Domestication of livestock in different regions of the world occurred as a gradual process starting 9,000 to 10,000 years ago. As trade routes were established, dairy animals became widely available in areas of the planet capable of sustaining herds, thus dairy products became important components of dietary and cultural diversity within global populations.
Lactase, the enzyme that facilitates digestion of lactose in breastmilk and other milks, is produced during infancy in mammals. While lactose is a sugar that provides energy for infants, most adults have lost the ability to digest lactose, leaving them lactose intolerant. However, in some populations, adults have maintained expression of lactase. This lactase persistence is associated with single nucleotide polymorphisms (SNPs)—in other words, replacements of a single nucleotide in the DNA sequence—near the gene that encodes for lactase. Specific global populations have different variants of SNPs, suggesting that adaptive responses to increased dietary dairy occurred independently following the adoption of agriculture in different regions and populations. Generally, lactase persistence occurred more frequently in northern latitudes, higher altitudes, or regions subject to periodic droughts—regions where non-dairy food sources may be impacted yearly or seasonally by the environment.
What are the consequences of extending milk consumption beyond infancy?
In the infant, milk provides a superfood that has high caloric content, transfers proteins and other nutrients from mothers to offspring, and assists with establishing the offspring’s microbiome. Beyond calories, dairy intake may have accelerated development, increased growth, and enabled earlier reproduction and wider birth canals in early humans. The availability of dairy therefore afforded better chances at individual and species survival.
Populations that maintained the ability to digest lactose had improved infant and adolescent growth, and better later-life health outcomes. Further, populations in areas with shorter growing seasons (and therefore limited ability to rely on plant-based foods) but with availability of dairy products were able to maintain body size. Contemporary populations also demonstrate a general correlation between increased average height and lactase absorption and dairy consumption in adults, suggestive of association between the two.
Evolution of genes that promote lactase persistence in different regions of the world but with similar timing suggests that milk played an important role culturally and nutritionally in regions where agricultural crops were difficult to establish or susceptible to environmental fluctuations. The persistence of lactase enables digestion of milk lactose throughout the lifespan allowing milk to provide an important dietary component, especially in areas with food insecurity.
This blog post is based on the article “Dairying and the evolution and consequences of lactase persistence in humans,” written by Jay Stock.