Why can some adults drink milk and others cannot?

Some adults can drink milk because a gene stayed switched on in them, and the reason is probably thousands of years of dairying. In most mammals the enzyme lactase, which digests the milk sugar lactose, is dramatically reduced after weaning. That is the normal pattern, and it is the one most people worldwide still follow: most people are lactase non-persistent and have some degree of lactose intolerance as adults. In some human populations, though, lactase persistence has recently evolved as an adaptation to consuming nonhuman milk and dairy beyond infancy.

The map of who can drink milk roughly follows the history of dairying. In Europe the share of lactase-persistent people rises from 15 to 54 percent in the south-east to 89 to 96 percent in the north-west. Roman authors recorded that people in northern Europe, particularly Britain and Germany, drank unprocessed milk. In Native Americans, East Asians and most Chinese populations, the share is 5 percent or less.

Before farming, hunter-gatherers were overwhelmingly lactose intolerant. The oldest persistence alleles only reached appreciable levels in the last 10,000 years, which matches the start of animal domestication. In the Neolithic, two aurochs domestications gave rise to cattle: one produced the taurine cattle of the Fertile Crescent, and the other, from the Indian aurochs, produced zebu cattle.

Scientists call it a textbook example of natural selection, possibly with the strongest selection pressure of any known human gene, yet exactly why it helped remains open to speculation. And people have workarounds: steppe peoples mainly drink fermented milk, kumis, which has almost no lactose.