Tracing how horse domestication turned the Eurasian Steppe into a highway

From Neanderthals to human hunter-gatherers to the mounted horde of Genghis Khan, the Eurasian Steppe has long been a crossroads of humanity. And for the last 5,000 years or so, domesticated horses have shaped how people moved through, lived in, and dominated that vast grassland stretching from Hungary and Romania to Northeastern China.

A paleogenomic study adds new evidence to the debate about where people first domesticated horses, and a related study reveals the impact of horsemanship on the peopling of the steppe.

The first evidence we have of domesticated horses comes from a site called Botai in Northern Kazakhstan, where archaeologists have found evidence of milking, corrals, and the use of harnesses. But there’s still debate about whether hunter-gatherers at Botai started domesticating horses—which they’d previously hunted for meat—in order to milk and ride them. It’s possible they learned from herders farther west, such as the people whose graves have been found at Khvalynsk dating from around 7,150 to 5,930 years ago.

To get a better picture, Copenhagen University evolutionary geneticist Eske Willerslev and his colleagues examined the ancestry of 74 people who lived on the Eurasian Steppe from 11,000 years ago up through the Medieval period. Among other things, they wanted to see whether the Botai had interbred with the Yamnaya, the pastoral descendants of the Khvalynsk people. If they had, that would be a clue that the Botai had interacted with the Yamnaya enough to perhaps exchange cultural ideas as well as genes.

Score one for the hunter-gatherers

According to genomes retrieved from the bones of three Copper Age skeletons from Botai, an early Bronze Age skeleton from a Yamnaya site in Kazakhstan, and 70 other sets of remains, the two groups hadn’t shared ancestry since at least 15,000 years ago during the Paleolithic. The Yamnaya genomes in the study showed signatures of both European and Caucasian hunter-gatherer ancestry, but the Botai genomes showed no trace of Caucasian ancestry at all. Instead, they carried the genetic signature of ancient East Asian populations, something the Yamnaya and their Khvalynsk ancestors didn’t have.

There had been, according to the study, almost no gene flow between the two groups. That means that the Yamnaya didn’t interbreed with the Botai when their migration took them through the Central Steppe. Of course, that doesn’t rule out other kinds of cultural interactions, but the archaeological record of the Bronze Age steppe doesn’t seem to bear much of a mark of Yamnaya influence, either.

Another piece of the evidence comes straight from the horse’s mouth, so to speak. A study published in the April edition of , by University of Copenhagen anthropologist Charleen Gaunitz and her colleagues, shows that the horses whose bones archaeologists found at Botai weren’t related to the horses whose remains turned up at later Yamnaya sites.

“Together these emerging facts indicate that the Botai people domesticated their own wild horse populations, and furthermore that they did not trade animals or apparently even steal animals from the Yamnaya horse herds,” said Hartwick College archaeologist David Anthony, who specializes in the history of horse domestication.

The prey path?

Willerslev and his colleagues suggest that horses took the “prey path” to domestication, much like reindeer in Northern Europe. At some point, the local hunter-gatherers realized that the horses they’d been hunting for meat could also be milked and even ridden. That makes horses, along with dogs and reindeer, rare examples of animals domesticated by hunter-gatherers rather than more settled farmers.

But there’s a wrinkle; the horses from Botai aren’t the ancestors of modern horses. Willerslev and his colleagues suggest that horse domestication may have arisen separately in two places, although we’re not yet sure where that second place may be; no archaeological evidence has been found.

Anthony suggests the region around Khvalynsk, southwest of the Ural mountains and significantly west of Botai. There, horses have been found as sacrifices in and near human graves, along with cattle and sheep. No wild animals, such as deer, were included in those burials. Horses are also a common motif in decorative bone plaques and stone mace-heads from sites in the area. Critically, these sites date back to 6,500 years ago, about a thousand years earlier than the signs of domestication at Botai.

None of that is direct evidence of domestication, like the corrals and the signs of bit-wear on horses’ teeth found at Botai, but Anthony contends that it’s an indication that people’s attitudes toward horses had begun to shift. Based on their horse genomic data, Gaunitz estimated that the lineage of today’s modern horses is about 4,500 to 5,000 years old, which would coincide with the spread of the Yamnaya people.

