The genetic adaptation of humans to the consumption of animal milk is a textbook example of gene-culture coevolution. Taking advantage of the accumulated ancient DNA data, this Unsolved Mystery article explores where and when lactase persistence emerged. The mutation rapidly spread throughout Eurasia between 4000 and 3000 BP, but if Asian herders managed to culturally adapt to milk consumption, then why have Europeans genetically adapted at all?

The genetic adaptation of humans to the consumption of milk from dairying animals is one of the most emblematic cases of recent human evolution. While the phenotypic change under selection, lactase persistence (LP), is known, the evolutionary advantage conferred to persistent individuals remains obscure. One informative but underappreciated observation is that not all populations whose ancestors had access to milk genetically adapted to become lactase persistent. Indeed, Central Asian herders are mostly lactase nonpersistent, despite their significant dietary reliance on dairy products