Many of these changes seem to be quite recent. The mutation
that appears to have the greatest effect on skin color among
Europeans and neighboring peoples, a variant of SLC24A5,
has spread with astonishing speed. Linkage disequilibrium—
that is, the degree to which the genome is surprisingly uniform
around this gene—suggests that it came into existence about
5,800 years ago, but it has a frequency of about 99 percent
throughout Europe and is found at significant levels in North
Africa, East Africa, and as far east as India and Ceylon. If it is
indeed that recent, it must have had a huge selective advantage,
perhaps as high as 20 percent. It would have spread so rapidly
that, over a long lifetime, a farmer could have noticed the change
in appearance in his village. Again, if it is that recent, it must
have had a more limited distribution in early historical times,
particularly in peripheral areas: In fact, this may explain the Roman
impression that the Picts of Scotland were dark-skinned.
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