Fresh evidence seems to firmly shift the focus of sustained early horse domestication on the Eurasian steppe to the Pontic-Caspian region, according to researchers.
The Pontic-Caspian region extends from northeastern Bulgaria and southeastern Romania, across southern Moldova, Ukraine, and through Russia and northwestern Kazakhstan to the Ural Mountains.
From the Xiongnu to the Mongols, the pastoralist populations of the wider Eurasian steppe have long been a source of fascination.
Among the earliest herding groups in this region were the Yamnaya, Bronze Age pastoralists who began expanding out of the Pontic-Caspian steppe more than 5000 years ago. These Bronze Age migrations resulted in gene flow across vast areas, ultimately linking pastoralist populations in Scandinavia with groups that expanded into Siberia.
Just how and why these pastoralists travelled such extraordinary distances in the Bronze Age has remained a mystery.
Now, a new study led by researchers from the Max Planck Institute for the Science of Human History in Jena, Germany, has revealed a critical clue and it might come as a surprise. It appears that the Bronze Age migrations coincided with a simple but important dietary shift – the adoption of milk drinking.
The researchers drew on a humble but extraordinary source of information from the archaeological record – they looked at ancient tartar (dental calculus) on the teeth of preserved skeletons.
By carefully removing samples of the built-up calculus, and using advanced molecular methods to extract and then analyse the proteins still preserved within this resistant and protective material, the researchers were able to identify which ancient individuals likely drank milk, and which did not.
Their results surprised them.
“The pattern was incredibly strong,” observed study leader and palaeoproteomics specialist Dr Shevan Wilkin. “The majority of pre-Bronze Age Eneolithic individuals we tested – over 90% – showed absolutely no evidence of consuming dairy. In contrast, a remarkable 94% of the Early Bronze Age individuals had clearly been milk drinkers.”
The researchers realized they had uncovered a significant pattern. They then further analysed the data in order to examine what kind of milk the herders were consuming.
“The differences between the milk peptides of different species are minor but critical,” explains Dr Wilkin. “They can allow us to reconstruct what species the consumed milk comes from.”
While most of the milk peptides pointed to species such as cow, sheep and goat, which was not surprising in light of the associated archaeological remains, calculus from a couple of individuals revealed an unexpected species: horse.
“Horse domestication is a heavily debated topic in Eurasian archaeology,” Dr Wilkin noted.
One site where early Central Asian milk drinking had been proposed was the 3500-year-old site of the Botai people in Kazakhstan. The researchers tested calculus from a couple of Botai individuals, but found no evidence of milk drinking.
“Although two samples are insufficient for drawing broad conclusions, this finding does not support widespread milk consumption at the site,” the study team wrote in their paper.
“However, two calculus samples from Early Bronze Age individuals of the Pontic-Caspian region do provide evidence for the consumption of horse milk.”
Combined with previously published evidence that the Botai equids were undomesticated Przewalski’s horses, this finding — if backed up by further sampling and analysis — would seem to firmly shift the focus of sustained early horse domestication on the Eurasian steppe to the Pontic-Caspian region, they said.
“Our identification of — to our knowledge — the earliest horse milk proteins yet identified on the steppe or anywhere else reveals the presence of domestic horses in the western steppe by the Early Bronze Age, which suggests that the region (where the first evidence for horse chariots later emerged at about 2000 BC) may have been the initial epicentre for the domestication of the DOM2 (domesticated) lineage during the late fourth or third millennium BC.”
Professor Nicole Boivin, senior author of the study and director of the Department of Archaeology at the Max Planck Institute for the Science of Human History, said the results won’t make everyone happy, but they are very clear.
“We see a major transition to dairying right at the point that pastoralists began expanding eastwards.”
Domesticated horses likely had a role to play, too. “Steppe populations were no longer just using animals for meat, but exploiting their additional properties – milking them and using them for transport, for example,” Professor Boivin said.
What precise critical advantage milk gave remains to be investigated. But it is likely that the additional nutrients, rich proteins, and source of fluids in a highly arid environment would have been critical to survival in the harsh and open steppe.
“What we see here is a form of cultural revolution,” Dr Wilkin says. “Early Bronze Age herders clearly realized that dairy consumption offered some fundamental benefits and once they did, vast expansions of these groups across the steppe became possible.”
Overall, the findings offer strong support to the notion of a revolution in secondary products in the Eurasian steppe by the Early Bronze Age.
This change in their subsistence economy, indicated by dietary stable isotopes in human bones as well as by proteomics, was accompanied by the widespread abandonment of pre-Bronze Age river-related settlement sites, the appearance of kurgan cemeteries in the previously unexploited arid plateaus between the river valleys, and the inclusion of wheeled vehicles and occasional horse bones in Yamnaya graves.
At the same time, the steppe Yamnaya population expanded westward into Europe and eastward to the Altai Mountains – a range of 6000km.
“Although we cannot offer direct insight into the question of horse riding or traction on the basis of our data, evidence for milked horses certainly makes horse domestication more likely, and may indicate that horses had a role in the spread of Yamnaya groups,” the study team wrote.
“The triad of animal traction, dairying and horse domestication appears to have had an instrumental role in transforming Pontic-Caspian economies and opening up the broader steppe to human habitation by the Early Bronze Age.
“If some or even all of these elements were present before the Bronze Age, it is only from this latter period that we witness their intensive and sustained exploitation amongst numerous groups.
“Although other factors will no doubt also have been important, the emergence of more mobile, pastoralist societies adapted to survival on the cold and arid steppe — where horses may have opened up snow-covered pasturage for other animals, and milk would have provided a sustained source of protein, nutrients and fluids — was undoubtedly critical to the expansion of Bronze Age pastoralists such the Yamnaya groups.”
The study team comprised Shevan Wilkin, Alicia Ventresca Miller, Ricardo Fernandes, Robert Spengler, William Taylor, Dorcas Brown, David Reich, Douglas Kennett, Brendan Culleton, Laura Kunz, Claudia Fortes, Aleksandra Kitova, Pavel Kuznetsov, Andrey Epimakhov, Victor Zaibert, Alan Outram, Egor Kitov, Aleksandr Khokhlov, David Anthony and Nicole Boivin.
Wilkin, S., Ventresca Miller, A., Fernandes, R. et al. Dairying enabled Early Bronze Age Yamnaya steppe expansions. Nature (2021). https://doi.org/10.1038/s41586-021-03798-4