All the world's wild horses have gone extinct, according to a study Thursday that unexpectedly rewrites the horse family tree based on a new DNA analysis of their ancestry.
What most people thought were the last remaining wild horses on Earth -- known as Przewalski's horses -- were actually domesticated horses that escaped their owners, said the report in the journal Science.
"This was a big surprise," said co-author Sandra Olsen, curator-in-charge of the archeology division of the Biodiversity Institute and Natural History Museum at the University of Kansas.
"This means there are no living wild horses on Earth -- that's the sad part," said Olsen.
The study is based on archaeological work at two sites in northern Kazakhstan, called Botai and Krasnyi Yar, where scientists have found the earliest proof of horse domestication, going back more than 5,000 years.
To further dig into these roots, international researchers sequenced the genomes of 20 horses from the Botai -- based on teeth and bones unearthed from the sites -- and 22 horses from across Eurasia.
Then, they compared these ancient horse genomes with already published genomes of 18 ancient and 28 modern horses.
They discovered that Przewalski's horses descended from the earliest-known domesticated horses, kept by the Botai people of northern Kazakhstan some 5,500 years ago.
That means what people thought were wild horses were actually feral, meaning they had escaped from domestication but were not originally wild.
A new quest
According to Beth Shapiro, a professor of ecology and evolutionary biology at the University of California Santa Cruz, the findings are "super interesting."
"Certainly, it is surprising to see that Przewalski's horses are descended from early domestic horses, as this is not what people tended to believe," Shapiro, who was not involved in the study, told AFP.
"Swapping the word 'wild' for 'feral' is a semantic change that may better reflect their evolutionary history but should not change their status. We should continue to protect Przewalski's horses as a population of wild horses."
Przewalski's horses are considered an endangered species by the International Union for Conservation of Nature.
The round-bellied, short-legged, reddish brown to beige horses roamed Central Asia, Europe and China in prehistoric times.
They were listed as extinct in the wild in the 1960s, but a number of breeding programs and reintroductions have helped bolster their numbers.
The findings have also sparked a new quest -- to uncover the true origins of today's domestic horses.
"Current models suggest that all modern domesticated horses living now descend from those first tamed in Botai, in the north of present-day Kazakhstan," said a statement from the French National Center for Scientific Research (CNRS) scientist Ludovic Orlando.
"Yet this genomic analysis yielded unexpected results."
Since the Botai horses did not give rise to today's domesticated horses, "the origin of modern domestic horses must be sought elsewhere."