The Milk Mutants

Milk and dairy products are some of my favorite foods, so lactose intolerance has always struck me as a particularly awful affliction. No more milk with my chocolate cookies? Unthinkable! Well, the other day I found out that it’s not the lactose intolerance that’s the affliction, but the lactose tolerance!

According to the latest genetic research, all humans were originally lactose intolerant. The enzyme that’s responsible for the digestion of lactose ceases to be produced in the human body once children are weened off of breast feeding. Based on genetic studies of bone fragments, neolithic Europeans were all lactose intolerant. This, however, changed drastically with the advent of agriculture some 7,000 years ago. Humans discovered that the milk of their cows was a great source of nutrition that provided much needed sustenance. Initially, drinking cow milk caused my neolithic ancestors great pains, badly bloated stomachs, and a lot of diarrhea.

Enter evolution and the process of positive selection. The ability to drink milk provided humans with an enormous nutritional advantage. Scientists have figured out that humans who became lactose tolerant were able to raise ten times the amount of offspring than those who weren’t. Guess who lived to see their great-grand children romp around in the dark forests of central Europe? You guessed right. The lactose tolerant bunch. A single mutation had turned the neolithic farmers of Europe into dairy lovers. The mutation responsible for lactose tolerance is located on the gene that produces the lactose “digesting” enzyme. As a consequence of this mutation, this enzyme continues to be produced even when humans are no longer breast fed. Similar mutations on the same gene were found among herding tribes in Eastern Africa, where they developed about 6,800 years ago.

The rapid spread of lactose tolerance among neolithic Europeans and the close association with the development of prehistoric agriculture is one of the most convincing examples of the power of evolution. In only 400 generations, a lactose intolerant population in central and northern Europe became almost 100% lactose tolerant. The chart below illustrates the worldwide distribution of lactose intolerance today. Not surprisingly, areas with predominantly European ancestry have the lowest rates of lactose intolerance.