For example, if you’re descended from people who once herded cattle and consumed cow’s or mare’s milk, you will probably be able to digest their milk even as an adult because you can keep producing lactase, the enzyme that digests lactose (milk sugar). Hunter-gatherers consumed only breast milk till about four years of age, and after childhood, they no longer needed to produce lactase. But where cow’s or mare’s milk became a major food source, humans began to produce lactase into adulthood—a genetic mutation had occurred.
