An ancient bone found eroding out of a riverbank in the Yukon is the oldest bison fossil ever discovered in North America, researchers say.

What’s more, genetic evidence extracted from the bone reveals that all of the continent’s bison, living and extinct, descended from a single female ancestor that crossed into the Americas less than 200,000 years ago — and not millions of years ago, as some experts had thought.

“There has long been a controversy about the timing of bison arrival in North America,” said Dr. Beth Shapiro, of the UC Santa Cruz Paleogenomics Lab, in a press statement.

“Until recently, the fossil records from different parts of North America disagreed with each other, with a few fossil localities suggesting that bison arrived millions of years ago but most old fossil sites showing no evidence of bison at all.”

The newly found fossil is limited to a single foot bone, found protruding from a riverside cliff known as Ch’ijee’s Bluff in western Yukon Territory along the Porcupine River, not far from the Alaska border.

The exposed strata of the bluff allowed the researchers to date the bone to around 130,000 years old.

“We were fortunate with a fossil recovered from northern Yukon that was found adjacent to a well-dated volcanic ash,” said Dr. Alberto Reyes, the University of Alberta geologist who made the find.

Analysis of the fossil showed that it belonged to a now-extinct species known as the steppe bison, or Bison priscus, an ungulate that roamed Europe and Asia for millions of years.

And its discovery provided a unique opportunity to compare the genetics of this ancient steppe bison with those of nearly four dozen other fossils from around the continent, which together could help reconstruct North America’s bison family tree.

The first step was to isolate the bone’s mitochondrial DNA — the genetic code that’s found inside each cell’s mitochondria.

This is distinct from nuclear DNA, which contains the instructions for making an entire organism.

By contrast, mitochondrial DNA is only passed down from the mother to her offspring, which allows researchers to trace lineages by birth, going back millennia.

(Learn more about what mito DNA can teach us about human history: “Elite ‘Dynasty’ at Chaco Canyon Got Its Power From One Woman, DNA Shows“)

The scientists compared the mitochondrial DNA from the fossil found at Ch’ijee’s Bluff to that taken from 45 other bison remains, including one of the oldest and most interesting specimens, the fossil of a giant, long-horned bison — belonging to the species Bison latifrons — found in Snowmass, Colorado.

“Bison latifrons is an interesting beast,” said Dr. Duane Froese, a geologist with the University of Alberta, in a separate statement.

“Its horns measured more than two meters across at the tips, and it was perhaps 25 percent larger than modern bison.”

(Read about a long-horned bison recently found in San Diego: “Giant Ice-Age ‘Longhorn Bison’ Unearthed in San Diego“)

The Colorado specimen was a mere 15,000 years or so younger than the one from Ch’ijee’s Bluff, the researchers said, but their analysis showed that the two were indeed distinct species, although they shared a very close genetic relationship.

Moreover, the comparison across all 45 specimens allowed the researchers to estimate the timeline of bison evolution in North America, by tracking the mutations that have periodically shown up in the animals’ DNA.

The rate at which mutations take place can serve as a kind of “molecular clock,” the researchers said, which provides an estimate of how old a particular maternal lineage is.

And the results revealed that all the bison’s mitochondrial DNA stemmed back to a single female who made her way into the Americas very — at least in geologic terms — recently.

“[It] showed that all of the bison had a common ancestor between about 130,000 and 195,000 years ago,” Froese said.

These dates seem likely, the researchers noted, because they coincide with peaks during the last Ice Age, when sea levels were low enough to expose the Bering Land Bridge between what’s now Alaska and Russia, which would have made migrations possible.

“The land bridge forms during ice ages, when much of the water on the planet becomes part of growing continental glaciers, making the sea level much lower than it is today,” Shapiro said.

After the bison arrived, the rest is natural history.

North America had not seen such giant bovines like bison before, and the animals quickly made their mark on the landscape.

“After they arrived in Alaska, they spread quickly across the continent, taking advantage of the rich grassland resources that were part of the ice age ecosystem,” Shapiro said.

“They were so well adapted to the plains ecosystem, they quickly colonized the continent.” Froese added.

(Read about another record-breaking fossil: “700,000-Year-Old Horse Found in Yukon Permafrost Yields Oldest DNA Ever Decoded“)

“In a way, North American bison are really an invasive species, though perhaps not in the common sense of the term,” he noted.

“They were remarkably successful when they entered North America and interrupted a grazing ecosystem that had been dominated by horses and mammoths for at least the previous million years.”

The team reports its findings in the Proceedings of the National Academy of Sciences.