A fragment of a bear skull housed for several years in a Canadian museum may be rewriting the history of North America's brown bears.
Scientists say the skull, which was retrieved from a gravel pit in central Alberta in 1997, is at least 25,000 years old.
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The finding suggests that brown bears, having crossed the Bering Strait from Asia, must have ventured south, deep into North America, far earlier than previously thought.
The discovery also sheds light on the ancestry of modern brown bears. A genetic analysis of the skull fragment indicates its owner was closely related to the brown bears that inhabit southern Canada and the northern United States today.
"It's like finding a missing piece of a puzzle," said Paul Matheus, a paleontologist at the Alaska Quaternary Center at the University of Alaska in Fairbanks. Matheus is the lead author of the report, which is described in this week's issue of the journal Science.
Ice-Free Corridor
Based on the fossil record, researchers say brown bears migrated from Asia to the edge of North America between 50,000 and 100,000 years ago. However, ice sheets were believed to have prevented the predators from moving beyond Alaska and Yukon, then a part of Beringia. (A land bridge formed during the last ice age, Beringia joined Asia to Alaska and northwestern Canada.)
About 13,000 years ago, an ice-free corridor opened up, allowing brown bears to travel south. Until now, the oldest brown bear fossils in southern Canada and the northern United States were about 13,000 years old, an age paleontologists equated with the bears' arrival.
However, continental glaciers only blocked the route south between roughly 23,000 and 13,000 years ago.
"This has always been a mystery," Matheus said. "If brown bears came over to eastern Beringia at least 50,000 years ago, why didn't they go all the way down if there was no ice blocking their way?"
The new skull discovery suggests that brown bears actually did make that journey before the ice sheets arose.
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