Background Information
Lactose Intolerance: Lactose intolerance is a common condition in which individuals cannot digest lactose, the sugar in milk and dairy products, due to a deficiency in lactase, the enzyme responsible for breaking down lactose. In these individuals, intestine bacteria break down the lactose, producing methane, and consuming milk and dairy products with lactose results in bloating, gas, abdominal pain, and diarrhea.
Lactase persistence refers to the continued activity of the lactase enzyme into adulthood. It is a genetic trait resulting from a mutation that allows for the continued production of lactase beyond infancy, allowing individuals to digest lactose without discomfort. It is primarily caused by a single gene, LCT, where the allele that allows persistence is dominant.
A recent study of ancient proteins found evidence of using milk products in the form of several individual skeletons from East Africa as far back as 6,000 years ago (Johnson 2022, 12), and ancient DNA studies show that lactase persistence only evolved about 5,000 years ago in Europe. However, it would not reach even moderate frequencies for thousands more years. Thus, humans were using milk products well before evolving lactase persistence. Paul Kindstedt, a cheese historian, noted, "It was cheese and butter making that enabled dairying to gain a foothold among Neolithic populations and allowed the genetic selection for adult lactase persistence to take place" (Kindsted 2012, 14).
As noted by Johnson, “The ability to continue to digest lactose into adulthood has evolved in multiple human populations from changes at the same gene but at different locations of the gene” (Johnson 2022). Lactase persistence evolved independently in multiple populations, including East Africa and Europe, and the genome responsible for the trait shows a strong signature of positive selection from these populations. The use of milk products also substantially predates the evolution of lactase persistence in East Africa.
Today, Lactase persistence prevalence varies across populations, reflecting historical dietary patterns and genetic evolution. Understanding the genetic basis of lactose tolerance has practical implications in fields such as medicine and genetics, making it a relevant and engaging topic for High School students.
Cultural and Genetic Factors: As mentioned above, the ability to digest lactose into adulthood is a relatively recent evolutionary adaptation, about 5,000 years ago. It is closely associated with the domestication of dairy animals and the cultural practice of dairy farming. In regions where dairy farming became significant, individuals with the LCT gene mutations allowing continued lactase production had a selective advantage, and the trait spread rapidly due to the nutritional benefits of dairy consumption.
It is important to note that this beneficial mutation provided a reliable source of calories and essential nutrients, particularly in environments where other food sources might have been scarce or seasonal. For instance, in Northern Europe and certain African and Middle Eastern populations, the genetic mutation for lactase persistence spread rapidly due to cow's milk's high availability and nutritional value, allowing individuals to ensure a stable food supply and improved survival and reproductive success (Tishkoff et al. 2007, 1037). Conversely, in populations where dairy farming was not a prevalent cultural practice, such as in East Asia and many Indigenous populations in the Americas, the mutation for lactase persistence was less common. As a result, lactose intolerance remains more widespread in these populations. Understanding the intersection of cultural practices and genetic adaptations helps elucidate how human populations have evolved in response to their environments, and learning about the relationship between genetics and culture highlights the dynamic nature of human evolution and adaptation.
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