[Governor Dinwiddie]

People who are genetically from the British Isles have the least lactose intolerance of any group. Scotland, England, Ireland, Wales — folks from these backgrounds have genetics which tolerate dairy very well. In general European stock as a whole tolerate lactose in their diets.

[SauronOfMordor]

Domesticating cows, goats, and sheep allowed early humans to have something that would turn grass into nutritious milk. Initially, infants would have been fed milk, reducing infant mortality. Those who could tolerate cow’s milk into adulthood would survive better.