Selection and Lactase Persistence

Early humans probably did not have the lactase persistence mutation. Like other mammals, they did not drink milk as adults. So it was wasteful for them to continue producing lactase after weaning.

Archaeological evidence shows that in Europe, people started raising livestock about 8,000 to 9,000 years ago.

A heat map of Europe and western Africa with a dairy cow in northern Europe indicates high lactase persistence.
Lactase persistence in northern Europe coincides with archaeological evidence of livestock.