The majority of the world's people become lactose-intolerant as they mature into adulthood. The exceptions are mostly people of European ancestry, who continue to drink milk all their lives. Why? It's a case of genetic mutation, in which an adult continues to produce lactase, the enzyme that digests the milk sugar lactose. But why did the mutation become so prevalent in the population so quickly (about 20,000 years)? Evolutionary geneticist Mark Thomas says there had to be something about drinking milk in Europe that led to increased chances of survival or higher fertility. Here are a couple of possibilities:
First, the farmers that settled there came from the Fertile Crescent, and they brought crops native to that region, like wheat and barley. But with Northern Europe's shorter growing season, these crops were more likely to fail, causing famine.
Additionally, the colder Northern European climate lent itself to natural refrigeration. "If you're a farmer in Southern Europe, and you milk a cow in the morning and you leave the milk out, it will be yogurt by noon. But if you do the same thing in Germany, it'll still be milk," says Thomas. A healthy lactose-intolerant person who drank that still-fresh milk would get a bad case of diarrhea. "But if you're malnourished, then you'll die," Thomas says.
In times of famine, milk drinking probably increased. And the very people who shouldn't have been consuming high-lactose dairy products — the hungry and malnourished — would be the ones more likely to drink fresh milk. So, with milk's deadly effects for the lactose intolerant, individuals with the lactase mutation would have been more likely to survive and pass on that gene.