About 4200 years ago, a few herders on the western Eurasian steppe got a brand-new mount. They were experienced at herding wild horses for food, but their new steeds had a calmer disposition and a stronger back, making the horses easier to train and ride, perhaps for the first time. The new model galloped across Eurasia within a few centuries, triggering major shifts in Bronze Age human cultures. “Once we domesticated this new kind of horse, suddenly they were everywhere,” says molecular archaeologist Ludovic Orlando of the French national research agency CNRS and Paul Sabatier University.
Orlando is lead author of a landmark study that pinpoints where the new horse first appeared and how it replaced earlier equids to become the ancestor of modern horses, from Shetland ponies to massive Clydesdales and sleek thoroughbreds. He and a giant interdisciplinary team analyzed 300 ancient horse genomes from 121 archaeological sites in Eurasia. In a paper in this week’s issue of Nature, they conclude that the ancestor of all modern horses made its first appearance by 4200 years ago in the western Eurasian steppe.
“They have found the original homeland of modern horses,” says molecular archaeologist Christina Warinner of Harvard University and the Max Planck Institute for the Science of Human History, who was not part of the study.
The first evidence of horse domestication comes earlier, from Kazakhstan, where herders of the Botai culture corralled mares for meat and perhaps milk about 5500 years ago. Researchers haven’t proved the Botai horses, whose teeth show wear likely from bits, were actually ridden, but archaeologists assumed for years that they were ancestral to modern horses. Then in 2018 Orlando and colleagues tested ancient DNA from the Botai horses and got a surprise: The horses were not the forerunners of modern horses. Instead, they were the ancestors of today’s Przewalski’s horses, endangered “wild” horses found only in Mongolia that escaped domestication long ago.
This put geneticists back at the starting gate in their quest to find the ancestor of modern horses, which they call DOM2 for “second domestication” horse, although they can’t be sure whether the genetic change happened in wild or previously domesticated horses. “We asked, where were they? The where was huge, because it was all of Eurasia,” Orlando says.
He traveled from Kazakhstan to Siberia and Mongolia to gather every sample he could get of bone from horses that lived 10,000 to 2000 years ago. After the genomes were sequenced, he and postdoc Pablo Librado sorted them into a family tree, to see how they were related to each other and to modern horses. The equids that lived more than 4200 years ago showed a rich genetic diversity. “The horses living in Anatolia, Europe, Central Asia, and Siberia used to be genetically quite distinct,” Orlando says.
Close cousins of the ancestor of all of today’s horses—the early DOM2 horses—first appeared in the western Eurasian steppe. Once the DOM2 horse itself arose, it spread rapidly throughout Eurasia.
But starting by about 4200 years ago, that diversity began to disappear. The genetic profile of one type of horse, closely related to modern horses, began to spread rapidly across Eurasia, replacing the others. By 4000 years ago, the new horse dominated a region from central Anatolia to central Russia, where people of the Sintashta culture buried horses with the earliest spoked wheels and chariots in mounds called kurgans. By 3000 years ago, the replacement was complete. The new horse became an icon of late Bronze Age art, carved in the bone hilts of daggers and cast into bronze figurines, says archaeologist Pavel Kuznetsov of the Russian Academy of Sciences (RAS).
To learn what was so special about this new horse, researchers sought gene variants present in all DOM2 horses but not in earlier equids. They found one in the ZFPM1 gene that helps control anxiety and aggression, and one in the GSDMC gene that’s associated with chronic back pain in humans. The researchers suggest the new horse was calmer and less likely to have back problems, traits that would have made it easier to train and ride into battle.
To find out where in the vast Eurasian steppe herders first began to favor this valuable new horse, Orlando put out a call for more horse bones from archaeological sites. The DNA samples he got back solved the mystery. Although the team didn’t find the very first DOM2 horse, they identified several closely related ancestors that lived from 5400 to 4600 years ago.
Those early cousins came from three sites in a center of horse domestication near the lower Volga and Don rivers in the northern Caucasus, which is now southern Russia (see map, above). Here, pastoralist cultures of the time herded goats, sheep, cattle, and perhaps horses in the dry, open steppe, says archaeologist Sabine Reinhold of the German Archaeological Institute (DAI), a co-author. The horses of the region didn’t yet have the two winning genetic adaptations, but they were part of the wild or managed herds that gave rise to that coveted horse, says DAI archaeologist Svend Hansen, another co-author.
One of the oldest dates, about 5400 years ago, comes from a horse’s leg bone buried with a small child in a kurgan built by the Maykop culture at Aygurskiy, in southern Russia. Another early relative of the DOM2 horse was found at Repin Khutor, a site on the Don River that was home to the famed Yamnaya herders who make up a significant part of Europeans’ genetic ancestry. Some researchers had thought the Yamnaya rode into Europe 5000 years ago on the ancestors of modern horses. But samples from the new study show that once they got to Europe, the Yamnaya used earlier European horses rather than the DOM2 variety.
The new horse’s spread and takeover between 4200 and 3000 years ago coincide with a “cultural genesis” during the late Bronze Age in the Volga-Don region, Kuznetsov says. First came the DOM2 horse, followed by the spoked wheel and chariots. Together, those innovations gave people the ability to travel off well-trodden paths, allowing them to find new sources of tin, gold, and other metals; set up long-distance trade networks; and herd other livestock farther to new pastures, says Natalia Roslyakova of RAS.
Climbing on top of the DOM2 horse was a “complete game changer,” says University of Oxford archaeologist Greger Larson, who wasn’t part of this study. “You now have an advantage over anyone else who wants to get around quickly.” This paper also shows, he says, that data from animals can reveal major cultural changes in the past. “I love the part that it’s animals that can be used to test what happens to people … to understand their culture.”
Source: Science Mag