An international team of researchers unveiled at least 125 genes that aided in the domestication of horses 5,500 years ago. The genes could also help determine the taming process used by humans.

The study published on Monday in the U.S. journal Proceedings of the National Academy of Sciences also discovered that extinct and previously unknown population of wild horses contributed a high number of genes to modern horses.

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These wild horses produced traits so desirable that ancient breeders selected horses to mate with them. Among these traits were as skeletal muscles, balance, coordination, and cardiac strength,  said geneticist and study lead author Ludovic Orlando of the Natural History Museum of Denmark.

Researchers sequenced the genomes of two ancient horse specimens from 29 horse bones found in Russia that dated from 16,000 and 43,000 years ago. This was the period before horses were domesticated.

The study revealed that breeding domestic horses with wild ones, a practice known as restocking, appears to account for at least 15 percent of the domestic horse genome, and possibly up to 60 percent.

It also suggests genes of modern horses were not present in ancient horses and only showed-up because of recent mutations. Among the modern traits is a short-distance "speed gene" that propels every Kentucky Derby winner.

Researchers also found a high level of harmful genetic mutations in modern horses that supported an earlier theory concerning "the cost of domestication."

"Domestication is generally associated with repeated demographic crashes. Yet, mutations that negatively impact genes are not eliminated by selection and can even increase in frequency when populations are small," said co-author Laurent Excoffier, a professor at the University of Bern, Switzerland.

Tagshorses, ancient horses, ancient animals, animal domestication, domestication, genetic mutation, inbreeding, restocking, genes, horse domestication, taming horses