Wоrldwide, оnly аbоut 35% of аdults cаn digest lactose, the primary sugar in milk. Breakdown of lactose requires the enzyme lactase; while it is normal for all mammals to express lactase as an infant, in most species, including humans, the gene is turned off at weaning. Thus, in most people, consumption of dairy products in adulthood is less than pleasant, a condition termed lactose intolerance. Lactose intolerance is caused by the non-persistence of lactase expression. The mutation that led to lactase-persistence (continued expression of lactase) is thought to have risen in frequency in pastoralist populations – those that started domesticating and farming animals. Lactase persistence (LP) arose independently in both African and European populations. The African and European populations have different LP mutations, but all LP mutations are in the regulatory region of the lactase gene. . Thus, lactase persistence demonstrates: