Though humans are known to lose lactase ability post-weaning, some people maintain lactase production into adulthood, a condition known as lactase persistence. Global distribution patterns of lactase persistency are inconsistent; the condition is much more prevalent in some populations than others. Using results across independent studies, the existence of such a phenomenon can be attributed to positive selection on ancestral pastoralist populations. Two different hypotheses explain this positive selection: milk-independent economies, and colonic bacteria adaption.

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