In an evolutionary eye-blink, 80 percent of Europeans became milk-drinkers; in some populations, the proportion is close to 100 percent. (Though globally, lactose intolerance is the norm; around two-thirds of humans cannot drink milk in adulthood.) The speed of this transformation is one of the weirder mysteries in the story of human evolution, more so because it’s not clear why anybody needed the mutation to begin with.

…A “high selection differential” is something of a Darwinian euphemism. It means that those who couldn’t drink milk were apt to die before they could reproduce. At best they were having fewer, sicklier children. That kind of life-or-death selection differential seems necessary to explain the speed with which the mutation swept across Eurasia and spread even faster in Africa. The unfit must have been taking their lactose-intolerant genomes to the grave.

…The rise of civilization coincided with a strange twist in our evolutionary history. We became, in the coinage of one paleoanthropologist, “mampires” who feed on the fluids of other animals. Western civilization, which is twinned with agriculture, seems to have required milk to begin functioning. No one can say why.