The horse was the heart of the mysterious Iron Age Xiongnu empire in Mongolia, as suggested by a decorative bronze belt plate showing two horses grappling.
NATIONAL MUSEUM OF MONGOLIA
Until now, the only accounts of the Xiongnu came from their enemies. Chinese records from 2200 years ago describe how these fierce mounted archers from the wide-open steppes of today’s Mongolia clashed with armies in what is now northwestern China. Their onslaughts spurred the Chinese to build what would become known as the Great Wall of China on their northern border, as protection against the mounted nomads. They also started to raise cavalry armies of their own.
The equestrian empire of the Xiongnu left no written records. But biology is now filling out their story, and those of other Central Asian cultures in antiquity. Two studies—a sweeping survey of ancient DNA from more than 200 individuals across 6000 years and an analysis of horse skeletons from just before the rise of the Xiongnu—trace population movements across Central Asia and the key role played by horsemanship. The results “show the horse was probably the driver of some of the ancestry shifts we see in the human population,” says Ludovic Orlando of Paul Sabatier University, who was not involved in the paper. “The horse provided new range in patterns of human mobility and allowed people to travel long distance faster.”
Horses were probably domesticated by the Botai culture around 3500 B.C.E. near what is modern Kazakhstan. Horses may have been mainly used for meat and milk at first, and later began to pull wheeled chariots.
To learn more about human migration across Central Asia, a team led by Choongwon Jeong of Seoul National University and Harvard University’s Christina Warinner sampled and sequenced DNA from human remains found in Mongolia. The results, which they report today in Cell, span the period from 5000 B.C.E. all the way to the heyday of another horse-riding culture—that of Genghis Khan’s Mongol Empire, around 1000 C.E.
Genetic studies of Western European populations have shown that around 3000 B.C.E., the Yamnaya—mobile herders of cattle, sheep, and goats—pushed west from the steppes of what is today Russia and Ukraine and triggered a dramatic genetic turnover in Europe. Skeletons from Bronze Age Mongolia had shown the Yamnaya also moved east and introduced their dairy-oriented pastoralist lifestyle there. But they left no lasting genetic traces in Mongolia, the oldest samples in the new study show.
The ancient DNA does show that 1000 years later, another group from the steppes, called the Sintashta, left a lasting imprint. They also brought fateful cultural changes to Mongolia’s grasslands, as earlier archaeological studies had shown. Starting in about 1200 B.C.E., equestrian innovations including selective breeding for size and endurance, plus bridle bits, riding pants, and even early saddles, appeared in the record, says archaeologist William Taylor of the University of Colorado, Boulder, a co-author on both papers.
N. Desai/Science
Mongolians of the time were obviously riding horses, as vividly confirmed by the second paper, in the Proceedings of the National Academy of Sciences. The authors, Chinese and U.S. archaeologists, report that horse skeletons buried around 350 B.C.E. in the Tian Shan mountains, now part of China’s Xinjiang province, show bone abnormalities from riding, including spinal damage from the weight of a rider and changes to the bones of the mouth from bits and bridles. “Put the lower back pathologies together with evidence for a bridle, and it all suggests horses were being ridden,” says Sandra Olsen, an archaeologist at the University of Kansas, Lawrence, who was not part of either study.
Not long after, the Xiongnu emerged. They translated their skills on horseback into a sophisticated means of waging war and organizing an empire over vast distances. Starting in about 200 B.C.E., the Xiongnu marshaled nomadic tribes from across Eurasia into a formidable force, turning the steppes into a political center rivaling neighboring China. “The Xiongnu have been a source of constant worry and harm to China,” one contemporary Chinese historian wrote. “They move about in search of water and pasture and have no walled cities or fixed dwellings, nor do they engage in any kind of agriculture.”
Jeong’s study of DNA from 60 human skeletons from the Xiongnu’s 300-year-run shows how the region was transformed into a multiethnic empire. After more than 1000 years in which three distinct, stable human populations lived side by side on the Mongolian steppe, genetic diversity rose sharply around 200 B.C.E. Populations from western and eastern Mongolia mixed with each other and with people carrying genes from as far away as present-day Iran and Central Asia. Such wide-ranging mixing has “never been seen before at that scale,” Jeong says. “You can see the entire Eurasian genetic profile in the Xiongnu people.”
The results suggest mastery of the horse made possible stunning long-distance voyages on Central Asia’s sea of grass. Archaeological finds in the graves of Xiongnu elites, such as Roman glass, Persian textiles, and Greek silver, had suggested distant connections. But the genetic evidence suggests something more than trade. Eleven Xiongnu-period skeletons showed genetic signatures similar to those of the Sarmatians, nomad warriors who dominated the region north of the Black Sea, 2000 kilometers across the open steppe from Mongolia.
“There’s no written evidence of [Xiongnu] contact with Sarmatians, and it’s not well-attested archaeologically. It’s really surprising they’re mixing over these long distances,” says Tsagaan Turbat, an archaeologist at the Mongolian Academy of Sciences’s Institute of Archaeology. “This kind of information is really a game changer.”
In the future, researchers hope the genomes will help reveal how the mysterious nomad empire worked. The Xiongnu are “doing the things that empires do—forcing or enticing people to move,” says University of Michigan, Ann Arbor, archaeologist Bryan Miller. “Are people sent out to rule, or are local elites allowed to continue?” he asks. “Only genetics could answer that.”