Answer:
Adult humans capable of digesting lactose were selected, thereby this phenotypic trait became common in dairy consuming societies
Explanation:
From the first moment after birth, babies can digest lactose in milk because they produce lactase, which is an enzyme capable of hydrolyzing lactose into glucose and galactose. In human societies that non consume milk products, the level of the lactase enzyme abruptly decreases during the first years of life due to the inactivation of the gene encoding this enzyme (i.e., lactase gene). However, in dairy consuming societies, a genetic mutation in the lactase gene that appeared among early cattle-raising people in Europe was selected, nowadays, this mutation is mostly present (i.e., in a higher frequency) in these societies, thereby adult people is generally adapted to consume lactose-based products.