Scientists have reconstructed how the ability to digest lactose evolved. They rely on residues in clay pots and gene banks.
Today’s dairy farming in Switzerland would not exist without the events of thousands of years ago.
On average, every inhabitant of Switzerland consumes 370 kilograms of milk and milk products a year, more than a liter a day. Of course, that wasn’t always the case. It is the preliminary result of a development that began 9000 years ago. At that time, people started milking cows. The amount of milk consumed was lower then, the cows were smaller. And: The people could not tolerate the milk. They were lactose intolerant.
It was only several thousand years later that they showed the genetic ability to digest lactose. So far, it has been assumed that this ability prevailed primarily in regions where a lot of milk was consumed, and that consumption and tolerability were mutually reinforcing. So the more milk people drank, the more they developed the ability to digest it, and the better they could digest milk, the more they drank. In a new, broad study in the journal “nature” However, scientists come to the conclusion that this simple connection cannot be proven. Instead, they suggest two other possible drivers of development: crises and diseases.
Milk residues in clay pots provide clues
Richard Evershed, a biogeochemist at the University of Bristol in England and a specialist in archaeological milk research, is in charge of the study. Evershed helped develop a method years ago that makes it possible to prove whether milk was stored in a clay pot that was thousands of years old. It uses a specific carbon isotope.
The oldest examples of such milk residues are 9000 years old and come from Northwest Anatolia. People are now sedentary farmers. They drink milk even if they can’t digest it well and get stomach cramps, diarrhea and bloating. Only through a mutation in the genome does the allele – i.e. the gene variant – arise that enables the formation of the enzyme lactase. Only lactase can break down milk sugar.
This allele first occurs around 4700 BC according to the available DNA samples. on. However, until it can be ascertained with some regularity, another almost three millennia pass, until about 2000 BC. Chr.
Milk consumption and lactose intolerance are not related
In addition to milk fat residues in clay pots from Europe and the Middle East, Evershed and his colleagues also included entries from a modern gene database. It was found that the presence of the lactase allele seems to have little effect on whether the person consumes milk or not. In China, too, milk consumption has risen sharply in recent years, although the vast majority of people there do not have the lactase allele.
The scientists therefore propose two interrelated mechanisms. On the one hand, famine could have led to the lactase gene being passed on to an increased extent: for those who are already malnourished, diarrhea caused by milk consumption can be fatal. This would be exacerbated by the fact that in times of food shortages, more unfermented foods, which were particularly high in lactose, would be consumed.
Another factor may have reinforced selection for the ability to digest lactose: the greater abundance of pathogens. Sedentary agriculture is primarily associated with zoonoses, i.e. diseases transmitted from animals to humans. In addition, people live closer together.
Evershed and colleagues write that these conditions would have turned the rather mild consequences of lactose intolerance such as diarrhea into very serious ones. This led to a higher mortality rate for these people – and thus to a genetic selection in the direction of the lactase allele.
Lactose intolerant people did not survive
The paleogeneticist Shevan Wilkin from the University of Zurich has written an accompanying article on the study in “Nature”. She thinks the hunger hypothesis is particularly convincing, she says when asked. However, there are probably regional differences: milk was probably important for survival in areas such as the Baltic States or Great Britain, where agriculture was more difficult. There, lactose-intolerant people might have died before they could reproduce.
It is quite possible that in today’s China, over time, the ability to digest lactose will also spread. But there are already countries like Mongolia or Kazakhstan where the allele is very rare but milk consumption is still high. It is therefore assumed that there are other ways than the genetic one to be able to digest lactose: through bacteria in the milk that help with fermentation – or through an adapted intestinal biome.