Submitted/Julius Csotonyi
Ancient horses crossed over the Bering Land Bridge in both directions between North America and Asia multiple times during the Pleistocene, represented in an illustration.

Submitted/Julius Csotonyi Ancient horses crossed over the Bering Land Bridge in both directions between North America and Asia multiple times during the Pleistocene, represented in an illustration.

Links between ancient North American horses and modern species examined in new study

The genetic analysis could strengthen a campaign to consider the horse a native species

Wild horses roaming the forests of the Yukon are considered an introduced species — their most recent ancestors from Europe — but a new research paper based on ancient DNA is revealing that they may have shared many genes with Beringian ancestors.

“We have mustangs in North America today, so what are they? They are domestic horses that were brought here by Europeans,” explained Alisa Vershinina, a postdoctoral scholar working at UC Santa Cruz, who recently presented her research during a talk at the Beringia Centre.

“Horses in North America went extinct around 11,000 years ago and the mustangs that we see here today are sometimes considered an invasive species. They are considered absolutely not from here and something different from the horses that were inhabiting this place back in the day.

“This research shows that might not actually be the case. Eurasian horses were present in Alaska and present in the Yukon, and it weakens the argument that mustangs are invasive.”

Vershinina was the lead author on a new paper published in the journal Molecular Ecology on May 18.

By looking at ancient horse DNA retrieved from samples across the northern hemisphere – including fossils found in the Yukon – researchers discovered that the early horses who roamed North America and Eurasia may have intermingled more than originally thought.

If modern horses share the same genetic lineage as ancient Yukon horses, as the paper suggests, it’s been quite the journey home for a population that mysteriously disappeared from the continent around the end of the last ice age.

The researchers examined DNA records from hundreds of ancient horses. Each of the genetic samples also has a date and a location, allowing researchers to track their movement and evolution.

The new study suggests that early horses moved back and forth between Asia and North America over thousands of years when the two continents were connected by a land bridge.

While a prevailing view of the horse evolution was that two separate and distinct species developed in Asia and North America — the American species eventually going extinct — the new research paper suggests that both regional populations interbred freely and shared genetic material.

A separate North American species would have been an evolutionary dead end, but the paper shows instead that ancient Yukon horses might not be an entirely separate branch of the family tree after all.

Vershinina notes that there isn’t an extreme diversification of genes between the two populations.

“There may not have been as many species as people think,” she said.

Paleontologist Aisling Farrell holds a mummified frozen horse limb recovered from a placer gold mine in the Klondike goldfields in Yukon Territory, Canada. Ancient DNA recovered from horse fossils reveals gene flow between horse populations in North America and Eurasia. (Submitted/Yukon Government)

Why the North American population disappeared is still a mystery for scientists to figure out. Yukon government paleontologist Grant Zazula, also a co-author on the paper, said the well-preserved fossils found in the territory are helping researchers build theories about ancient species.

“It’s exciting because it shows how important the Bering Land Bridge was for the continuity of these populations and all that was happening in the Yukon,” said Zazula.

“It also further raises the question of why they went extinct in North America but not on the other side of the Bering Land Bridge. That’s another question to tackle,” he said. “The fact that they prevailed in Asia is really the only reason why we have them today, because if not, they would have died out at the end of the Ice Age like the woolly mammoth. But horses, for some reason, survived. Luckily they did, because they were later domesticated.”

Domesticated horses have played such a pivotal role in human history that it’s a shame that wild horses are now viewed by many people across the continent as a pest species that doesn’t belong and needs to be disposed of, said Ross MacPhee.

MacPhee is a paleontologist at the American Museum of Natural History and another co-author on the paper. He is also the scientific advisor for the Cana Foundation, an organization based in the United States that advocates for the protection of wild mustangs.

He said the new research will contribute to recent efforts to have the modern horse declared a native species, rather than an invasive one. If the American government recognizes the species as wildlife it would likely change how the animal is managed.

“The bottom line is that horses survived in northern North America and places like Yukon up until comparatively recently, maybe just a couple of thousand years or so before the Europeans showed up with their own horses. To any rational person that should be sufficient to indicate that horses are still part of our fauna,” he said.

“They had the misfortune to disappear for a while, but that’s an effective case for citizenship, as I like to put it,” he said.

MacPhee compared the wild horses to another iconic American mammal — the bison — whose evolutionary ancestors are actually from Asia. While modern horses may have managed to survive on that same continent, their genetic origins are actually in North America.

“If there’s anything that should be regarded as native among mammals around here, it should be horses,” said MacPhee.

Contact Haley Ritchie at haley.ritchie@yukon-news.com

Science

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