http://www.sciencemag.org/news/2018/11/how-can-you-eat-dairy-if-you-lack-gene-digesting-it-fermented-milk-may-be-key-ancient

A woman milks a cow on the flat plains of Mongolia. People living here have ingested dairy products for millennia despite being lactose intolerant, a new study of ancient genes and proteins shows.

More than 3000 years ago, herds of horses, sheep, and cows or yaks dotted the steppes of Mongolia. Their human caretakers ate the livestock and honored them by burying animal bones with their own. Now, a cutting-edge analysis of deposits on ancient teeth shows that early Mongolians milked their animals as well. That may not seem surprising. But DNA analysis of the same ancient individuals shows that as adults they lacked the ability to digest lactose, a key sugar in milk.