The ability for adult Europeans to drink milk was inherited from Russian herders just 4,000 years ago, a genetic study has shown.
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Without a key mutation, the enzyme that enables us to digest lactose becomes deactivated after weaning.
The findings come from the largest ancient DNA study of its kind published in the journal Nature.
"Everyone assumed it came to Europe with the first farmers," co-author Dr Bastien Llamas, from the Australian Centre for Ancient DNA at the University of Adelaide, said.
"But you actually had a 4,500-year period when European farmers could not actually drink milk."
The study of DNA from 230 Eurasians who lived between 6500 BC and 300 BC showed that Russian herders from the Great Steppes brought the enzymes for lactose tolerance into Europe.
"Suddenly 4,000 years ago there's a revolution when the Steppe herders brought the enzymes they needed," Dr Llamas said.
Earlier this year, Dr Llamas and colleagues found Europeans descended from three groups: Stone-Age hunter-gatherers, farmers that migrated from Anatolia (modern-day Turkey), and nomadic herders that migrated west from the Great Steppe in Russia.
For this study, the researchers analysed patterns in the genomes of these groups to look at how human traits had changed since the advent of agriculture in Europe around 8,500 years ago.
One of the most surprising findings related to the emergence of the genetic mutation that enables humans to drink raw milk.
This mutation enables the enzyme lactase, which digests lactose in milk, to remain active long after weaning occurs.
Most people had assumed the mutation, which is widespread in Europe today, would have been introduced by the Anatolian farmers, who had been keeping animals such as cows since around 6500 BC.
However, Dr Llamas and colleagues found the mutation did not enter the European population until 4,000 years later, when the Russian herders arrived.