Horses had a huge impact on the success of many human societies. Now, scientists have found two key gene variants that helped paved the way for that equine role in human history. The pair made horses tamer and more rideable, researchers now report.

Ancient horse DNA suggests modern domesticated horses came from southwestern Russia more than 4,200 years ago. This research, published in 2021, revealed where and when humans had domesticated the animals. Ludovic Orlando led that study. A molecular archaeologist, he works at the Centre for Anthropobiology and Genomics. That’s in Toulouse, France.

What that work hadn’t shown was precisely what genetic changes in horses — mutations — might have led to these new traits.

Orlando and a team of scientists from China and Switzerland have now done that. They analyzed horse genomes, the full set of genetic instructions making up their DNA. In all, they compared the genomes of 71 horses from a range of breeds and time periods.

The team focused on 266 places in the genomes. From these, nine genes showed strong signatures of have been selected, or altered. That suggests the traits these genes produced in the horses may have been targeted by human breeders.

Two of these genes appear to have been heavily selected very early in horse taming.

Homing in on genes linked to ‘selected’ traits

One gene, ZPFM1, underwent strong selection some 5,000 years ago. That suggests one of the first steps in horse domestication involved selecting for horses with this gene. In mice, ZPFM1 is known to influence anxiety levels. In people, it’s related to overall well-being. In horses, changes to this gene appear to have made the animals tamer.

Another spot in the genome stuck out near a gene called GSDMC. It went through strong selection between 4,700 and 4,200 years ago. Mutations here in humans are linked to back pain. In horses, they are linked with the ratio of body length to height.

To find out more about what this gene does, the researchers ran experiments on mice. (These smaller, more quickly developing animals are easier to study in the laboratory than horses or people.) The mice were genetically modified to “silence” their GSDMC gene. Mice with this gene turned off grew straighter spines and stronger forelimbs. 

The researchers now think changes in GSDMC would have altered how horses move and bear weight. That may have made them more rideable. Over just a few hundred years, one variant of this gene exploded in frequency. It went from barely detectable to present in nearly all horses.

“That means people intended to put that variant more frequently in the [horse] population,” Orlando concludes. Horses with this variant had 20 percent more offspring than those without. “When you see something like that,” Orlando says, “you know you’re onto something that was really a game changer for horse biology.”

Rideable horses were also a key shift for human societies. People could now travel much farther. These long-distance journeys changed the face of war and transportation. 

Stable results

The findings are “really resounding,” says Samantha Brooks. She’s a geneticist at the University of Florida in Gainesville. The appearance of these selected gene mutations, she says, aligns with the existing archeological record of taming horses.

Orlando notes there may be other genes that were missed in the new analysis. Some crucial cultural innovations — such as tricks for training horses — may not have left a mark in the genome.

But a few genetic traits in horses may well have fed into the success of ancient human cultures.

Notes Orlando, “We are sequencing a lot of [ancient] horses to understand what kind of horses [breeders] developed to make the societies we read about in history.”