Famines and exposure to zoonotic pathogens are the factors that “best explain” the evolution of lactose tolerance
The towns prehistoric of Europe drank milk thousands of years before developing the genetic adaptation that allows adults to digest lactosea characteristic that evolved not so that they could take more dairy, but that it would be related to famines and infectious diseases.
A study published by Nature led by the University of Bristol and University College London (United Kingdom), with Spanish participation, has outlined a map of milk consumption patterns over the last 9,000 years.
For this they have analyzed animal fat residues in ceramic fragments from 554 archaeological sitesincluding the Portalón de Cueva Mayor in Atapuerca (Burgos), and have analyzed the DNA of ancient and modern individuals to understand how tolerance arose and evolved.
Humans already drank milk in the Neolithic, although its consumption varied by region and timealthough almost all adults were intolerant to its sugar, lactose.
Mammals in their infancy can digest lactose using an enzyme called lactase, but when they become adults most of them stop producing it, however, a mutation in the DNA allows the persistence of lactase.
Analysis of DNA data from prehistoric Eurasian individuals over time indicates that that genetic trait was not common until around 1,000 BC That’s almost 4,000 years after it was first detected, around 4,700-4,600 BC.
Lactase persistence was a genetic trait absent in the Neolithic and Chaolithicwhich spread during the Bronze Age, becoming increasingly common to the present day.
“It is surprising that, despite having domesticated goats, sheep, cows or camels for so long and consuming milk and its derivatives, genetic adaptation was not fixed until several millennia later and very quickly”, José Miguel Carretero, from the Laboratory of Human Evolution of the University of Burgos and signer of the study, tells Efe.
The genetic changes that favored lactase persistence are one of the most influential and rapidly evolving genetic adaptations in human populations over the past 10,000 years, the researchers write.
Until nowit was assumed that lactose tolerance arose because it allowed more milk and dairy to be consumedbut this new research paints a different story.
Famines and exposure to zoonotic pathogens are the factors that “best explain” the evolution of lactose tolerancesince modeling of genetic and archaeological data did not show a strong relationship between drinking milk and increased lactase persistence.
They also analyzed data from current Europeans taken from the UK Biobank to see the relationship between milk consumption and health, says Carretero.
The result was that its consumption “provides no advantages” in people who are tolerant to lactose compared to the otherexplains the anthropologist and member of the Atapuerca project, that is why there had to be “other reasons that made lactase-persistent individuals more common”.
Milk consumption in Europe was widespread for at least 9,000 years and healthy people, even the intolerant, could take it without too much troublealthough in these it can cause cramps, flatulence or diarrhea.
Nevertheless, in famine situations, when a crop failed or available livestock decreasedthe consumption of raw or slightly fermented milk was more compulsory, indicates Carretero.
At that time, the non-tolerant were at a disadvantage, because if you are malnourished, weakened, and -he emphasizes- “you also have diarrhea from drinking a lot of raw milk, then you have problems that endanger your life.”
Something similar would happen in times of pandemic, which “require high population densities for the pathogen to proliferate”. From the Neolithic, large population centers began to form, where the space was also shared with domestic animals.
Thus, in times of famine, infectious epidemics or both, the high consumption of raw milk, almost by necessity, would have made the lactose intolerant more likely to die before or during their reproductive years, which would increase the frequency population of the lactase persistence gene to current levels.
The team led by Mark Thomas of University College London introduced indicators of past famines and exposure to pathogens into a new statistical method, and the results supported that theory.
To delve into the co-evolution of dairy farming and lactase persistence, the team led by Richard Evershed of the University of Bristol created a comprehensive map of prehistoric milk consumption, analyzing animal fat residues from 13,181 pottery shards.
The researcher from the University of Burgos, Marta Francés, who participated in the analyses, explains that they identified the lipids that are generally preserved inside ceramics, which are sought through chemical processes.
The “easiest” to identify are of terrestrial animal origin and, in general, it is possible to know which groups of animals they belong to, for example ruminants or non-ruminants, although -he specified- they cannot be separated by species.
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