Homo Sapiens Became Black To Beat Cancer

SHAVE a chimpanzee and you will find that beneath its hairy coat its skin is white. Human skin, though, was almost always black—at least it was until a few thousand years ago when the species began settling in parts of the world so far north that the sunshine was too weak to allow dark skin to synthesise enough vitamin D. This means that, sometime after chimps and people parted ways, the colour of human skin changed. And that, in turn, must have required an evolutionary pressure.

One suggestion often proferred is that the melanin in black skin, by absorbing ultraviolet light which might otherwise damage DNA and cause mutations, protects against skin cancer. Certainly, white-skinned people who move to the tropics are more at risk of such cancers than they would have been had they stayed at home. But critics of this hypothesis point out that most types of skin cancer, specifically the basal-cell and squamous-cell carcinomas that are the commonest varieties of the disease, tend to affect older people (who have already reproduced and are thus, in Darwinian terms, expendable) and are often not lethal anyway. Malignant melanoma, the one variety which is both lethal and affects all age groups, is rare.

However, a study by Mel Greaves, of the Institute of Cancer Research, in Britain, just published in the Proceedings of the Royal Society, settles the question in favour of cancer being the driving force. Dr Greaves does so by reviewing the clinical data about those Africans who do not have black skin because they are albino.

Albinism has a variety of genetic causes, but they all have the same consequence—a restricted or non-existent ability to synthesise melanin. The phenomenon is not well studied in Africa, not least because of widespread prejudice against albinos, who are ostracised in many parts of the continent. Dr Greaves nevertheless managed to assemble 25 relevant studies, and they do not make pretty reading.

One, conducted in Nigeria and published in 1980, found that half of the 512 albinos whom the researchers followed had developed skin cancer of some sort by the time they were 26. Another, carried out a few years later in Tanzania, showed that half of 125 participants were afflicted by the age of 20. A third, from Soweto in South Africa, suggested that that an albino African has a thousandfold greater risk of developing skin cancer than does his normally pigmented neighbour. And a fourth estimated that fewer than 10% of albinos in equatorial Africa survive into their thirties—with the strong inference that what is killing them is skin cancer.

Nor is the cancer in question always malignant melanoma. Basal-cell and squamous-cell carcinomas are not, for African albinos, the relatively harmless diseases of old age which data collected in the rich world suggest. In Africa, they kill—quickly. Presumably they would have done the same to any human forebear who had had the evolutionary temerity to shed his hairy coat without replacing it with a suitably dark undercoat of melanin-laden skin.

Why humans became Unclad apes is still a mystery. Explanations range from ease of heat loss to the selection of mates by the quality of their (now visible) skin. Dr Greaves’s study, though, removes any doubt about why, having done so, a change of skin colour was essential.

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