The genetic mutation that allows adult humans to digest milk became established in European populations over a remarkably short period of time, suggesting that drinking milk must have once brought significant survival advantages. According to a new study in the journal Current Biology, just 7.1 percent of Bronze Age warriors were able to digest milk, yet this figure rose rapidly over the next few thousand years due to natural selection.

All human babies produce an enzyme called lactase, which breaks down lactose in their mother’s milk. These days, about a third of the world’s population – predominantly those heralding from Europe and North America – have inherited a genetic mutation for lactase persistence, which allows them to continue digesting milk throughout their lifetime.

Those who lack this gene typically stop producing lactase soon after weaning, and can experience anything from stomach cramps to flatulence if they consume dairy products as adults. Exactly when lactase persistence became an established genetic trait in certain parts of the world has remained a mystery for some time.

To investigate, researchers analyzed genetic material retrieved from the bones of 14 Bronze Age soldiers who died in battle around 3,200 years ago in Tollense. Located in modern-day Germany, Tollense is thought to have witnessed the very first large-scale battle in northern Europe, contested by some 4,000 warriors.

Results suggested that only a fraction of those who fought at Tollense were lactase persistent, despite the fact that the battle occurred more than 4,000 years after the agricultural revolution, when the raising of cattle became common. Even more striking is that less than 2,000 years later, some 60 percent of the population in this area had become lactase persistent, as are 90 percent of northern Europeans today.

The study authors also examined the genetic records relating to other Bronze Age skeletons from eastern and southern Europe, and found that they too were overwhelmingly lactose intolerant.

This means that, since the Bronze Age, the genetic mutation giving rise to lactase persistence has spread through the population at an astonishing rate, becoming commonplace in just 120 generations.

“This is actually an incredibly fast rate of change for the gene that controls milk digestion,” explained study author Krishna Veeramah in a statement. “It appears that by simply possessing this one genetic change, past European individuals with the ability to digest lactose had a 6 percent greater chance of producing children than those who could not. This is the strongest evidence we have for positive natural selection in humans.”

Exactly why milk drinkers were so much more likely to survive into adulthood and pass on their genes is unknown, although study co-author Joachim Burger speculates that milk may have provided an important source of nutrition during tough times.

“With milk being a high-energy, relatively uncontaminated drink, its ingestion may have provided greater chances of survival during food shortages or when supplies of drinking water may have been contaminated,” he said.