Yukon fossil helps scientists pinpoint when bison arrived in North America
Government of Yukon
Scientists have discovered that an unprepossessing fossil found near Old Crow likely belonged to one of the earliest bison that lived in North America.
The new research, published in the journal Proceedings of the National Academy of Sciences, used the 130,000-year-old Yukon fossil to show that steppe bison, the now-extinct ancestor of modern bison, first travelled across the Bering land bridge from Asia between 195,000 and 135,000 years ago.
“Stories like this further reinforce that Yukon is a pretty amazing place to study history,” said Yukon paleontologist Grant Zazula, one of the study’s authors.
The fossil, which Zazula calls a “broken little foot bone,” was first discovered in Ch’ijee’s Bluff along the Porcupine River outside Old Crow in 2006.
It was a good find. Most fossils in that area are found washed up along the riverbank after eroding out of the cliffs, Zazula said. When that happens, it’s hard to gauge how old they are.
But this bone was found where it was deposited eons ago, alongside a bed of volcanic ash that paleontologists know to be about 130,000 years old.
“Being able to find a fossil right alongside this volcanic ash bed provided us a really clear idea of how old the fossil was,” Zazula said.
Still, the researchers didn’t know how significant the find was until a couple of years later. After looking through the literature, they realized it was “probably the oldest, earliest evidence of bison in North America,” Zazula said.
Paleontologists have long been wondering when bison first arrived on the continent. Now, the Old Crow fossil and another 120,000-year-old fossil recently discovered in Colorado have finally provided an answer.
Using DNA analysis, the researchers found that the two fossils were “very, very, very closely related,” Zazula said.
He explained that DNA undergoes mutations all the time. Most of these mutations don’t lead to visible changes, but they do produce small differences in the genetic code.
Tom Patrick/Yukon News
If bison had first come to North America long before these two animals were alive, their DNA would have looked quite different, because they were from two populations that lived far apart.
But their similarity suggests their ancestors were part of the same population not long before.
“So these were probably some of the earliest family members of the founding population in North America,” Zazula said.
The estimated timing of that first crossing — between 195,000 and 135,000 years ago — matches a period when sea level was low and the Bering land bridge existed between what is now Russia and Alaska. There was likely a second group of bison that crossed over much more recently, between 45,000 and 21,000 years ago.
It’s an important finding, according to Zazula, because of the impact steppe bison had after they arrived.
“Once they entered North America, they basically exploded across the continent,” he said. “And they quickly became the dominant large mammal.”
He suspects that bison elbowed their way in amongst the mammoths and ice-age horses that were already here, and may have even caused their decline by out-competing them for food and space.
“They’re very aggressive, they form huge herds … and the population could reproduce rapidly.”
Today, for instance, about 80 per cent of the fossils found in the gold-mining district around Dawson City belong to bison.
But eventually, even the steppe bison met their end. Zazula said records suggest the animals died out in the Yukon about 400 years ago, and it’s not clear why. It’s possible that the Little Ice Age, a period between 1300 and 1870 when the climate cooled suddenly, may have played a role.
It’s possible, too, that humans had an impact. But that doesn’t quite make sense to Zazula.
“There have been people in the Yukon for 15,000 years and bison were living alongside them for most of that time,” he said. “It doesn’t seem likely to me that people would have killed them off 400 years ago.”
So there are unanswered questions, still. As Zazula puts it, “we’re trying to build a puzzle with only a few of the pieces that are remaining.”
But the Old Crow fossil is one major piece of that puzzle. Zazula said future research will focus on the impact bison had on soils and plant communities after they arrived, in part by trampling down shrubs and trees and helping grasslands expand.
“We suspect that there were probably some dramatic environmental effects,” he said.
Zazula believes this type of research can help scientists understand present-day ecology. For instance, the 140 wood bison that were reintroduced to the Yukon in the 1980s have since grown to a herd of 1,500. “That tells us that bison are incredible colonizers,” he said — just like their ancestors.
For William Josie, director of natural resources with the Vuntut Gwitchin First Nation, the new research helps validate oral history, passed down for generations, that tells of Indigenous people living alongside ice-age mammals. He’s not surprised by the results.
“I grew up listening to stories of large animals in our country,” he said. “We always knew we had history with these large animals.”
The First Nation is involved in the paleontological research in Vuntut Gwitchin traditional territory, and Josie said it’s a good opportunity for young people to learn more about their history. The bison fossil and many other fossils found around Old Crow are stored in a facility in the community.
“In my view, it’s really important,” he said. “Not only to validate our history, but just to give a better understanding for the rest of Yukon, the rest of Canada — the rest of the world, for that matter.”