour or more, a potentially heavy strain on the heat-regulating mechanism under conditions of severe stress. Baker (19) reported a greater rise in rectal temperature accompanied by greater sweating in Negroes than in White soldiers while exercising in the nude, a difference which disappeared when exercising in clothing. Hence disadvantage of the pigmented skin in thermoregulation can be imagined in heat stress either where there is little chance of replenishing water lost in sweating, or in conditions of high humidity where evaporative heat loss is very difficult indeed, in humid heat stress conditions, it was found that in nineteen matched pairs of negro and white males heat casualties claimed twenty-two victims and only one of them was Negro. With this the distribution of pigmentation in Africa appears to conform. But on the world scale the temperature association appears to be the wrong way round. Perhaps the deeper pigmentation triggers earlier activation of heat loss mechanisms such as sweating, or perhaps the heat load is insufficient for the disadvantages to operate. Another possibility comes from the earlier discussion. Dark skins are much less suscep- tible to sunburn damage than are light skins. Severe sunburn damage to the skin involves the sweat glands, disturbs their functioning and hence heat regulation. Under conditions in which efficient sweating is critical, the survival value of any feature such as pigmentation which may prevent sweat gland damage will be great. Though not under- stood fully, acclimatization to heat as a component of the adaptive role of melanin against solar radiation appears reasonable. Another possibility is that pigmented skin may act as a more efficient dissipator of the solar heat load by radiation (20). Here the evidence is conflicting, and several studies (e.g. 21, 22) suggest small differences of about 2-3• in the emissivity coefficients of black

340 D. F. Roberts and white skins between 5000 nm and 8000 nm. But even a small physical effect of this magnitude may be significant from the point of view of adaptation or survival. Vitamin D synthesis The slight pigmentation of European populations poses a different problem. The role of melanin in vitamin D synthesis was suggested by Murray •.s long ago as 1934. Vitamin D is synthesized in the epidermis from a precursor 7-dehydrocholesterol, by photoactive reaction to UVR, medicated by melanin and keratin in the epidermis. Excessive or deficient synthesis can result respectively in hypervitaminosis or rickets in children. Fremon & Loomis (23) suggested that while the heavily melanized skin of the Negro screens out excessive UVR by absorption, and so prevents excessive amounts of vitamin D, the less melanized and keratinized skin of the northern Asians and north-west Euro- peans enables the available amounts of ultraviolet light to be fully utilized for synthesis of adequate quantities of vitamin D. On this hypothesis there would be dual selection for heavy pigmentation to prevent toxic overdoses of vitamin D where there is heavy ultra- violet radiation, and for slight pigmentation in regions of low ultraviolet. The first half of this hypothesis does not hold, for example Fremon (23) found no cases of hypervita- minosis amongst the Whites in the tropics. But selection for fair skin may well have occurred in north-west European conditions of coolness, heavy cloud cover, and rela- tively little radiation and indeed the suggestion from recent genetic studies of overall dominance of the genes from the white parent over those from the black parent carries the implication that selection for depigmentation may have been strong. In conditions of minimal sunshine where dietary vitamin D is low, increased access of these rays to the deeper skin layers could well be selectively advantageous in preventing the development of rickets. There is little direct evidence, but it is interesting that black children dwelling in the great cities in the northern part of America and Pakistani children in Glasgow are more susceptible to rickets than are white children. Certainly the occurrence of rickets amongst the more deeply pigmented inhabitants of some cities of the United Kingdom is today causing concern (24). This may indicate something more than a dietary deficiency. It may be due to the fact that, of the lessened amount of ultraviolet light reaching the skin in the temperate latitudes, again diminished by atmospheric pollution and indoor living, what is left after absorption by the melanin of black skin is insufficient to stimulate the production of vitamin D in the dermal layers. Disease protection An intriguing suggestion is that of Wasserman (25, 26), who argues that the main selec- tive factor in the evolution of darker pigmentation in the tropics is disease and not climate. The primary need for survival in tropical peoples must have been protection against many infective and parasitic diseases. 'In their defence, the tropical dwellers evolved a mechanism of increased reticuloendothelial activity and elevated gamma- globulins. These features are related inversely to the size and acitvity of the adrenal cortex, so that through the decrease in adrenal cortical activity there would be increased MSH and ACTH levels which in turn result in increased pigmentation. Thus according to this hypothesis the dark skin pigmentation is only a byproduct of successful adaptation of a more efficient reticuloendothelial system. Such a hypothesis apart from ignoring other adaptive values of melanin, is certainly inadequate to explain the lighter pigmenta- tion of whites.