The ability of humans to digest lactose as adults likely originated via natural selection and adaptation. Populations with a propensity to retain lactase into adulthood (i.e., lactase persistence) could better exploit nutritious milk beyond childhood without experiencing discomfort resulting from lactose intolerance—an inability to fully break down lactose due to low lactase production, often leading to digestive issues after consuming lactose-containing foods.

Over countless generations and spanning across different cultures, this advantageous genetic mutation was selectively favored and increased in regions where livestock herding and animal milk consumption played central roles, especially given the relatively short period during which young mammals naturally produce sufficient lactase while consuming their mother's milk during a nursing period lasting a few months.