The following is an executive summary for one of my book proposals. The book itself will probably take me a year to write and I’m sure I’ll have to update the manuscript continually as new information comes in. Comments are welcome.
Humans look strikingly different in Europe, particularly within a zone centered on the East Baltic and covering the north and the east. Here, skin is unusually white. Hair is not only black but also brown, flaxen, golden, or red. Eyes are not only brown but also blue, gray, hazel, or green.
This pattern also stands out chronologically. It arose very late during the time of modern humans and long after their arrival in Europe some 35,000 years ago. Such is the conclusion now emerging from genetic studies of skin, hair, and eye color.
Europeans owe their light skin to alleles that go back only c. 11,000 years at one gene and 12,000–3,000 years at another. As a Science journalist remarked: “the implication is that our European ancestors were brown-skinned for tens of thousands of years.” They were also uniformly black-haired and brown-eyed. Then, just as recently, their hair and eye color diversified as new alleles began to proliferate at two other genes.
The challenge now will be to narrow the time window. If these changes happened after 7,000 BP, the cause might be northern Europe’s shift from hunting and gathering to cereal agriculture. The change in diet may have reduced the intake of vitamin D, thus favoring the survival of paler Europeans whose skin could synthesize more of this vitamin.
This theory explains how European skin could have turned pale almost at the dawn of history. It leaves unexplained, however, why selection for lighter skin would have multiplied the number and variety of alleles for hair or eye color, especially when so many have little effect on skin color.
If these changes had happened earlier, before 10,000 BP, the cause might involve the last ice age. At that time, the tundra ecozone ran further south in Europe than in Asia, having been pushed down on to the plains of northern and eastern Europe by the Scandinavian icecap. The lower, sunnier latitudes created an unusually bioproductive tundra that could support large herds of game animals and, in turn, a substantial human population—but at the cost of a recurring shortage of male mates. Among present-day hunter-gatherers, similar environments raise the male death rate because the men must cover long distances while hunting migratory herds. The man shortage cannot be offset by more polygyny, since only a very able hunter can provide for a second wife (tundra offers women few opportunities for food gathering, thus reducing their self-reliance in feeding themselves and their children). With fewer men altogether and fewer being polygynous, the sex ratio is skewed toward a female surplus.
In this buyer’s market, men will select those women who look the most feminine. Since human skin color is sexually dimorphic (women are the ‘fair sex’), this sexual selection would eventually whiten the entire population. Where pigmentation has no female-specific form, as with hair and eye color, sexual selection would favor women with color variants that stand out by their novelty, the outcome being an increasingly diverse polymorphism.