Only a few of the bones from remains found on the oldest known battlefield in Europe show evidence of milk tolerance. Some 120 generations later, almost all people living in the same area are now able to tolerate milk. Studies carried out by a co-led research team at the University of Fribourg shed light on the evolution of this adaptive trait.

People in Central Europe have only been able to digest milk after infancy for a few thousand years. This is the conclusion of results published in the current issue of the journal Current Biology. Researchers examined genetic material present in the bones of warriors killed around 1200 BC in the Battle at Tollense, a river in the north-eastern part of present-day Germany. Only one of the 14 warriors examined was able to break down lactose and thus digest milk.

«90 per cent of today’s population living in the same area has this trait, known as lactase persistence», says Joachim Burger, a population geneticist at Johannes Gutenberg University Mainz. «This is an enormous difference when you consider that not much more than 120 generations separate them.»

Better chances of survival with milk tolerance
In Switzerland today around 80% of the population can tolerate milk. «The only way to explain the difference between the Bronze Age and today is natural selection», says biologist Daniel Wegmann of the University of Fribourg. The researchers have calculated an astonishing advantage: those who tolerated milk had on average 6% more children than those who could not digest milk. «This is probably also because these children have better chances of survival than those without this trait», says Vivian Link of the University of Fribourg, who led the bioinformatic analyses. «No other gene has resulted in such strong selection.»

In 2007, Burger and his team demonstrated that almost none of the first farmers to settle in Europe could tolerate milk. Even more than 4,000 years after agriculture was introduced in Europe, this trait was still very rare in adults. As an energy-rich liquid, milk could have offered better chances of survival in times of food scarcity or contaminated drinking water. This inability to tolerate milk may have been decisive in the Bronze Age, especially in early childhood.

Trait missing entirely in some cases
For comparison, the researchers also analysed genetic material in bones from the Bronze Age in Eastern and South-eastern Europe. There, too, they seldom found evidence of milk tolerance. In bones examined from the Eastern European steppes, the trait was missing entirely. «We were very surprised», explains Link, since earlier studies suggested that its origin might be there.

The Battle at Tollense is the oldest known battle in Europe. Remains from it were first discovered in the 1990s. For ten years, archaeologists have been scanning a kilometre-long section along the river. So far, the bones of more than a hundred bodies have been discovered, many of them showing traces of combat: some still have arrowheads embedded in them; some skulls are split by blows. It is estimated that several thousand people took part in the battle – mostly men; but among the 14 skeletons examined, the researchers also found two women.

More information
Link to publication: Burger et al., Low prevalence of lactase persistence in Bronze Age Europe indicates ongoing strong selection over the last 3,000 years, Current Biology, https://doi.org/10.1016/j.cub.2020.08.033

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Bronze Age skull along the Tollense river, © Joachim Krüger
So far, the bones of more than a hundred individuals have been discovered on the battlefield, © Stefan Sauer / Tollense Valley Project
Archaeologists have been systematically scanning a section along the Tollense for ten years, © Stefan Sauer / Tollense Valley Project