Lactase persistence describes the ability of adults to digest lactose, the sugar found in milk. While nearly all human infants produce the lactase enzyme to digest milk, this ability declines after weaning in most of the world’s adult population, leading to lactose intolerance. However, in certain populations, the capacity to digest milk persists into adulthood, a recent evolutionary adaptation. The trait’s uneven global distribution reflects a fascinating aspect of human dietary evolution.
The Genetics of Digesting Milk
Digesting milk relies on the lactase enzyme, which breaks down lactose into simpler sugars, glucose and galactose, for absorption. The LCT gene directs this enzyme’s production. In most mammals and a majority of humans, the LCT gene’s activity decreases after infancy, effectively “switching off” lactose digestion in adulthood.
Lactase persistence occurs when specific genetic changes prevent this “switch-off.” These changes are not within the LCT gene but in a nearby regulatory gene, MCM6. Variants in a specific DNA sequence within MCM6, a regulatory element, influence LCT gene expression. These single nucleotide changes, such as the common -13910T variant found in Europeans, lead to sustained lactase production in the small intestine, allowing adults to digest lactose.
Global Distribution of Lactase Persistence
Lactase persistence reveals distinct geographical patterns. Northern European populations show the highest frequencies, ranging from 89% to 96%, with nearly all individuals in areas like Ireland, the UK, Scandinavia, the Netherlands, and Germany retaining the ability to digest lactose. Southern Europe contrasts sharply, with much lower frequencies, for instance, around 17% in Greeks and 14% in Sardinians.
Other hotspots include pastoralist groups in East Africa and the Middle East. East African populations like the Tutsi show high rates, up to 90%, and pastoralist Arab populations in the Middle East exhibit frequencies around 50%. In contrast, lactase persistence is low in East Asia, often below 10%, and in most non-pastoralist sub-Saharan African populations, as low as 1% to 20%.
The Evolutionary Advantage of Dairy Consumption
The global patterns of lactase persistence are linked to the historical practice of dairy farming, illustrating gene-culture co-evolution. The domestication of milk-producing animals and the subsequent consumption of their milk created selective pressure favoring individuals who could digest lactose. This ability offered a nutritional advantage, especially where other food sources were scarce or unreliable.
Milk provided a rich and readily available source of calories, protein, and fat, crucial for survival and reproduction. In northern latitudes, where sunlight exposure is limited, milk also provided calcium and vitamin D, helping prevent bone diseases like rickets. Consuming raw milk without discomfort allowed access to these nutrients, contributing to better health and potentially higher reproductive success compared to those with adverse symptoms from undigested lactose.
Convergent Evolution in Different Populations
Lactase persistence did not arise from a single global genetic event. Instead, the ability to digest milk evolved independently multiple times in different human populations. This is convergent evolution, where distinct genetic mutations lead to the same beneficial trait in response to similar environmental or cultural pressures.
For instance, the primary mutation for lactase persistence in most European populations is the -13910T variant. However, in East African and Middle Eastern pastoralist groups, different mutations, such as -14010C, -13915G, and -13907G, are responsible for the trait. These distinct variants arose on different genetic backgrounds, yet all confer the same advantage of continued lactase production. This highlights the selective force exerted by dairying, driving parallel evolutionary paths across geographically separated human communities.