The Evolution of Lactase Persistence

by Abigail Wang

An ingredient in most dishes, milk is a food that humans are born with the ability to digest. However, it is also the source of one of the most common food intolerances. For many people, the ability to digest milk is a short- lived faculty, gradually diminishing with the advent of adulthood. In fact, despite the popularity of milk products in today’s society, those who can digest it normally are in the minority. The majority—those who are sensitive to dairy—possess the trait commonly known as lactose intolerance.

Lactose intolerance is the inability to break down dairy sugar, or lactose. To be lactose intolerant is to lack sufficient amounts of lactase, an enzyme present in the small intestine. Lactase breaks lactose into smaller molecules which are more easily absorbed during digestion. Without lactase, individuals may experience nausea, abdominal pain, bloating and other digestive issues upon ingestion of dairy products.

Less common than one might think, lactase persistence—or the ability to digest lactose—is only abundant in a few populations. Individuals with lactase persistence have the lactase enzyme and are therefore able to consume dairy products without any discomforting problems. Studies of the lactase persistence trait have shown that it most frequently occurs in populations from northern Europe, appearing in about 90 percent of individuals. However, only 50 percent of other Europeans and Middle Easterners and less than 10 percent of Asians possess the lactase persistence trait. While the trait is common in parts of Africa that rely on pastoral farming (where they use milk as a primary nutrient source), other African populations experience relatively low rates of lactase persistence. In all, only 35 percent of the world population can digest dairy.

Though lactose intolerance is widely perceived as an allergy that developed over hundreds of years, the inability to digest lactose into adulthood is a chronologically older condition. That is, lactase persistence appeared later, in evolutionary terms, particularly in populations that relied heavily on agricultural means of food production. Early humans could not digest dairy into adulthood at all, and the appearance of lactase persistence evolved by natural selection over a span of thousands of years.

A dominant trait, lactase persistence arises from the expression of a particular gene that encodes for lactase. The most common root of the gene’s expression is a variation in a single nucleotide—or one link in the long chain responsible for protein production—that found its way into the human race and nurtured the dependence on dairy that exists in many societies today.

As noted above, lactase persistence occurs most commonly in populations that rely heavily on dairy farming. Thus, dairy consumption and lactase persistence grew side by side.

The question remains: was dairy farming adopted because these populations had the ability to digest milk products? Or did the lactase persistent trait become favored by natural selection in response to the rise in dairy farming? A 2006 study conducted by genetic researcher Sarah Tishkoff favors the latter hypothesis.

Analyzing DNA from ancient Neolithic and Mesolithic hunter-gatherer skeletons, Tishkoff and her team found a low frequency of the lactase persistence gene, suggesting that early pastoralist populations had not yet shown a high prevalence of lactase-digesting ability. It is likely that lactase persistence became more common in populations after dairy farming was introduced, and the ability to drink milk and garner its health benefits resulted in a selective advantage for the lactase persistence trait.

Despite the prevalence of lactose intolerance, most people continue to relish dairy products, whether in deliberate disregard or simple ignorance of their hypersensitivity. Lactose intolerance is not an allergy; food allergy involves an immune reaction in response to an ingestion of something normally harmless, whereas a food intolerance, conversely, indicates an inability (or decreased ability) to digest a certain food. Food intolerance can be mild enough to be almost unnoticeable with small amounts of the food, so many people ignore the adverse side effects and continue to consume lactose. Furthermore, popular dairy products like cheese and yogurt have already undergone fermentation in their production processes, and much of the lactose is already broken down, rendering these products agreeable with the digestive systems of lactose intolerant individuals.

While lactose intolerance is still widely prevalent today, the modern availability of much-cherished products like milk, ice cream and butter is nevertheless thanks to the emergence of lactase persistence that has its roots in ancient populations. Appreciating the fact that the ability to drink milk has only been around for a few thousand years can raise questions about the prevalence of other food sensitivities at the intersection of history, culture and biology.