Our earliest ancestors were lactose intolerant, because the enzyme that breaks down lactose—called lactase—was made only during infancy. Adults, who were no longer exposed to breast milk, did not need the enzyme.

When humans began to get milk from domesticated cattle some 10,000 years ago, variant genes—the product of random mutation—enabled those who carried the variation to continue to express lactase as adults, and thus take advantage of nutrition provided by cow’s milk.

We now know that people who retain the ability to digest milk as adults contain a point mutation in the regulatory DNA sequence of the lactase gene, allowing it to be efficiently transcribed throughout life. In a sense, these milk-drinking adults are “mutants” with respect to their ancestors

It is remarkable how quickly this adaptation spread through the human population, especially in societies that depended heavily on milk for nutrition. These evolutionary changes in the regulatory DNA sequence of the lactase gene occurred relatively recently (10,000 years ago), well after humans became a distinct species.