In today’s northern and central Europe, most people can still digest the milk sugar lactose as adults. However, this was not the case with individuals who died in the battle of the Tollense River in present-day Mecklenburg-Western Pomerania around 1200 BC. Researchers have now been able to show this by examining the genetic material from the bones of the fallen. Accordingly, the gene variant that is responsible for lactose tolerance has spread across Europe within a few millennia.
Milk is the primary source of food for the offspring of mammals. After infancy, however, almost all mammals – including most humans – lose the ability to digest the sugar lactose contained in milk. But when people began to keep and use cattle in the Neolithic around 7,500 years ago, it became an advantage for them to still be able to drink milk as adults. To do this, they need the enzyme lactase, which breaks down milk sugar. In order for the body to produce this enzyme in adulthood, a certain gene variant is required. An international research team led by the population geneticist Joachim Burger from Johannes Gutenberg University Mainz has now investigated how widespread this variant was in the Bronze Age around 3200 years ago.
Archaeological finds from the oldest battlefield in Europe
To do this, the researchers used archaeological material from the oldest known battlefield in Europe: around 1200 BC, thousands of warriors met on the Tollense River in what is now Mecklenburg-Western Pomerania. Their bones provide valuable information to today’s archaeologists. Burger and his colleagues examined the genome of 14 fallen people, two of them female. To a large extent, the genome of the examined individuals was similar to that of the people who live in northern Germany today. The gene variant that is responsible for lactase production was an exception. Only one of the 14 examined people from the Bronze Age carried the necessary gene. Everyone else was apparently lactose intolerant. “Of today’s population in the same area, 90 percent have this characteristic, known as lactase persistence,” says Burger. “This difference is enormous when you consider that there are not much more than 120 generations of people in between.”
How did the gene variant spread so widely in such a short time? Theoretically, it is conceivable that migrations of peoples have led to more people with lactase persistence living in Mecklenburg-Western Pomerania than in the Bronze Age. However, since the rest of the genome largely matches, this possibility is unlikely, according to the researchers. The most plausible explanation, then, is strong natural selection. Co-author Daniel Wegmann from the Université de Friborg in Switzerland explains: “We conclude that lactase-persistent individuals have had more children over the last 3,000 years, or that these children had a better chance of survival than those without this characteristic.” according to the required gene variant meant a selection advantage of six percent. This means that for every 100 lactose intolerant offspring there were 106 offspring who could still digest milk as adults. “This means that the corresponding gene is the most positively selected in the entire human genome,” says Joachim Burger.
Milk as a vital advantage
What evolutionary advantages did it have to be able to drink milk after infancy? This question has not yet been fully clarified. Various factors have probably come into play here: Milk provides many calories and nutrients and, compared to the drinking water available at the time, is relatively little contaminated with pathogens. In addition, people in northern latitudes with little solar radiation could particularly benefit from consuming higher amounts of vitamin D and calcium with their food.
“It is astonishing that at the time of the conflict on the Tollense, more than 4,000 years after the introduction of agriculture in Europe, milk tolerance was still so rare in adults,” says Burger. Co-author Krishna Veeramah of Stony Brook University in New York added: “Less than 2,000 years later, in the early Middle Ages, more than 60 percent of the people in Europe could drink milk as adults. It’s an incredibly quick change. ”One explanation for this could be that the ability to digest milk only became a particularly strong selection factor with increasing population density. In times of food shortages or contaminated drinking water, milk, as an energy-rich, uncontaminated liquid, could have offered higher chances of survival, says Burger: “Especially in early childhood, i.e. in the years after weaning, this may have always been decisive in prehistoric populations.”
Source: Joachim Burger (Johannes Gutenberg University Mainz) et al., Current Biology, doi: 10.1016 / j.cub.2020.08.033