Modern polar bears are genetically related to an extinct species of Irish brown bear, according to a study published Thursday in Current Biology.  The two share a distinct genetic marker not present in other brown bears, strongly suggesting that all modern polar bears are descended from the same extinct species.
The study found evidence of polar bear hybridization with a now-extinct population of brown bears that lived in Ireland and Britain during the last ice age. Brown bears are no longer found on the British Isles, and it had been thought that polar bears interbred more recently with brown bears living around Alaska. However, the samples used in the analysis came from the past 120,000 years and show that interbreeding of the two species occurred a great deal earlier. Climate change is thought to have presented opportunities for the two species to mate now and then in the past 100,000 years. 
The evidence comes from a DNA analysis of fossil bones collected from the teeth and bones of seventeen bears from eight Irish caves. Ten Irish brown bears from between 10,000 and 38,000 years ago carried a distinct sequence of mitochondrial DNA, originating from a specific female brown bear, and this DNA sequence has been passed down the female polar bear line and is found in all polar bears today. It is not found in modern bears from Europe.
Fossils from the last species of ancient brown bears in Ireland, living from 3,000 to 5,000 years ago, have a different genetic fingerprint, matching neither that of polar bears nor of modern brown bears.
—Authors, Ancient Hybridization and an Irish Origin for the Modern Polar Bear MatrilineA member of the international team conducting the study, Dr Ceiridwen Edwards from Oxford University, said: "Hybridisation between ancient Irish brown bears and polar bears has led to the complete replacement of the original polar bear mitochondria. This maternal lineage is now present in all modern polar bears."
The authors of the paper conclude that their evidence shows that matrilineal history of brown and polar bears suggest that "a likely genetic exchange with extinct Irish brown bears forms the origin of the modern polar bear matriline." They suggest that hybridization may be more common than previously recognized and may be a way species deal with loss of their  habitats when the environment is changing.

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Polar bears related to extinct Irish bears, DNA study shows