How Wild Horses First Became Domesticated: Scientists Sequence Whole Genomes

Most people know that the horses of today are different from the wild horses that they once were. Now, though, researchers are learning a bit more about how horses first became domesticated. They've sequenced the whole genome of modern and ancient horses, revealing that the genes in horses have been selected by humans through the last 5,500 years and what the cost of this domestication has been.

Horses were first domesticated about 5,500 years ago. This helped revolutionize human civilization and societies, since horses were used for transportation as well as the circulation of ideas, languages and religion. These animals were also used for war and agriculture. Yet their use and the encroachment of human civilization resulted in the near extinction of wild horses.

In fact, there's only one surviving wild horse population: the Przewalski's horses from Mongolia, which are descended from just 13 individuals. Because of this loss of genetic diversity, it's been difficult to unravel the effects of horse domestication.

Yet now, scientists may have a few answers. Humans helped shape the horse as it is today. In fact, researchers detected a set of 125 candidate genes involved in a wide range of physical and behavioral traits, by comparing the genomes of two ancient horses with those of the Przewalski's horse and five breeds of domesticated horses.

"We provide the most extensive list of gene candidates that have been favored by humans following the domestication of horses," said Beth Shapiro, one of the researchers, in a news release. "This list is fascinating as it includes a number of genes involved in the development of muscle and bones. This probably reveals the genes that helped utilizing horses for transportation."

Domestication, though, also had negative impacts. Increasing levels of inbreeding were discovered among domesticates. In fact, there was an enhanced accumulation of deleterious mutations in their genomes relative to the wild horses.

"Domestication is generally associated with repeated demographic crashes," said Laurent Excoffier, one of the researchers. "Yet mutations that negatively impact genes are not eliminated by selection and can even increase in frequency when populations are small. Domestication thus generally comes at a cost, as deleterious mutations can accumulate in the genome. This had already been shown for rice and dogs. Horses now provide another example of this phenomenon."

The findings reveal a bit more about how wild horses first became domesticated. In addition, the researchers found that humans likely restocked from wild population descendants during the domestication processes that eventually lead to modern horses.

The findings are published in the journal Proceedings of the National Academy of Sciences.

For more great science stories and general news, please visit our sister site, Headlines and Global News (HNGN).