A glass of cow’s milk on an adult table looks biologically out of place. In mammals, the enzyme that breaks down milk sugar, lactase, usually fades after weaning; digestive biochemistry expects a shift to other foods. Yet in several human populations, mutations near the LCT gene keep lactase switched on, a rare extension of infant metabolism into adulthood.
That persistence did not spread by chance. In communities that herded cattle, goats or camels, milk became a dense package of calories, protein and calcium when crops failed. The evolutionary payoff was a higher survival rate, a textbook case of strong selection pressure reshaping allele frequencies. Where genes did not cooperate, culture improvised: fermenting milk into yogurt or cheese lets microbial lactase and lactic acid bacteria pre-digest lactose, turning a problematic sugar into a manageable nutrient.
The result is a layered adaptation that fuses DNA, microbiome engineering and social practice. Lactase persistence marks an unusually fast shift in the human genome, while dairy rituals, taboos and entire economies reveal how cultural evolution can override baseline mammalian rules and keep an infant food in permanent circulation on the adult menu.
