The ability to digest milk in adulthood spread among the inhabitants of Europe in just three thousand years. This conclusion was reached by researchers who studied the remains of soldiers who fell in a battle in the valley of the Tollensee river about 3200 years ago. The vast majority of them have not yet had a mutation that allows them to break down lactose. Additional analysis has confirmed that over the past millennia the corresponding allele has been under the influence of strict natural selection, which has significantly increased its frequency. The research results are published in the journal Current Biology.
Unlike most mammalian species, many people can digest milk even when they grow up. This is helped by the mutant allele rs4988235-A, responsible for the production of the enzyme lactase — thanks to it, the milk sugar lactose is broken down. It is believed that this mutation became fixed in the population after the domestication of livestock, but the history of its distribution has not yet been studied enough.
A team of researchers led by Joachim Burger from the Johannes Gutenberg University of Mainz decided to investigate when the ability to digest lactose appeared in the inhabitants of Europe. To do this, they analyzed the remains of people who died in the battle in the valley of the Tollensee river. This battle, which is considered the first large-scale conflict North of the Alps, took place in what is now Germany in the bronze age, around 1200 BC. After studying 14 samples from the battlefield, the authors concluded that the dead soldiers, including both men and women, came from Northern and Central Europe. From a genetic point of view, they represented a homogeneous population, which has a high continuity with the modern inhabitants of the region.
The prevalence of the rs4988235-A allele among warriors in the Tollensee river valley was 7.1 per cent. For comparison, among 18 people whose remains were found in a burial site near the Serbian village of Mokrin, this mutation was noted in 4.6 per cent. This means that three thousand years ago, most adult Europeans could not digest lactose. For comparison, today 90 per cent of the inhabitants of Northern Europe have this ability.
Genetic analysis of more recent remains shows a higher prevalence of rs4988235-A. For example, according to samples from early medieval burial sites, 57 per cent of southern German and 73 per cent of Hungarian residents had the ability to digest lactose during this era.
After comparing the genomes of soldiers who fell in the Tollensee valley with samples obtained from modern inhabitants of Northern Europe, the researchers concluded that the rs4988235-A allele had been under the pressure of strict selection for three thousand years. This allowed it to spread throughout the population in just 120 generations, that is, from an evolutionary point of view, very quickly. The only other allele that shows equally clear signs of selection over a similar period is the innate immunity-related rs5743810.
For a long time, it was believed that Europeans acquired the ability to digest milk in adulthood from nomads from the steppes of Eastern Europe and Central Asia about five thousand years ago. However, analysis of 37 samples from burial mounds located in this region, Dating from the copper and early bronze ages, did not reveal the presence of the rs4988235-A allele. These data do not agree with the hypothesis of a steppe origin of the mutation among Europeans.
Photo: Sesame Street / Sesame Workshop, 1969