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Matt Ridley is the author of provocative books on evolution, genetics and society. His books have sold over a million copies, been translated into thirty languages, and have won several awards.

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Where blue eyes came from

My latest Mind and Matter column for the Wall Street Journal is on gene-culture co-evolution:

Human beings, we tend to think, are at the mercy of their genes. You either have blue eyes or you do not (barring contact lenses); no amount of therapy can change it. But genes are at the mercy of us, too. From minute to minute, they switch on and off (i.e., are actively used as recipes to make proteins) in the brain, the immune system or the skin in response to experience. Sunbathing, for example, triggers the expression of genes for the pigment melanin.

As a recent study confirms, on a much longer time scale, genes are even at the mercy of culture. The paradigmatic example is lactose tolerance. All mammals can digest lactose sugars in milk as babies, but the lactase gene switches off at weaning when no longer needed. In much of Europe and parts of Africa, by contrast, most people can digest lactose even as adults, because the lactase gene remains switched on. (About 90% of East Asians and 70% of South Indians are lactose-intolerant to some degree.)

This "lactase persistence" is caused by one of four genetic mutations that occurred in different regions and at different dates, one in Europe and three in Africa. Coincidentally, these regions also saw the domestication of cattle and the adoption of milk-drinking by adults around the same time. Of course, it's plausible that the culture came first-drinking milk gave some advantage to all, because milk has nutrition other than lactose in it, but it especially benefited adults with mutations that allowed lactose digestion. So such mutations spread.

The new study, of 5,000-year-old bones from the Basque region of Spain, catches this evolutionary event in the act, finding that just 27% of individuals were then lactose tolerant, much lower than today. Concepción de la Rúa of Spain's University of the Basque Country and her fellow authors conclude that the genetic change most probably happened after cattle domestication, at a time "when fresh milk consumption was already fully adopted as a consequence of a cultural influence." Here we have genes at the mercy of culture.

Could blue eyes be another example of the same phenomenon-"culture-gene co-evolution"? Thanks to the work of the appropriately named (and blue-eyed) Danish geneticist Hans Eiberg and his colleagues, we now know that the chief mutation that causes blue eyes is a single letter change, from A to G, at the 26,039,213rd position on chromosome 15, within a gene called HERC2.

HERC2 has no effect on eye color, but it contains an unexpressed segment of DNA that is needed for the switching on of a nearby gene called OCA2, as demonstrated by newly published work by Robert-Jan Palstra and others at Erasmus University in the Netherlands. The mutation that causes blue eyes reduces the expression of OCA2 and hence reduces pigment concentration. Paler eyes look bluer.

Why did this mutation become so common somewhere around the shores of the Baltic sea around 6,000 years ago? The answer may lie in the fact that the date coincides with the arrival of agriculture in the area. When people began relying heavily on a diet of bread at such a northern latitude, they probably became chronically deficient in vitamin D, for bread is generally low in vitamin D.

This wouldn't matter in a lower latitude, because the body can synthesize vitamin D if exposed to ultraviolet sun rays. But in northern Europe, diseases related to vitamin D deficiency, such as rickets, would have become common. Any individual who had a genetic mutation that lightened his or her skin (and eyes) would absorb more sunlight, boosting health and the ability to survive and breed. Paleness was selected.

When Nordic people started depending more on bread than on fish, they got less vitamin D from their diet. As a result, they got paler, improving the capacity of their skin to generate this crucial nutrient just from scarce sunlight. How they lived changed, in effect, how they looked.