For most of the time humans have lived in Europe, the inhabitants have had quite dark skin, a genetic study has concluded. The genes for light skin that came to dominate the continent were a relatively recent arrival. Like the capacity to digest milk as an adult, pale skin was sufficiently advantageous at high latitudes that it spread rapidly through the population.
While it is basic to natural selection that advantageous mutations spread, it is not always a quick process. Most new genetic variations confer only a limited advantage, and their progress through the community can be slow. This makes the traits that spread rapidly particularly interesting since such success demonstrates the advantage must be very large.
The 1000 Genomes Project is searching for such examples by comparing the genomes of modern individuals from specific regions in Europe with 83 samples taken from seven ancient European cultures. Harvard University's Dr. Iain Mathieson has identified five features whose spread through the population of Europe indicates a strong selection advantage.
At the annual conference of the American Association of Physical Anthropologists, Mathieson said his team distinguished, “between traits that have changed consistently with population turnovers, traits that have changed apparently neutrally, and traits that have changed dramatically due to recent natural selection.”
Five traits were identified that fit into the last category. Previous studies have identified one of these: the capacity to digest lactose in adulthood. Although this is far from universal today, most people of European descent are lactose tolerant, to the extent that milk products not only form a major source of nutrition but are a defining feature of European cultures.
Yet 8,000 years ago, Europeans lacked this ability, a fact that advocates of “Paleo” diets prefer to ignore when marketing to a very white audience. Mathieson expanded on the previous work by showing that the capacity to digest lactose as an adult appeared in the population after the development of farming. Two waves of farmers settled Europe 7,800 and 4,800 years ago, but it was only 500 years later that the gene for lactose tolerance became widespread.
Unsurprisingly, the first modern humans to leave Africa were black. Moreover, Mathieson found that more than 30,000 years later hunter-gatherers in what is now Spain, Luxumberg and Hungary had dark-skinned versions of the two genes more strongly associated with skin color. The oldest pale versions of the SLC24A5 and SLC45A2 genes that Mathieson found were at Motala in southern Sweden 7,700 years ago. The gene associated with blue eyes and blond hair was found in bodies from the same site.
The light SLC24A5 gene swept through southern and eastern Europe with farmers who arrived from the east, but the pale SLC45A2 only became common around 5,800 years ago.
Mathieson and colleagues expressed their surprise at not finding strong selection for immunity to transmissible diseases with the spread of agriculture, but did identify patterns in height, with shortness favored among the first farmers on the Iberian Peninsula, while genes associated with tallness taking over 5,000 years ago.