Adult lactase tolerance arose twice...

Natural selection appears to have favored humans tolerant to lactose in their adulthood, especially in early cow-raising societies. Different genetic mechanisms appear to have arose independently in Europe and Africa.


Study Detects Recent Instance of Human Evolution - New York Times: "After testing for lactose tolerance and genetic makeup among 43 ethnic groups of East Africa, she and her colleagues have found three new mutations, all independent of each other and of the European mutation, which keep the lactase gene permanently switched on."