Polar bears have been chilling on the ice far longer than is generally thought, new research suggests, and they probably interbred with brown bears at one point after the two species separated. The new German study contradicts data from a study published last July in the journal Current Biology that suggested polar bears separated from brown bears150,000 years ago. The new study analyzed the bears' mitochondrial DNA, a special "additional genome" that lives in the cell's energy factories and is passed down only from the mother. The new study concludes that the bears became separate species closer to 600,000 years ago.
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