Milk-Drinking Mutation Changed Human History

To repurpose a handy metaphor, let's call two of the first Homo sapiens Adam and Eve. By the time they welcomed their firstborn, that rascal Cain, into the world, 2 million centuries of evolution had established how his infancy would play out. For the first few years of his life, he would take his nourishment from Eve's breast. Once he reached about 4 or 5 years old, his body would begin to slow its production of lactase, the enzyme that allows mammals to digest the lactose in milk.