The ability to digest lactose, the sugar found in milk, after infancy is known as lactase persistence. While most mammals and a significant portion of the human population typically lose this ability after weaning, some human groups retain it into adulthood. Researchers have made considerable strides in uncovering the specific genetic changes responsible for this unique human adaptation.
Understanding Lactase Persistence
Lactase is an enzyme produced in the small intestine that breaks down lactose into simpler sugars: glucose and galactose. These sugars are then absorbed by the body.
In most mammals and human populations, lactase production decreases significantly after infancy, typically around weaning. This reduction leads to lactose non-persistence, often resulting in symptoms like abdominal pain, bloating, or diarrhea when milk products are consumed.
Lactase persistence, by contrast, refers to the continued production of the enzyme into adulthood, allowing individuals to comfortably digest lactose. The prevalence of lactase persistence varies greatly across human populations, being very high in some Northern European groups and much lower in others.
The Specific Genetic Mutations Discovered
The genetic basis for lactase persistence lies not within the lactase gene (LCT) itself, but in a regulatory region upstream of it. This region is within an intron of the adjacent gene, minichromosome maintenance complex component 6 (MCM6). Specific single nucleotide changes (SNPs) in this MCM6 region act as “enhancers” to ensure the LCT gene remains active throughout life.
One well-known mutation in European and Middle Eastern populations is C>T-13910. This change, where a cytosine (C) is replaced by a thymine (T) at position 13910 upstream of the LCT gene, is strongly associated with lactase persistence. Individuals with at least one T allele generally maintain high lactase levels, while those with two C alleles (CC genotype) often have very low or undetectable lactase production.
Other distinct mutations have been identified in various African and Middle Eastern populations, demonstrating independent origins. Examples include G>A-14010, found among some Tanzanian and Kenyan groups, and T>G-13915, a founder mutation in urban Saudi populations. Another variant, C>G-13907, has also been noted. These different SNPs, while distinct, all sustain lactase production beyond infancy.
The Evolutionary Story of Lactase Persistence
The emergence of lactase persistence is closely linked to the historical development of dairy farming and pastoralism. Genetic studies indicate that the oldest mutations appeared in human populations within the last 10,000 years, coinciding with the Neolithic Revolution when humans began domesticating animals and consuming their milk.
The ability to digest fresh milk offered a survival advantage to early dairy-farming communities. Milk provided a consistent, calorie-rich food source, offering fats, proteins, and essential nutrients like calcium, especially where other food sources were scarce or unreliable. This led to the increased frequency of these genetic variants in dairying populations.
Different lactase persistence mutations arose independently in various parts of the world, such as Europe, the Middle East, and Africa. This pattern is a compelling example of convergent evolution, where distinct genetic changes lead to the same beneficial trait in response to similar environmental pressures—the cultural practice of consuming milk. The spread of these mutations often mirrors the historical migrations of pastoralist groups.
Beyond Digestion: Broader Implications
The genetic discoveries surrounding lactase persistence offer unique insights into human biology and history. Understanding these mutations provides a clear example of gene-culture co-evolution, where dairying directly influenced human genetic adaptation. This trait highlights natural selection’s role in shaping human genetic diversity over relatively short evolutionary timescales.
The study of lactase persistence also contributes to fields beyond basic biology. It offers clues about ancient human migrations, as the distribution of specific lactase persistence alleles can trace past population movements. This research also has implications for personalized nutrition, as knowing an individual’s lactase persistence genotype can inform dietary recommendations and approaches to lactose consumption.