What Made Horses Rideable

When humans began to charge across plains and mountains on the sleek backs of horses, it changed the course of history. The earliest definitive evidence of equestrianism dates to 4,000 years ago: In the Ural Mountains of Russia, archaeologists unearthed the remains of bridles and chariots. But some scholars say humans began relying on horses for transport a couple of millennia earlier than that, in the Eurasian steppes near the Black Sea.

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Regardless of when this horse-human bond first emerged, it soon promoted the sharing of culture, language, and commerce between people from distant lands and transformed agriculture and warfare.

Now a team of researchers from France, China, and Switzerland say they have identified the specific genes in horse lineages that may have made it possible for humans to start taming and mounting these muscular animals. The genes they identified began to appear around 5,000 years ago, and are involved in temperament, movement, and the way horse bodies are shaped. The researchers published their results in the journal Science.

Understanding horse evolution helps us understand ourselves.

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“These genetic changes allowed horses to become rideable and fast-moving, which transformed human societies by accelerating transport, warfare, and cultural exchange,” says lead author Xuexue Liu, a postdoctoral researcher affiliated with the Centre for Anthropobiology and Genomics of Toulouse in France. “In short, horse genetics and human social development co-evolved in a mutually reinforcing process.”

Liu and colleagues analyzed ancient horse DNA collected from archaeological sites, tracking how 266 trait-associated genetic markers changed over the period when humans were breeding them. One pair of genes known as GSDMC and ZFPM1 also appear in other animals, such as mice, which helped researchers isolate and observe the effects of the genes. GSDMC is linked to spinal anatomy, motor coordination, and strength in mice, they found, while ZFPM1 is associated with anxiety and learning.

Understanding horse evolution helps us understand ourselves, said William Taylor, an archaeozoologist at the University of Colorado Boulder, who was not involved with the study.

“I think this study shows very clearly that horse domestication was paired with, and probably motivated by, a desire for horse transport,” Taylor wrote in an email. “The first horse herders were raising horses with only a few things in mind, namely their behavior—friendliness, aggression—and their movement/roles in transport.”

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Selective pressures for characteristics such as horses’ heights appears later in their genetic record, which suggests to Taylor that earlier people were not breeding for specific coat colorings or other traits important to contemporary riders.

“These finds suggest that early horse transport—relying heavily on chariot teams—was probably much different than the kind of mounted riding we know today, with different values and different logistics,” said Taylor.

Taylor also pointed out that the findings of the study suggest that humans had little relationship with horses before 3,000 B.C. and that in those early years, humans used them almost exclusively for transport. “That’s exciting, and points us toward a very different understanding of horse domestication than has been the status quo.”

In a related Perspective, also in Science, Laurent Frantz, who studies zooarchaeology and evolutionary genomics at Oxford University, pointed out the impressive significance to history of the very first horseback riders—and these tiny bits of DNA the researchers identified.

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“Although the precise circumstances and the cultural identity of the people responsible for this early, intensive breeding remain a mystery, they must have had the necessary ingenuity, technology, and foresight,” writes Frantz. “What is certain is that these first riders kick-started a revolution that changed the world, demonstrating how the immense currents of history can turn on the smallest of biological changes.”