It only took a few thousand years for the human ability to digest the milk sugar lactose, after infancy, to spread throughout Central Europe, a study suggests.

Researchers analysed genetic material from the bones of individuals who had fallen in a conflict around 1200 BC on the banks of the Tollense, a river in present-day Germany.

They found that around one in eight of the assumed warriors had a gene variant that enabled them to break down the lactose in milk.

Population geneticist Professor Joachim Burger, of Johannes Gutenberg University Mainz (JGU), said: “Of the present-day population living in this same area, around 90% have this lactase persistence.

“This is a huge difference when you consider that there cannot be many more than 120 human generations between then and today.”

The study, published in Current Biology, found that aside from lactase persistence and a few other genetic variants, the genomes of the Tollense people are similar to those of today’s inhabitants of northern Germany and the Baltic Sea region.

Professor Daniel Wegmann, of the University of Fribourg in Switzerland, said: “The only way to explain this difference between these Bronze Age people and those of today is very strong natural selection.

“We conclude that over the past 3,000 years, lactase-persistent individuals had more children or, alternatively, those children had better chances of survival than those without this trait.”

Prof Burger added: “In each generation lactase-persistent individuals have a 6% greater chance of surviving to reproductive age than non-lactase-persistent individuals.”

But researchers say there is still no answer to why being able to digest the sugar in milk after infancy provided such a big evolutionary advantage.

The study also analysed the genetic material in Bronze Age bones found in eastern and south-eastern Europe for comparison. Scientists found these also showed low frequencies of lactase persistence.

Researchers say the conflict in the Tollense valley is considered the oldest known battle in Europe.

Remains were first discovered there in the 1990s, and for more than 10 years archaeologists have been systematically searching a kilometre-long section along the river.

The bones of more than 100 individuals have been discovered, many of them exhibiting signs of violent combat.

Many still contain arrowheads, while some skulls look to have been crushed by blunt objects.