Drinking Milk in Adulthood Is an Evolutionary Anomaly: Two-Thirds of Humanity Genetically Loses the Lactase Enzyme
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20.08.2026 17:38
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2 min read
The ability of adults to drink fresh milk without problems is the result of a relatively recent mutation, not a basic biological norm. According to the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), about 68% of the world’s population experience a decline in the activity of the gene responsible for producing the lactase enzyme after childhood.
For all mammals and human ancestors, the natural process is the gradual decline in the synthesis of lactase — the enzyme needed to break down milk sugar (lactose) — after the transition to solid food.
Gene-Culture Evolution and Herder Mutations
The situation changed about 10,000 years ago with the emergence and spread of animal husbandry. In populations that practiced dairy farming, inherited changes in the regulatory regions of the MCM6 gene arose and became established, allowing the body to produce lactase throughout life.
According to the genetic reference MedlinePlus, this mechanism is one of the most striking examples of convergent evolution: European populations and herding tribes in Africa developed completely different genetic markers that independently led to the persistence of lactose tolerance.
Protection Against Dehydration During Epidemics
According to a large-scale study in the journal Nature, the rapid spread of this mutation was driven by survival in extreme conditions. During crop failures and pathogen outbreaks, people with lactose intolerance who consumed raw milk suffered from diarrhea, which, in conditions of clean water scarcity, led to fatal dehydration. For those with lactase persistence, milk remained a sterile and safe source of calories.
At the same time, scientists emphasize that reduced enzymatic activity in two-thirds of the world’s adult population is not a pathology or disease. Most people without the mutation tolerate fermented dairy products like hard cheeses, yogurts, or small amounts of milk without significant digestive symptoms.