“The idea of domesticated horses then diffused eastward, but not the actual animals,” said Anthony. “Eventually the usefulness of riding horses became apparent to the Botai people, and they domesticated their own wild stock and adopted a new economy. It was a prey path to domestication locally, but with stimulus from the West, where horseback riding developed first.”

But archaeologists haven’t yet found direct evidence supporting that model, and Willerslev and his colleagues’ genetic data definitely raises doubts about how much cultural exchange happened between the Yamnaya and the Botai. That debate is likely to remain unresolved until more evidence turns up.

Turning the steppe into a highway

But if horses were domesticated a thousand years earlier in the Ural Foothills, their influence didn’t begin to be felt on the steppe until around the time of their domestication at Botai. Up through the end of the Copper Age, around 5,000 years ago, genetic records show that the Central Steppe was fairly stable (unlike Europe, which was one big churn of migrations and population turnover). But somewhere around the cusp of the Bronze Age, everything changed.

Willerslev and his colleagues mapped those changes in a second study involving genomes from 304 sets of remains taken from sites across the Eurasian Steppe.

“The horse has transformed the world of humans in a very short time,” said Willerslev. Domesticated horses made it possible to cross the vast expanse of the steppe much more quickly, and they helped pastoral societies on the steppe to grow and expand. Suddenly at the start of the Bronze Age, people were on the move across the grasslands at a massive scale. The Bronze Age was the start of the time of the mounted warrior cultures that most people imagine when they think about the Eurasian Steppe: the Scythians, the Huns, and Genghis Kahn’s Mongolian horde.

“I think it’s the most extreme example to have ever been reported in terms of one population replacing each other in a very short time period, and I think it’s really key that this is a result of the domestication of the horse,” said Willerslev. “This steppe area is like a short grassland, very flat for some parts of it, so it’s kind of a massive highway, but you don’t have a car yet. And suddenly you’re getting that car—and that’s basically the horse—and then everything just goes completely crazy.”

Population turnover

The Scythian culture, a confederation of mounted tribes speaking Iranian Indo-European languages, dominated the Western Steppe from 2,800 to about 2,200 years ago. Scythian sites from one end of the steppe to the other seem to reflect a single culture, but the newly sequenced genomes of people buried there indicate the Scythians themselves were pretty genetically diverse. At the western end of the steppe, individuals tended to have more ancestry from Eurasian groups; farther east, Asian ancestry was more dominant.

“Later, moving into the late Bronze Age, you have the invention of the spoked-wheel chariot, which connects South Asia with Europe and establishes these huge trade routes, and that is really what transforms the steppes into becoming like a highway,” said Natural History Museum of Denmark paleogeneticist Peter de Barros Damgaard, “and then at the southern fringes, you have the Silk Trade route.”

By 2,200 years ago, the Xiongnu culture swept westward from the Eastern Steppe, absorbing and replacing the Scythians and other populations they encountered. The Xiongnu evolved into the Hun Empire, which lasted until the 500s CE. In its place, new waves of migration and conquest, interbreeding and replacement, swept westward across the steppe. The most famous, of course, are the Mongols of the 1100s and 1200s CE, first ruled by Genghis Khan, but there were several smaller waves of other groups.

“As we move into Medieval times, you have these empires and khanates that are changing constantly because it is such an interesting place to dominate, so you have these groups coming in, taking over the world for a couple of hundred years, until they are defeated by another incoming group,” said Damgaard.

Those movements shaped the modern demographics of Europe, Anatolia, and Southwest Asia. To reconstruct that process, his team combined the paleogenomic data with archaeological and written records, such as ancient Chinese sources describing the westward movements of the Huns and other groups.

“This process is changing the steppe, you could say, from being mainly a Western Eurasian ancestry to becoming mostly of East Asian genetic ancestry, and also changing the steppe in terms of being Indo-European speakers into becoming Turkish-speaking people,” said Willerslev.

, 2018. DOI: 10.1126/science.aar7711; , 2018. DOI: 10.1038/s41586-018-0094-2  (About DOIs).

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