Hardened dental plaque coating the ancient teeth of Bronze Age pastoralists suggests the emergence of dairying and milk-drinking coincided with a wave of migration across the Pontic-Caspian steppe.

For thousands of years, Eurasia has played host to human movement and mixing, with recent surveys revealing genetic links between Scandinavia and Siberia.

Now, research published Wednesday in the journal Nature suggests some of the region's earliest herders were drinking milk as they moved eastward.

"When the data came in we were really blown away," senior author Nicole Boivin told UPI in an email.

"We did not expect such a massive shift to milk consumption right at the time of a known massive human migration. But once we had our data, the correlation was too strong to ignore," said Boivin, a professor of archaeology at the Max Planck Institute for the Science of Human History in Germany.

After carefully extracting samples of tartar, or dental calculus, researchers used sophisticated molecular analysis techniques to identify ancient proteins trapped in the hardened layers.

The analysis showed 90% of pre-Bronze Age individuals were not drinking milk. However, by the Early Bronze Age, 94% of the people living and moving across Eurasia were milk drinkers.

Previous genetic and archaeological surveys suggest the Yamnaya people, a group of pastoralists from the western steppe began expanding across Pontic-Caspian steppe around 3000 B.C.

Researchers found evidence that Yamnaya herders were migrating vast distances, moving from western Russian to Mongolia. Previous studies also showed some of these people were drinking milk.

"I had the idea that it was the use of milk, a renewable resource of both calories and hydration, that allowed these people to expand across the arid steppe," lead study author Shevan Wilkin told UPI.

"The steppe is unusual as it is covered with ground forage that human cannot eat, but animals can," said Wilkin, a palaeoproteomics specialist and MPI researcher.

"Through proteins and ancient DNA we were able to see that western steppe people and milk were present on the Far Eastern steppe 5,000 years ago," Wilkin said. "Now from the data we have from this paper we can see that these vast expansions coincided with the incorporation of milk into the diets of the Yamnaya people on the Pontic-Caspian steppe."

The latest findings have implications not just for studies of human movement across Eurasia, but also the search for the origins of the domestic horse -- a hot topic in Eurasian archaeology.

Some researchers have pointed to Botai, Kazakhstan, as the birthplace of the domestic horse, but more recent archaeogenetic analysis has cast doubt on such claims.

By analyzing the ratios of peptides preserved in Bronze Age tartar, the authors of the latest study were able to identify the different sources of milk being consumed by Yamnaya herders and their relatives.

Cows, sheep and goats provided early pastoralists with most of their milk, but researchers found evidence that Yamnaya people were drinking horse milk too.

"We also extracted proteins from Botai, a site in northern Kazakhstan which has been suggested as a place of horse domestication, but we did not find any milk proteins in those samples," Wilkin said.

The discovery suggests the earliest domestic horses likely originated on the Pontic-Caspian steppe.

Today, non-urban people living on the Eurasian steppe still practice mobile dairy pastoralism. Researchers suspect milk was even more important to the Bronze Age herders moving across the steppe thousands of years ago.

"The steppe is a pretty harsh environment -- it's wide open, very arid, not much grows there and water is limited," said Boivin. "It's a tough environment for humans to inhabit."

"So milk benefited people as it meant animals gave them a moving source of high quality fluid, proteins and vitamins -- and they didn't have to kill the precious animals to get it," Boivin said.

The study's authors said there is still much they don't know about the pastoralists that migrated across the Eurasian steppe during the Bronze Age.

They hope, however, that additional surveys of animal remains and dental calculus will help them better understand the logistics of human expansions and the spread of dairying across the region.

Researchers are also keen to understand how these pastoralists developed the stomach for all that dairy.

"Lactase Persistence, the genetic adaptation to break down the lactose in milk as adults, was not common during this time. So it is a bit of an unanswered question of how all these people were drinking milk," Wilkin said.

It's possible they were processing milk through some sort of manual fermentation process, or that their gut microbiomes had adapted and contained bacteria that would ferment milk in the digestive track, she said."I think this will also be a future avenue of research," Wilkin said.

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