Origins of modern horses traced to breeding revolution 4200 years ago

Ancient breeders dramatically shortened the natural generation times of horses starting about 4200 years ago, according to a genetic study of hundreds of ancient horses. This intensive breeding led to a massive expansion of those bloodlines across Eurasia within a few centuries, says Ludovic Orlando at the Centre for Anthropobiology and Genomics of Toulouse, France.

“In other words, they controlled the reproduction of the horse,” he says. “So this tells us something about the process of breeding that was underlying the success of the expansion of horses around the world.”

Horses were first domesticated 5500 years ago by the Botaï people in what is now Kazakhstan, but they didn’t spread their horse culture elsewhere, says Orlando. The Botaï eventually died out and their horses returned to the wild.

More than a thousand years later, however, a different line of horses became domesticated in the Pontic-Caspian steppes of southern Russia. It was this line that ultimately spread across the planet, leading to every domestic horse in the world today, he says.

To chart the history of horse husbandry, Orlando and his colleagues analysed the genomes of 475 ancient horses from Eurasia dating up to 50,000 years ago. They compared those with the genomes of 71 modern domestic horses representing 40 breeds worldwide, as well as six endangered Przewalski’s horses – which are a different sub-species.

The team confirmed that horses prior to the third millennium BC weren’t being bred or domesticated – except among the Botaï. This means horses didn’t contribute to human migrations and cultural expansions before that time, contrary to some theories, says Orlando.

The DNA analysis revealed significant inbreeding 4200 years ago in the Pontic-Caspian steppe horses, probably because people aimed to develop specific traits that make high-quality riding and chariot horses, he says.

Then, using a new technique combining genome sequencing and carbon dating, the scientists were able to estimate the average number of years between two successive generations, which Orlando calls the generational time interval. That interval got remarkably shorter – half as long as in the wild – during the same period of massive inbreeding in the Pontic-Caspian steppes.

“Right at the time of the domestication bottleneck, around 2200 BC, this is when breeders managed to control the reproduction of the horse so much that generations were ticking faster and faster,” says Orlando.

Orlando suspects the breeders were probably shortening generations by having them mate at younger ages than they would in the wild, he said at the International Havemeyer Foundation Horse Genome Workshop, which took place last month in Caen, France.

Christine Aurich at the University of Veterinary Medicine Vienna suspects the shortened generations were probably due to better survival rates rather than younger breeding ages. Horses give birth lying down in open grasslands, making them highly susceptible to predators until the foal can run, several hours later. Plus, any disturbances could prevent the foal from drinking its first milk – which always leads to death.

“It must be assumed that for horses living in the care of humans, losses of mares and their newborn foals were considerably reduced in comparison to horses living under wildlife conditions,” says Aurich.