oldest known bison fossil in North America was discovered
at Chijees Bluff along the Porcupine River near
Old Crow, Yukon. The fossil was below a layer of 130,000-year-old
volcanic ash, which allowed scientists to date it. (Alberto
Reyes / University of Alberta)
After humans, the mammals most successful at colonizing North
America were the bison that thundered across the Great Plains.
Just when they arrived on the continent from Asia, however,
has long been a mystery.
Now genetic and geologic information has helped pinpoint the
time of their migration across the Bering Land Bridge. Bison arrived
between 135,000 and 195,000 years ago, a new
study finds. Humans were believed to have been much more recent
travelers, with the first wave of migrants about 20,000 years ago,
though that arrival time is facing increasing contention among experts.
After the first bison migrations, the animals multiplied, diversified
and became the dominant grazers, displacing mammoths, Pleistocene
horses and other mammals that arrived earlier, said Duane
Froese of the University of Alberta, lead author of the study.
"They became very successful very quickly," Froese
said. "Outside of humans, it's pretty much the most successful
mammal invasion into North America."
The study is published in the Proceedings of the National Academy
Its findings rely in large part on DNA extracted from the oldest
known bison fossil, a bone found a decade ago near the Gwich'in
village of Old Crow in Canada's Yukon territory.
view of Chijees Bluff shows the Yukon site where
North Americas oldest known bison fossil was found.
(Alberto Reyes / University of Alberta)
The Yukon fossil was found near the Porcupine River. It was
just below a layer of ash that was spread in a massive eruption
about 130,000 years ago from a southwestern Alaska volcano that
has since been transformed into what is now called the Emmons
Lake Volcanic Center.
The ash spread by the eruption is a time marker in the earth
that allowed scientists to determine the Yukon bison fossil's age
and confirm it as the oldest known bison fossil in North America,
Froese said. Radiocarbon dating does not work beyond ages of about
40,000 to 50,000 years, so scientists needed other methods to determine
the date, he said.
Thanks to new technology, DNA from the Yukon fossil was extracted
and analyzed, he said. It was compared with that from a slightly
younger fossil found near Snowmass, Colorado, and both were compared
with dozens of younger bison fossils, including some from Yukon
soil above the volcanic ash, where there were "lots and lots
of bison all over the place," Froese said.
Bluff is the site of the oldest known bison fossil found in
North America, a specimen from a steppe bison. (Alberto Reyes
/ University of Alberta)
From that genetic material, the scientists determined that all
the bison had a common ancestor 135,000 to 195,000 years ago, a
period when the Bering Land Bridge was exposed, he said.
The genetic information also showed that bison migrated over
the land bridge in a second wave 21,000 to 45,000 years ago, the
Once bison were in North America, they quickly developed into
new genetic forms. The first species on the continent the
species that left the Yukon bone was the steppe bison, Froese
said. The Colorado bison fossil, though only about 10,000 to 20,000
years younger, was of a different but related species, the giant
long-horned bison, he said. That is a species known for its huge
size, with fossils not found north of the Lower 48 states, he said.
Steppe and giant long-horned bison are Pleistocene species now
extinct, but modern bison are
The study results answer what has been a nagging question: Why
do many assemblages of fossils made up of bones from mammoths, horses,
rodents and other animals not include bison?
"It's been kind of a long puzzle in the world of paleontology
and paleobiology," Froese said.
Yukon wood bison (Yukon government photo)
About this Author
Yereth Rosen has been a journalist in Alaska since 1987. For most
of that time, she was the sole Alaska-based reporter for Reuters.
She has been reporting on energy issues, the environment, politics
and all things Alaska ? from oil spills to sled-dog races. She enjoys
running, skiing and other outdoors pursuits. She lives in Anchorage
with her family.