Milk: it does some bodies good

When I was little, I watched the movie Annie over and over. Since we had recorded the movie from television onto videocassette, this meant that I saw the same commercials over and over, as well. The experience left me with a deep belief that redheaded orphans get lucky and that milk does a body good. Unfortunately, milk may not be the ultimate cure-all. The underlying biology illustrates the dynamic interaction between genes and the environment. Dr. Bill Durham at Stanford University calls the phenomenon co-evolution, to denote the interplay between biological and cultural evolution.

Humans and other species have the ability to control our environment and, theoretically, increase our reproductive fitness. This adaptation gains another layer of complexity over time: previous generations have molded the environment in ways that benefit us today. Interplay between the inherited niche and inherited genes results in adaptation and - la fromage! As you'll see below, cheese and our ability to digest it illustrate the way that genes and culture intertwine.

To gain any nutritional benefit from milk, our bodies have to break lactose down into simple sugars that can easily travel through the small intestine to enter the bloodstream. This process requires the enzyme lactase. But all mammals, including most humans, can't digest lactose very well after weaning. There is only a small amount of lactase in the adult intestine, so adults can end up with indigestion and diarrhea as bacteria in the gut attack the undigested lactose. Interestingly, people can usually digest milk products like cheese and yogurt, which contain little lactose. The manufacturing process involves microorganisms that break down most of the lactose into less formidable simple sugars. So today and in previous generations, our genetics led to cultural practices like cheese-making.

The gene that regulates lactase activity usually switches off shortly after weaning. The gene is dominant, which makes it easier to study, and its frequency among the world's populations is not random. Amazingly, there are clusters of people with perfectly functional lactase activity who live in northern Europe, as well as certain regions of the Middle East and Africa. Anthropologist Bill Durham wondered what was different about the environmental niche that these people inherited.

About 6000 years ago, after human populations domesticated cattle, humans began to rely more heavily on milk products for food. Northern European populations in particular had well-established herds. People who had a variant of the lactase gene that left the gene on for longer periods of time were able to digest milk. These people were able to use milk from the same cows, season after season. So in northern Europe, people with the lactase-producing gene had an advantage over their competitors. This explains the increased frequency of the lactase-producing gene in northern European populations.

Similarly, in the Middle East and Africa, high frequencies of the gene coding for the ability to digest lactase in adulthood were associated with dairying practices. Groups with a history of dairying were typically pastoral nomads, who roamed to find hospitable environments. Pastoral nomads may have been more successful at finding habitable conditions that supported domesticated cows. With cows around, someone who could derive full nutritional benefit from a serving of milk would be more likely to thrive and reproduce.

So cattle domestication is one example of a change in human lifestyle that influenced the frequency of a human gene. This interrelationship reminds us of the environment's impact on human history, and of the lasting effect that our changes to the environment will have on the lives of future generations. 

Jablonska, E. and M. Lamb. Evolution in Four dimensions: Genetic, Epigenetic, Behavioral, and Symbolic Variation in the History of Life. Cambridge, Mass: MIT Press, 2005.

Lactose intolerance linked to ancestral environment. ScienceDaily. June 2, 2005.