Sara Wood and John Pickrell in Cosmos Magazine:
Though all people can digest milk in infanthood, most of the world’s population lose that ability at between two and five years of age.
At this time their bodies stop producing an enzyme called lactase, which is essential for digestion of lactose sugars found in dairy products. Most Asians, sub-Saharan Africans, native Americans and Pacific Islanders remain lactose intolerant today.
However, a mutation in many European and some African populations allows them to produce lactase into adulthood, and in these cultures dairy products – such as milk cheese and yoghurt – traditionally form a key component of the diet.