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Geneticists working in East Africa have discovered a gene mutation that enables certain adults to digest milk. The finding is a striking example of how a cultural innovation—raising livestock—can cause unrelated people in different geographic locations to evolve in a similar direction.

Less than half of the world’s population is “lactose tolerant” into adulthood. Sarah Tishkoff of the University of Maryland discovered that in some East Africans, this trait is caused by a mutation of a single nucleotide on the DNA of chromosome 2. The East African mutation is different from a previously known lactose tolerance mutation seen mostly among white Europeans and Middle Easterners.

The East African mutation became common when livestock farming arose there, 2,700 to 6,800 years ago. The nutrients and water from dairy animals helped humans survive in such arid regions. Lactose tolerance likely evolved in groups that practiced pastoralism, and was passed down to their ancestors.