Warrior skeletons reveal Bronze Age Europeans couldn’t drink milk
It doesn't really surprise me much that lactase persistence should be so strongly selected for in a relatively short period of time. Cows and other ungulates possess a rather magical ability to turn something that can't be digested by humans (cellulose) into something that can (milk). Opening up the huge caloric and nutrient resource of cellulose to humans in a relatively replenishable way had to be a giant benefit to those who could digest milk.