Scientists turn to human skeletons to explore origins of horseback riding

The answer, according to archaeologists from the University of Colorado Boulder: It's complicated. In a new study, the team drew on a wide range of evidence -- from medical studies of modern equestrians to records of human remains across thousands of years.

The researchers concluded that horseback riding can, in fact, leave a mark on human skeletons, such as by subtly altering the shape of the hip joint. But those sorts of changes on their own can't definitively reveal whether people have ridden horses during their lives. Many other activities, even sitting for long periods of time, can also transform human bones.

"In archaeology, there are vanishingly few instances in which we can tie a particular activity unequivocally to skeletal changes," said Lauren Hosek, lead author of the study and an assistant professor in the Department of Anthropology at CU Boulder.

She and her colleagues reported their findings Sept. 20 in the journal Science Advances.

The results may have implications for researchers who study the origins of when humans first domesticated horses -- and also cast doubt on a long-standing theory in archaeology known as the Kurgan hypothesis.

The first equestrians

The research lies at the center of what is among the old debates in archaeology, said William Taylor, a co-author of the new study and curator of archaeology at the CU Museum of Natural History.

He explained that the earliest, incontrovertible evidence of humans using horses for transport comes from the region around the Ural Mountains of Russia. There, scientists have uncovered horses, bridles and chariots dating back to around 4,000 years ago.

But the Kurgan hypothesis, which emerged in the early 20th century, argues that the close relationship between humans and horses began much earlier. Proponents believe that around the fourth millennium B.C., ancient humans living near the Black Sea called the Yamnaya first began galloping on horseback across Eurasia. In the process, the story goes, they may have spread a primordial version of the languages that would later evolve into English, French and more.

"A lot of our understanding of both the ancient and modern worlds hinges on when people started using horses for transportation," Taylor said. "For decades, there's been this idea that the distribution of Indo-European languages is, in some way, related to the domestication of the horse."

Recently, scientists have pointed to human remains from the Yamnaya culture dating back to about 3500 B.C. as a key piece of evidence supporting the Kurgan hypothesis. These ancient peoples, the group argued, showed evidence of wear and tear in their skeletons that likely came from riding horses.

Hips can lie

But, in the new study, Hosek and Taylor argue that the story isn't so simple.

Hosek has spent a lot of time poring over human bones to learn lessons about the past. She explained that the skeleton isn't static but can shift and change shape over an individual's lifetime. If you pull a muscle, for example, a reaction can emerge at the site where the muscle attaches to the underlying bone. In some cases, the bone can become more porous or raised ridges may form.

Reading those sorts of clues, however, can be murky at best. The hip joint is one example.

Hosek noted that when you flex your legs at the hip for long periods of time, including during long horse rides, the ball and socket of the hip joint may rub together along one edge. Over time, that rubbing can cause the round socket of the hip bone to become more elongated, or oval in shape. But, she said, other activities can cause the same kind of elongation.

Archaeological evidence shows that humans used cattle, donkeys and even wild asses for transport in some areas of western Asia centuries before they first tamed horses. Ancient peoples likely yoked these beasts of burden to pull carts or even smaller, two-wheeled vehicles that looked something like a chariot.

"Over time, this repetitive, intense pressure from that kind of jostling in a flexed position could cause skeletal changes," Hosek said.

She's seen similar changes, for example, in the skeletons of Catholic nuns from the 20th century. They never rode horses, but did take long carriage rides across the American West.

Ultimately, Hosek and Taylor say that human remains on their own can't be used to put a date on when people first started riding horses -- at least not with currently available science.

"Human skeletons alone are not going to be enough evidence," Hosek said. "We need to couple that data with evidence coming out of genetics and archaeology and by looking at horse remains, too."

Taylor added that the picture doesn't look good for the Kurgan hypothesis:

"At least for now, none of these lines of evidence suggest that the Yamnaya people had domestic horses."