Most mammals stop being able to digest lactose — the main sugar in milk — past infancy. But an estimated 35 percent of the world's population retains lactose tolerance, or lactase persistence, past childhood. During infancy all mammals produce lactase, an enzyme that breaks lactose down into simpler sugars and allows its digestion, but around weaning time they stop producing it.

This lactase persistence is due to genetic mutations that keep bodies producing lactase well past childhood. There are several mutations known to have arisen independently in human populations the last 10,000 years. Today the trait is unevenly distributed in the world and can be more or less frequent in the populations depending on their ancestry. Previous studies have found that the evolution of lactase tolerance shows signs of positive selection — indicating it must have conferred an advantage to its bearers. 

A new study published in Current Biology using genetic analyses and bones from Bronze Age archeological sites dated from 5980 to 3200 years ago brings new information about the evolution of this trait.

Northern Europeans show a high frequency of a specific mutation, called rs4988235-A, that confers lactase tolerance. An international team led by Joachim Burger from Johannes Gutenberg University in Germany, studied gene variants from DNA extracted from bones of warriors found in the Tollense battlefield archeological site in Germany dated to 3200 years ago. The analysis revealed they belonged to Central/Northern European individuals. They also analyzed bones from an archeological site in Serbia dated 4100 to 3700 years before present, and from sites from the Eastern Europe Pontic-Caspian Steppe dating back 5980 to 3980 years.

Lactose tolerance in the Central/Northern European samples was low (7.1 percent), contrasting with present-day levels of 90 percent lactose tolerance in the same region, and it was also low in the South Eastern samples (4.6 percent). The trait was absent in the samples from the 37 individuals studied from the Eastern Europe steppes. This does not support an existing hypothesis that proposes that this mutation, rs4988235-A, originated at the Eastern European steppes. Although the sample sizes were small, the results indicate an big increase of lactose tolerance frequency in just the past 120 generations.