Roughly 4,500 years ago, humans forged a bond that would shape the history of our species. The domestication of horses led to significant advancements in transportation, hunting, and warfare, literally carrying human society into the modern era.
Despite how influential horses have been for humans, scientists still have many fundamental questions about their domestication. New research published Thursday, August 28, in the journal Science, offers new insight into the genetic shifts that helped them become tame and rideable. The findings not only elucidate the history of a critical human-animal bond but could also help guide horse breeding and conservation efforts today, according to the researchers.
“By traveling back in time, you [can see] the impact of such breeding practices on the modern world,” co-author Ludovic Orlando, research director at the French National Centre for Scientific Research and head of the Anthropobiology and Genomics Center of Toulouse, told Gizmodo.
Clues hiding in ancient DNA
Orlando’s lab has spent the past 15 years meticulously sequencing ancient horse genomes dating back almost a million years ago and up to the 19th century. For this study, he and his colleagues analyzed hundreds of genomes from the 6,000-year domestication period. They identified 266 genetic markers linked to key traits such as behavior, coat color, body shape, locomotion, athleticism, and disease susceptibility to see how selective breeding influenced them over time.
The researchers found that about 5,000 years ago, during the early stages of horse domestication, selective breeding favored a genetic region (or locus) associated with the ZDPM1 gene. This gene is a known behavior modulator in mice, suggesting that taming was one of the earliest steps toward the domestication of horses.
About 250 years later, humans began intensively selecting for the GSDMC locus, a DNA region linked to body confirmation in horses and spinal anatomy, coordination, and strength in mice. The timing of this aligns with the “domestication bottleneck,” when increasingly selective breeding led to a sharp reduction in genetic diversity.
“The selection was not only very strong, but the timing was bang on,” Orlando said.
Rewriting the domestic horse’s history
To further validate the function of the GSDMC locus, co-author Lin Jiang, a professor at the Chinese Academy of Agricultural Sciences and longtime collaborator of Orlando, modified this DNA region in mice. She and her colleagues then used CT scans to look for physiological changes and conducted strength and coordination tests.
The modified mice proved to be much stronger and displayed enhanced locomotion, reflecting changes in the horse phenotype that supported riding or carrying heavier loads on their backs, Jiang told Gizmodo.
Demonstrating these biological effects and linking them to the point during selective breeding that immediately preceded the globalization of domesticated horses was “mind-blowing,” Orlando said. What’s more, the findings contradict a longstanding hypothesis that humans started domesticating horses by selecting for diverse coloration, making it easier to tell them apart.
“What was really surprising to me in our data is that we don’t see evidence of color being the trigger in the very beginning,” Orlando said.
Unraveling the genetic changes that shaped equine domestication also helps scientists understand their consequences for horses today. “We have realized that about 16% of the genetic diversity of horses has been lost in the last 200 years, simply because of the way we have bred those animals since the 19th century,” Orlando said. Ancient DNA analysis, he added, can tell scientists a lot about history but can also guide modern breeding practices.