Tiny babies drink their mother's milk. However, many humans also drink the milk of other mammals, especially cows. For much of human existence only children could drink milk, as adults did not produce the substance (enzyme) needed to digest the sugar called lactose in milk. But when milk was made into curd, butter and cheese, most of the lactose was removed and so adults could digest all of these products. This is why cheese and butter were used as food long before liquid milk.

However, many thousands of years ago, people in Europe evolved in a way that meant that lactose carried on being made by adults. This allowed milk to be drunk by adults. It was important because milk is an additional source of food, and contains many nutrients.

In some parts of the world people have not evolved this enzyme, which explains why some people can drink milk and others can't.