Brown bears migrated south through Canada much earlier than first thought, Science magazine has reported.
The find clears up some niggling inconsistencies in the history of brown bears
The bears reached Alaska about 50,000 to 100,000 years ago, after crossing a land bridge from Asia.
But, according to traditional wisdom, thick ice blocked their route into Canada until roughly 13,000 years ago.
Now, new fossil evidence suggests brown bears actually made their way through Canada more than 25,000 years ago, well before ice sealed their path.
Land of opportunity
Fossilised brown bear fragments dating back about 25,000 years have been uncovered in Alberta, Canada, for the first time.
The remains prove that brown bears must have made it through into mainland Canada before heavy ice closed the corridor from Beringia about 23,000 years ago.
Beringia is the name given to a land bridge that emerged periodically to connect Asia and America.
"This is the first fossil to be found in the region that is older than 13,000 years old," said co-author Jaco Weinstock from Oxford University.
The find is particularly important because it clears up some niggling inconsistencies in the history of brown bears.
Unravelling their past movements has been a frustrating task for researchers. Until now, it has been a tale that just did not add up.
There is clear evidence that bears crossed Beringia and continued to live in the region of the land bridge until about 35,000 years ago.
But, until now, the fossil record suggested they did not venture any further south for thousands of years. Palaeontologists hunting for bear fossils in Canada and the US came up with nothing older than 13,000 years old.
"Because people did not find any fossil remains older than 13,000 years old, they assumed that the bears had not migrated south," explained Dr Weinstock.
Bears reached North America about 50,000 to 100,000 years ago
But the theory that the bears stayed in Beringia was never completely satisfactory.
Firstly, ice did not clog up the route until about 23,000 years ago, so they had a clear passageway south for many thousand years.
Secondly, the genetics of modern brown (grizzly) bears living in southern Canada make the traditional theory particularly unconvincing.
These grizzlies are apparently all descended from a particular clade of brown bears, called clade four.
Mitochondrial evidence shows that clade four bears were present in Beringia before 35,000 years ago. But then they disappeared from the region, along with all other clades of brown bear.
When the bears recolonised Beringia from Asia about 21,000 years ago, clade four was not among them. So the bears pushing south from the region when the ice cleared 13,000 years ago did not contain the ancestors of modern bears living in southern Canada.
Unravelling the story of ancient brown bears has been a frustrating task for researchers
If this was indeed the first major migration south, scientists found it very hard to explain how all modern grizzlies in southern Canada could have been descended from clade four.
The new evidence clears up the problem, because it suggests clade four bears were already in Canada when the ice closed the access route 23,000 years ago.
However, they apparently did not venture very far south for a long time, which explains why the early fossil record is thin.
"Once they arrived in Canada they did not migrate further south for some reason," said Dr Weinstock. "They lived just south of the ice in what was probably quite a small population.
"Nobody quite knows why they did not move south. It could be that food was not available, but we just don't know. It is very strange."