Climate change likely drove this mixing among bears, writes the research team, noting there is evidence this is happening again.
"Maybe we're seeing a hint that in really warm times, polar bears changed their lifestyle and came into contact, and indeed interbred, with brown bears," said study researcher Stephan Schuster, a professor of biochemistry and molecular biology at Pennsylvania State University, and a research scientist at Nanyang Technological University in Singapore, in a statement.
The study estimates polar bears split from brown bears between 4 million and 5 million years ago, after which they endured fluctuations in climate, including ice ages and warmer times.
Polar bears are currently facing the effects of climate change, this time caused by humans, as the Arctic sea ice upon which they live recedes to unprecedented levels.
"If this trend continues, it is possible that future [polar bears] throughout most of their range may be forced to spend increasingly more time on land, perhaps even during the breeding season, and therefore come into contact with brown bears more frequently," the researchers write in results published today (July 23) in the journal Proceedings of the National Academy of Sciences.
"Recently, wild hybrids and even second-generation offspring have been documented in the Northern Beaufort Sea of Arctic Canada where the ranges of brown bears and [polar bears] appear to overlap, perhaps as a recent response to climatic changes," they write.
Schuster and colleagues sequenced genomes (the complete genetic blueprint) of three brown bears and a black bear and compared them with the genomes of polar bears, one modern and the other obtained from remains from a 120,000-year-old polar bear.
Read more at Discovery News
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