Two key gene variants may have made early domesticated horses more tame and more physically resilient to bearing a rider, researchers report August 28 in Science. The resulting horses were among the most significant advances in Bronze Age biotechnology.

Ancient horse DNA suggests modern domesticated horses originated in southwestern Russia more than 4,200 years ago, Ludovic Orlando and his colleagues reported in 2021. While this revealed the where and when for the domestication of horses, says Orlando, a molecular archaeologist at the Centre for Anthropobiology and Genomics of Toulouse in France, there were still unanswered questions about precisely what horse genes changed in those early populations.

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Orlando and a team of scientists from China and Switzerland analyzed the genomes — the full set of genetic instructions — from 71 horses from a range of breeds and time periods. The researchers focused on 266 places in the horse genomes to track the history of these genes from the early domestication process onward. Of these, nine genes showed strong signatures of selection, meaning the traits they produced in the horses may have been targeted by human breeders. 

Two of these genes were particularly interesting because they showed heavy selection very early on in horse domestication. One gene, ZPFM1, influences anxiety levels in mice and overall well-being in humans. ZPFM1 underwent strong selection some 5,000 years ago, suggesting that one of the first steps in horse domestication involved making the animals tamer.

Another location in the genome, near a gene called GSDMC, experienced strong selection a bit later, between about 4,700 and 4,200 years ago. Mutations at this spot in humans are associated with chronic back conditions and pain. In horses, they are linked with body length-to-height ratio. The team ran experiments on mice genetically modified to have GSDMC inactivated, and found the mice had straighter spines and stronger forelimbs. 

Orlando and his colleagues think changes to GSDMC would have altered how horses move and bear weight, possibly making them more suitable steeds. Over just a few hundred years, a variant of this gene exploded in frequency and went from barely detectable to present in almost all horses.

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

Rideable horses were also a pivotal shift for human societies, setting the stage for far greater mobility and changing the face of war and transportation. 

The findings are “a really resounding case of circumstantial evidence,” says Samantha Brooks, a geneticist at the University of Florida in Gainesville. “We know from our archeological record that we can begin to see the types of changes that indicate these horses were used during domestication and then we can simultaneously see these strong shifts in the genome itself at two very specific locations.”

While GSDMC appears important to the rise of horse riding, Orlando notes there may be other genes that were missed in their analysis, or crucial cultural innovations — such as tactics for interacting with horses — that didn’t leave their footprints in the genome.

Orlando is interested in how particular genetic traits in horses may have fed into the success and development of horse-fueled steppe empires in Mongolia and China.

“We are sequencing a lot of those [ancient] horses to understand what kind of horses those societies and states developed to make the societies we read about in history.”