Ancient fossil offers new clues to brown bears past
Fairbanks, Alaska–While nosing around the Quaternary mammal collection at the Provincial Museum of Alberta two years ago, Paul Matheus, a paleontologist with the Alaska Quaternary Center, came across a brown bear fossil that seemed out of place. The fossil had been collected by Jim Burns, curator of Quaternary mammals at the PMA a few years earlier near Edmonton, Alberta, in gravels that date to before the last ice age (older than 24,000 years). If this was true, Matheus thought, it could be a very important find. Burns loaned the specimen to Matheus so he could take it back to the University of Alaska Fairbanks to confirm its age using radiocarbon dating methods. Results showed the bear was indeed about 26,000 years old, and the two researchers realized the fossil's signficance--the history of brown bears in North America would have to be rewritten.
The ancestors of modern brown bears in North America are believed to have migrated from Asia to Alaska and Yukon (then a part of Beringia) between 50,000 and 100,000 years ago, and old brown bear fossils are not particularly uncommon in Beringia. Between roughly 13,000-23,000 years ago, the route from Beringia to areas of the continent further south was blocked by continental glaciers, so brown bears were more or less bottled up in Beringia. The oldest brown bear fossils south of Beringia, in areas like southern Canada and the northern U.S., are about 12,000-13,000 years old, so paleontologists concluded that's when they first arrived.
"It's always been a mystery, though, why brown bears didn't migrate farther south if they were in Beringia as early as 100,000 years ago and the passage south wasn't blocked by glaciers until about 23,000 years ago," said Matheus. "The discovery of the Edmonton specimen indicates that brown bears migrated south much earlier than previously thought."
The recent findings and their implications, are the subject of an article in the Nov. 12 issue of the journal Science titled Pleistocene Brown Bears in the Mid-Continent of North America.
In order to really nail the significance of the find, Matheus and Burns needed one more piece of important information--they needed to know something about the fossil brown bear's genetic identity. So, they brought in colleagues from the University of Oxford and the Max Planck Institute in Germany to sequence mitochondrial DNA from the specimen and assign the bear to one of the known genetic populations of modern and ancient brown bears. This was possible because of a previous collaborative study by Matheus and the Oxford lab using ancient DNA to uncover the population structure of ancient brown bears in Beringia.
"One thing that earlier study could not explain was the ancestry of modern brown bears in the southern part of their range, in places like southern Alberta, British Columbia, Montana, and Idaho," said Matheus. "Those bears belong to a genetic population thought to be extinct in North America for as much as 35,000 years."
Consequently, paleontologists and geneticists have found it difficult to explain where the ancestors of southern brown bears came from when ice sheets retreated about 13,000 years ago--their genetic type did not exist in Beringia at that time. DNA results in the current study show that the new Edmonton specimen belongs to the same genetic group as modern southern brown bears.
The age and genetic identity of this bear mean that brown bears not only made it far south sooner than previously thought, but that those bears in the Edmonton area about 26,000 years ago were very close relatives of southern bears we see today.
"Its like finding a missing piece of a puzzle, or even a proverbial missing link," said Matheus. "Their ancestors must have been stuck south of the ice sheets at the peak of the last ice age, 13,000-23,000 years ago because Edmonton was covered with ice most of that time. That represents a real shift in ideas about brown bear evolution in North America."
Matheus is a research scientist at the Alaska Quaternary Center and a research associate at the Institute of Arctic Biology. Both are located on the University of Alaska Fairbanks campus.
Source: Eurekalert & othersLast reviewed: By John M. Grohol, Psy.D. on 21 Feb 2009
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