Mankind didn't do horses any favors by domesticating them, at least not genetically; selective breeding by humans created more harmful variations in the DNA of horses, a study suggests.

More of those harmful changes are found in modern domesticated horse than existed in ancient wild breeds, the researchers say.

Because modern domesticated horses have no surviving living ancestors, Ludovic Orlando of the University of Copenhagen and international colleagues sequenced the genomes a 16,000-year-old horse and a 43,000-year-old horse from fossil bones excavated in Russia, with their DNA preserved by the arctic conditions where they were unearthed.

Both of those wild horses lived long before the domestication of horses began around 5,500 years ago.

Domestic horses have more harmful genetic variants than the 43,000-year-old example did, the researchers say, calling it the "cost of domestication" as humans have overridden natural selection by picking animals with desirable traits (as humans consider them) and unwittingly selecting damaged genes.

That domestication revolutionized human civilizations and societies by facilitating transportation that drove the global circulation of ideas, religions and languages.

However, that domestication of horses and subsequent encroachment of human civilization led to the near extinction of wild horse populations, the researchers say.

That makes studies of the genetic history of modern horses difficult, the researchers point out, since the only surviving population of wild horses, Przewalski's horses of Mongolia, descends from just 13 individuals, surviving only by dint of a massive conservation effort.

That has led to a significant loss of genetic diversity, making horse domestication through time hard to trace on a molecular DNA level, the researchers point out, which is why they had to turn to ancient fossils to see what pre-domesticated horses looked like genetically.

By comparing the genomes of the two ancient horses with those of the Przewalski's horse and five breeds of domesticated horses, the researchers were able to detect a set of 125 candidate genes thought to be at play in a range of physical and behavioral traits.

"This list is fascinating as it includes a number of genes involved in the development of muscle and bones," says Beth Shapiro, head of the Paleogenomics Lab at the University of California, Santa Cruz. "This probably reveals the genes that helped utilizing horses for transportation."

The study has been published in the Proceedings of the National Academy of Sciences.

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