Ancient Ritually Sacrificed Stallions Reveal How Humans Changed Horse Genetics

Some 2,700 years ago, in the Tuva region of southern Siberia, over 200 domestic horses were ritually sacrificed to honor the funeral rites of a high-ranking member of the Scythian people, one of the first cultures known to have mastered mounted warfare. About 400 years later, at the turn of the third century BCE, Scythians ceremonially killed around a dozen stallions and interred them in a sepulchral chamber in Berel, Kazakhstan.

These horses were probably none too thrilled about their fates. But thousands of years later, their literal sacrifice is helping to unravel the mysteries of horse domestication, and its enormous impact on human civilization, as evidenced by new research published Thursday in Science.

An international team led by Ludovic Orlando, a professor of molecular archaeology at the University of Copenhagen and research director at the University of Toulouse AMIS laboratory, conducted whole genome sequencing on 14 exceptionally preserved horse remains from three sites: Two stallions from the Siberian royal mound (known as Arzhan I), 11 more from the Kazakh burial grounds, and a mare that lived alongside the Sintashta people, the first culture known to use chariots, in the Chelyabinsk region of Russia, some 4,100 years ago.

By mapping and cross-examining their genomes, Orlando and his colleagues were able to reconstruct key details about the appearance, characteristics, and genetic relationships between these early domestic horses, along with insights into the animal husbandry practices of the peoples who relied on them to build their empires. (Horse domestication is generally considered to have originated about 5,500 years ago in the Eurasian steppes.)

"We wanted to target a time period where humans interacted a lot with horses," Orlando told me over Skype, "but also a time period that would be meaningful for learning about the early and late stages of horse domestication. Because of those two constraints, we decided on selecting the Scythians, because they were living at about halfway into the domestication timeline."

Much like modern domestic horses, the Scythian stallions had a range of coat colors, including black, cream, bay, chestnut, and spotted patterns. The DMRT3 gene, associated with modern ambling gaits like the rack or the two-beat trot, was not present, so these horses probably only moved with "natural gaits"—walking, trotting, cantering, and galloping.

However, the team did isolate genes associated with sprinting performance in contemporary horses, suggesting that Scythians may have valued those characteristics.

One of the study's major findings is that Scythians seem to have allowed their horses to maintain natural herd structures, as opposed to selectively breeding several mares with a few high-valued studs, which is the norm today with race-horses and other competitive breeds.

The genetic result is that the Scythian horses are much less inbred than modern counterparts descended from a small number of cherished lineages. This corroborates the historian Herodotus' claims that Scythians sacrificed horses that had been presented as gifts from different tribes.

"The genetic diversity that was present in the horse population has declined a lot," Orlando told me. "We breed fewer diverse horses, or more of the same exact individuals, simply because we fancied that type more."

This selective breeding during the last 2,000 years has resulted in "an almost complete homogeneity" on the Y chromosome of modern horses, which has caused deleterious mutations and has negatively impacted horse health, according to the study.

But for all of the costs of domestication to the horse, the process may have ultimately saved the species. Fossil evidence suggests that Eurasian wild horse populations were collapsing at the onset of domestication, and their counterparts in the Americas were already long extinct.

"Some paleontologists have even claimed that if humans had never domesticated horses, the horse would be extinct by now, simply because it was on the verge of extinction" 5,500 years ago, Orlando said.

The intricate ways in which humans shaped horses into their modern form, and were in turn shaped by them, are at the heart of the ongoing PEGASUS research project, funded by the European Research Council and led by Orlando.

Read More: How the Last Wild Horses Can Be Saved by Cheap Gene-Sequencing

"We are interested in replicating this same study, not just for Scythians, but actually for every ancient human culture," he told me. "The main goal is to understand how the human-horse relationship evolved through space and time."

"Horses have been so essential to human history," Orlando added. "Who knows whether some particular civilization managed to build their empire because they had a superior horse? This is the type of hypothesis that we want to test. By really looking at the horse, we want to see some facets of ancient people that are generally neglected."

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