Where does the ability of some adult humans to digest milk sugar come from? The authors of a study that explores this question, published on July 27 in the journal Nature. According to the current hypothesis, this ability would have been favored by the extent of the consumption of animal milk, as agriculture spread in Europe from the Near East where it was invented about ten thousand years ago. . Individuals able to digest milk would have benefited from the additional calories provided by this resource. The new study goes against this hypothesis.
Human beings digest lactose thanks to an enzyme, lactase, which splits this sugar in two in the small intestine, to release glucose and galactose. Almost all infants produce lactase: they digest the sugar in the milk of their mothers or other mammals. But, in the majority of modern humans, the production of lactase drops rapidly throughout childhood, reaching a very low residual level in adulthood. These people are said to be “lactose intolerant”: if they consume fresh milk, they can suffer from cramps, diarrhea and flatulence.
This is the case for two thirds of the current world population. But the proportion of lactose intolerant people varies by region: it is 40% in Europe, ranges from 50% to 80% in South America, from 60% to 80% in sub-Saharan Africa and rises to 95% in Asia. Overall, a third of current adults therefore tolerate lactose well. This is because they have acquired a mutation – a single-letter change in DNA – in the lactase gene carried by chromosome 2. Thanks to this genetic variant, they continue to produce lactase, therefore to digest lactose, even in adulthood.
Traces of lipids on pottery
When did this lactose tolerance appear? How did it spread in Europe? To find out, teams from the University of Bristol and University College, London (United Kingdom), with researchers from twenty countries, brought together “largest dataset ever published” on the analysis of traces of lipids in shards of ancient pottery, notes Mark Thomas, professor of evolutionary genetics at University College, who coordinated the study. That is a total of 13,181 pieces of pottery, from 554 European archaeological sites, including “more than 5,000 never studied before”specifies Mélanie Roffet-Salque, co-author, French researcher from the University of Bristol.
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