Many people do not realize that human beings have not always drank milk past weaning nor have they always been able to tolerate lactose into adulthood. Studies of European and African cultures in connection to when they started keeping herds of cattle have shown separate mutations of genes allowing for the production of lactase to continue and the consumption of milk to continue past childhood.

These observation are fascinating to researchers since they paint a picture of how one species and separate cultures can evolve separately from each other, and it reveals the a striking genetic footprint of the process of natural selection in humans.

Human adults were never supposed to digest milk; milk was confined to the very young, until a genetic mutation changed all of that. Not everyone has the mutation though, those whose ancestors first domesticated cattle do in the greatest abundance; the northern Europeans. Those who did not raise cattle till much later in human history do not have the genetic mutation or only a small percentage of them do, whether this is because of interbreeding or because the mutation is starting to occur in those cultures no one is really sure yet, but one thing is for sure, in those cultures without the mutation, most adults are unable to digest milk.

Certain African cultures now have the gene mutation but it is determined to be a different mutation than that of northern Europeans leading researchers to think that this mutation occurred at a different time in history and completely independent of the European mutation.  Researchers believe the ability to digest milk is an adaptive trait not only because milk contains considerable nutrition but also because in some dry arid regions milk became a primary source of water.

European cattle domestication began about 10,000 years ago, and it is thought that the mutation to digest milk occurred about a thousand years after that, for the African culture cattle herding showed up about 7,000 years ago and again the mutation in this population occurred about 1,000 years later.