Pre-History Lactose Tolerance

These figures painted in the Manda Guéli Cave in central Africa in prehistoric times show humans amidst animals they have domesticated. They illustrate the importance of pastoralism in human history, which isn’t just something that changed some people’s food supply (instead of foraged plants and animals, pastoralists focus on the nutrients from their domesticated beasts). It changed the genetic history of humans today.

If you can drink milk, you probably have herders in your family tree. That’s because before pastoralism began, lactose intolerance was the norm. This makes great sense: it would help the human population to have mothers able to wean their children rather than having them continuously rely on breast milk — mothers who have space to wean in between birthing can produce greater numbers of healthier children. And so, after about age five, the ability to digest lactose declines, and that’s why grownups can’t drink milk.

Except, of course, many of us can: 88 percent of white people in the U.S., for instance. But being able to consume milk into adulthood is actually the product of a rare genetic mutation on chromosome 2. The default in human history was to generate lactose intolerance.

The reason why many of us eventually selected for this rare mutation is an example of “environmental niche theory” in evolution. This idea stresses the ways that organisms actively change their genetic makeup because of their choices in how they change their environment. And for our herding ancestors, this meant utilizing the milk (as well as all other consumable body parts) of their domesticated animals. When milk was a key food supply for adults, those who carried the gene that suppressed lactose intolerance “won out”. Among modern Chinese Americans, who have far fewer herdsfolk in their ancestry, only 7 percent can drink milk into adulthood.

A recent study from 2019 analyzed the DNA of ancient Africans living around the Rift Valley, a site known for its human genetic diversity, and was able to trace when pastoralists entered the scene (about 5,000 years ago), as when they began mixing and also isolating from hunter-gatherers.