Jared Diamond - The rise and fall of the third chimpanzee

Racial variation has characterized humans for at least the past several thousand years, and possibly much longer. Already around 450 BC, the Greek historian Herodotus described the Pygmies of West Africa, the black-skinned Ethiopians, and a blue-eyed red-haired tribe in Russia. Ancient paintings, mummies from Egypt and Peru, and bodies of people preserved in European peat bogs confirm that people several thousand years ago differed in their hair and facial features much as they do today. Origins of modern races can be pushed back still further, to at least ten thousand years ago, since fossil skulls of that age from various parts of the world differ in many of the same respects that modern skulls from the same regions differ. More controversial are the studies of some anthropologists, contested by others, reporting continuity of racial skull characteristics for hundreds of thousands of years. If those studies are correct, then some of the human racial variation that we see today may predate the Great Leap Forward, and may have gone back to the times of Homo erectus. Now let's turn to the question of whether natural selection or sexual selection has made the larger contribution to those visible geographic differences of ours. Take first the arguments about natural selection, the selection of traits that enhance survival. No scientist denies today that natural selection does account for many of the differences between species, such as why lions have paws with claws while we have grasping fingers. No one denies either that natural selection explains some geographic variation ('racial variation') within some animal species. For instance, Arctic weasels that live in areas covered by winter snow change colour from brown in summer to white in winter, while more southerly weasels stay brown all year. That racial difference enhances survival, because white weasels against a brown background would be glaringly conspicuous to their prey if they were not camouflaged against snow.

By the sajne token, natural selection surely explains some geographic variation in humans. Many black Africans but no Swedes have the sickle-cell haemoglobin gene, because the gene protects against malaria, a tropical disease that would otherwise kill many Africans. Other localized human traits that surely evolved through natural selection include the big chests of Andean Indians (good for extracting oxygen from thin air at high altitudes), the compact shapes of Eskimos (good for conserving heat), the slender shapes of southern Sudanese (good for losing heat), and the slit-like eyes of northern Asians (good for protecting eyes against cold and against sun glare off the snow). All these examples are easy to understand. Can natural selection similarly explain the racial differences that we think of first, those in skin colour and eye colour and hair? If so, one might expect that the same trait (for instance, blue eyes) would reappear in different parts of the world with similar climates, and that scientists would agree on what the trait is good for.

Seemingly the simplest trait to understand is skin colour. Our skins run the spectrum from various shades of black, brown, copper, and yellowish to pink with or without freckles. The usual story to explain this variation by natural selection goes as follows. People from sunny Africa have blackish skins. So too (supposedly) do people from other sunny places, like southern India and New Guinea. Skins are said to get paler as one moves north or south from the equator, until one reaches northern Europe, with the palest skins of all. Obviously, dark skins evolved in those people who were exposed to much sunlight. That is just like the skins of whites tanning under the summer sun (or in tanning salons!), except that tanning is a reversible response to sun rather than a permanent genetic one. It is equally obvious what good a dark skin does in sunny areas: it protects against sunburn and skin cancer. Whites who spend lots of time outdoors in the sun tend to get skin cancer, and they get it on exposed parts of their body like their head and hands. Does that not all make sense?

Yes, but… it is really not so simple at all. To begin with, skin cancer and sunburn cause little debilitation and few deaths. As agents of natural selection, they have an utterly trivial impact compared to infectious diseases of childhood. Hence many other theories have been proposed to explain the supposed pole-to-equator gradient in skin colour.

One favourite competing theory notes that the sun's ultraviolet rays promote vitamin D formation in a layer of our skin beneath the main pigmented layer. Thus, people in sunny tropical areas might have evolved dark skin to protect them against the risk of kidney disease caused by too much vitamin D, while people in Scandinavia with its long dark winters evolved pale skins to protect them against the risk of rickets caused by too little vitamin D. Two other popular theories are that dark skins are to protect our internal organs against overheating by the tropical sun's infrared rays, or—just the opposite—dark skins help keep tropical people warm when the temperature drops. And if those four theories are not enough for you, consider four more: that dark skins provide camouflage in the jungle, or that pale skins are less sensitive to frostbite, °r that dark skins protect against beryllium poisoning in the tropics, or that pale skins cause deficiency of another vitamin (folic acid) in the tropics.

With at least eight theories in the running, we can hardly claim to understand why people from sunny climates have dark skins. That in rtself does not refute the idea that, somehow, natural selection caused the evolution of dark skins in sunny climates. After all, dark skins could have multiple advantages, which scientists may sort out some day. Instead, the heaviest objection to any theory based on natural selection is that the association between dark skins and sunny climates is a very imperfect one. Native peoples had very dark skins in some areas receiving relatively little sunlight, like Tasmania, while skin colour is only medium in sunny areas of tropical Southeast Asia. No American Indians have black skins, not even in the sunniest parts of the New World. When one takes cloud cover into account, the world's most dimly lit areas, receiving a daily average of under three-and-a-half hours of sunlight, include parts of equatorial West Africa, southern China, and Scandinavia, inhabited respectively by some of the world's blackest, yellowest, and palest peoples! Among the Solomon Islands, all of which share a similar climate, jet-black people and lighter people replace each other over short distances. Evidently, sunlight has not been the sole selective factor that moulded skin colour.

The first response of anthropologists to these objections is to raise a counter-objection, the time factor. This argument tries to explain away the cases of pale-skinned people in the tropics by claiming that those particular peoples migrated to the tropics too recently to have evolved black skins. For example, the ancestors of American Indians may have reached the New World only 11,000 years ago (Chapter Eighteen): perhaps that has not been long enough to evolve black skins in the tropical Americas. But if you are going to evoke the time factor to explain away objections to the climate theory of skin colour, then you also have to consider the time factor for peoples who supposedly support that theory. One of the prime supports of the climate theory is the pale skin of Scandinavians, living in the cold, dark, foggy North. Unfortunately, Scandinavians have been in Scandinavia for an even shorter time than American Indians have been in the Amazon. Until about 9,000 years ago, Scandinavia was covered by an ice-sheet and could hardly have supported any people, pale-skinned or dark-skinned. Modern Scandinavians reached Scandinavia only around 4,000 or 5,000 years ago, as a result of the expansion of farmers from the Near East (Chapter Ten) and of Indo-European speakers from southern Russia (Chapter Fifteen). Either Scandinavians acquired their pale skins long ago in some other area with a different climate, or else they acquired them in Scandinavia within half the time that Indians have spent in the Amazon without becoming dark-skinned.