An interesting article in Nature magazine on how Europeans around 7500 years ago developed a mutation that allowed them to drink milk and consume other dairy produce including early cheese, evidence of cheese making has been found in fragments of pottery with tiny holes in it. The ability to drink milk up until then was lost in adulthood and lactose intolerance remains true for many people around the world.
Early farmers learnt how to reduce the lactose in dairy produce so it was not hazardous to adult health but the main breakthrough came when a genetic mutation called LP allele allowed adults to drink milk without ill-effect. The mutation is thought to have happened around 7500 years ago in what is now Hungary.
Those with the mutation had an advantage over those who remained intolerant, milk was a vital source of nutrition when harvests were poor. Those with the mutation were also more fertile. Most people who can drink milk now can trace their ancestry to Europe though in a few other areas similar mutations occurred such as in West Africa.