A breeding revolution 4,200 years ago shaped the origins of the modern horse.

A genetic study of hundreds of ancient horses suggests that ancient breeders dramatically shortened the horse’s natural development period, starting around 4,200 years ago. This intense breeding allowed the lineage to rapidly expand across Eurasia within a few centuries, according to researchers led by Ludovic Orlando at the Centre for Human Biology and Genomics in Toulouse, France.

“In other words, they controlled horse breeding,” he says, “so this tells us something about the breeding processes behind the success of horse breeding around the world.”

Horses were first domesticated 5,500 years ago by the Botai people in what is now Kazakhstan. The Botai, however, did not spread their horse culture to other regions and eventually went extinct. Horses released back into the wild.

More than 1,000 years later, a different lineage of horse was domesticated in the Pontic-Caspian steppes of southern Russia. This lineage eventually spread worldwide, giving rise to all the domesticated horses we see today, according to Orlando.

To trace the history of horse domestication, Orlando and his team analyzed the genomes of 475 ancient horses dating back 50,000 years in Eurasia. They compared these genomes with those of 71 modern domestic horses representing 40 breeds from around the world, along with six species of the endangered mullein genus (a separate subspecies).

The research found that, except for the Botai, horses were not domesticated before the third millennium BCE, indicating that horses did not play a significant role in early human migration or cultural expansion, as previously suggested, Orlando explained.

DNA analysis showed that horses in the Pontic-Caspian steppe underwent significant inbreeding around 4,200 years ago, likely in an effort to develop specific traits for high-quality riding or chariot horses, according to Orlando.

Through a combination of genome sequencing and carbon dating, scientists estimated that the average time between two successive horse generations, called the generation time interval, was significantly shortened during the same period of inbreeding in the Pontic-Caspian steppes, halving the interval seen in the wild.

“During the domestication bottleneck around 2200 BCE, breeders were able to control horse reproduction so well that generations became faster and faster,” Orlando said.

Orlando suggests that breeders may have achieved this shortening of generation times not by breeding horses at a younger age, but by increasing survival rates. Unlike wild horses, horses in human care are less susceptible to deaths among mares and newborn foals, as they are protected from predators and disturbances that could jeopardize their survival, according to researchers at the University of Veterinary Medicine in Vienna led by Kristin Orlich.