Archaeological and genetic evidence calls into question the idea that the ability to digest lactose after infancy evolved gradually.
According to the largest study yet on the evolution of lactose tolerance in humans, milk consumption was widespread thousands of years before people were able to properly break it down. The ability to break down lactose was most likely acquired during acute crises rather than gradually over time, according to the study.
Lactase, an enzyme that breaks down lactose into the more easily absorbed glucose and galactose, is produced by all humans as babies, but many people have much lower levels of lactase after weaning, implying that they cannot digest milk properly.
One of the best examples of natural selection in humans is the spread of lactase persistence, or the ability to break down lactose after weaning. In just a few thousand years, one-third of the world’s population acquired this trait.
The leading theory was that humans developed lactase persistence so quickly because their evolution was deeply intertwined with a dairy cycle. The spread of lactase persistence would have increased human reliance on milk, increasing the pressure to be lactose tolerant as humans were pushed to evolve lactase persistence due to the nutritional benefits of milk.
Richard Evershed of the University of Bristol, UK, and his colleagues investigated 7000 animal fat residues found in pottery at 500 locations across Europe. This revealed that dairy farming was common in the region beginning around 7000 BC.
Lactase persistence, however, was not common until around 1000 BC, roughly 4000 years after the trait was first discovered and 6000 years after milk production began.
There are many theories as to why lactose tolerance developed so quickly, ranging from the benefits of milk’s sugar content to vitamin D, but they “all go out the window because they are all pinned to milk use in some way,” said Mark Thomas, a co-author of the study, at a press conference.
Mapping milk consumption in Europe against the spread of lactase persistence revealed no correlation, while computer models suggest that famine and disease are more likely explanations.
Using population size changes as a proxy for malnutrition, the researchers discovered that a lack of food was 689 times more likely than constant selection pressure to explain the rise of lactose tolerance. Using population density as a proxy for pathogen spread, they discovered that disease was 289 times more likely to be to blame.
Flatulence, stomach cramps, or diarrhea experienced by lactose intolerant people probably became a life or death matter during these times of crisis, according to Thomas.
An examination of health and genetic data from the UK Biobank also indicated that the co-evolution of humans and lactose tolerance is unlikely.
Lactose intolerant people would be expected to have higher levels of certain health metrics, such as bone mineral density, height, or vitamin D, according to Thomas, but the researchers found none, except for slightly higher BMI numbers.
Surprisingly, there was little difference in the proportion of lactose intolerant and intolerant people who do not drink cow’s milk: 6.8 percent versus 8%.
The negative symptoms associated with milk consumption only occur in lactose intolerant people, most likely due to variations in bacteria in the colon. According to the researchers, some people who experience side effects may be confusing lactose intolerance with milk allergy.
Reference: Nature, DOI: 10.1038/s41586-022-05010-7