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12 April 2015
How Europeans evolved white skin

A new study shows that pale skin, as well as other traits such as tallness and the ability to digest milk as adults, arrived relatively recently in much of Europe.
     Comparing the DNA of 83 ancient individuals throughout Europe, the report says Europeans today are a blend of at least three ancient populations of hunter-gatherers and farmers who moved into Europe in separate migrations over the past 8000 years, and that a massive migration from the steppes north of the Black Sea may have brought Indo-European languages about 4500 years ago.
     Curiously, neither the farmers who came from the Near East about 7800 years ago nor the pastoralists who came from the steppes 4800 years ago had the version of the gene that allows adults to digest sugars in milk. It wasn't until about 4300 years ago that lactose tolerance spread through Europe.
     About 8500 years ago, early hunter-gatherers in Spain, Luxembourg, and Hungary had darker skin. In the far north, seven people from a 7700-year-old site in southern Sweden had both light skin gene variants, as well as a third gene which causes blue eyes and may contribute to light skin and blond hair.
     The first farmers from the Near East carried both genes for light skin. One of their light-skin genes spread through Europe, so that central and southern Europeans also began to have lighter skin, while the other gene remained at low levels until about 5800 years ago.
     Complex traits such as height are the result of the interaction of many genes. Selection strongly favoured several variants for tallness in northern and central Europeans starting 8000 years ago, with a boost from the later migration 4800 years ago. In contrast, selection favored shorter people in Italy and Spain starting 8000 years ago. Spaniards in particular shrank in stature 6000 years ago.
     Surprisingly, the team found no immune genes under intense selection, which is counter to hypotheses that diseases would have increased after the development of agriculture.
     People in northern latitudes often don't get enough UV to synthesize vitamin D, so natural selection has favoured two genetic solutions to that problem - pale skin that absorbs UV more efficiently, and lactose tolerance to digest the sugars and vitamin D naturally found in milk.

Edited from AAAS (2 April 2015)

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