The maternal ancestors of modern polar bears were from Ireland, according to a DNA study of ancient brown bear bones.
Scientists in the UK, Ireland and the US analysed the teeth and skeletons of 17 brown bears that were found at eight cave sites across Ireland.
The new research has been reported in the latest edition of Current Biology.
Previously, it was believed that today's polar bears were most closely related to brown bears living on islands off the coast of Alaska.
However, analysis of mitochondrial DNA - which is passed from mother to child - has shown the extinct Irish brown bears are the ancestors of all today's polar bears, the scientists said.
Their work provides evidence of the two species mating opportunistically during the past 100,000 years or more.
Hybridisation has been recorded recently in the wild where grizzly bears have encroached on polar bear territories.
The bears split from a common ancestor to become separate species between two million to 400,000 years ago.
However, just before or during the last Ice Age the two species came together and polar bears mated with female Irish brown bears, the scientists said.
The maternal lineage can still be traced to all polar bears today, they added.
Prof Daniel Bradley, of Trinity College Dublin (TCD) and Dr Ceiridwen Edwards, formerly of TCD and now at Oxford University, collaborated with Prof Beth Shapiro, of Pennsylvania State University, in the study.
Previously, Dr Edwards attempted to carry out DNA analysis of a sample taken from bones of a polar bear washed into caves in north west Scotland 18,000 years ago.
However, DNA had not survived in the remains from the Bone Caves at Inchnadamph in Sutherland.
Brown bear bones have been found across Ireland, with some of the best preserved examples recovered by cavers at Poll na mBear - Cave of the Bears - in County Leitrim, in May 1997.
Eoghan Lynch and Barry Keenan made the first finds, followed by later discoveries by other speleologists.
An adult bear's skull with the teeth still in place and the bones of young bears were among the finds made.
These have since been dated and are the last recorded bears in Ireland.
The scientists who carried out the DNA analysis said the caves' constant and cool temperatures protected genetic material within the bones.
Dr Edwards, the research paper's lead author, sequenced the mitochondrial DNA from different time depths and from bones recovered from the eight sites.
She found that the older bears in Ireland - from between 43,000 and 38,000 years ago and before the last Ice Age arrived - had the same genetic signature as brown bears living today in eastern Europe.
But DNA from bears that roamed Ireland in cooler times, 38,000 to 10,000 years ago, have sequences that are the closest match yet to modern polar bears.
Bone isotope analysis revealed that despite the maternal genetic link, the Irish ice bears did not share the polar bears' marine diet.
Prof Bradley said ancient samples offered a means of going back in time and measuring the movement of species in response to past climate change.
Dr Edwards added: "It's amazing to think that Irish brown bears are the ancestors of the modern maternal polar bear lineage.
"As the hybridisation between the two species occurred at a time when their home ranges overlapped, most likely during environmental stress, this has implications for polar bears in today's climate."
Prof Shapiro said the results of their research pointed to the bears hybridizing opportunistically throughout the past 100,000 years and probably longer.
She said: "While brown bears and polar bears are hybridizing today, our results suggest that a recent hybridisation led to the capture of a mitochondrial DNA sequence that was present in the population of brown bears that were living in Ireland before the peak of the last ice age.
"That mitochondrial sequence replaced the previous sequence across the entire polar bear population."
Previously it was thought modern polar bears were most closely related to brown bears living on the islands of Admiralty, Baranof and Chichagof in Alaska's Alexander archipelago.
What are believed to be the only polar bear remains to have been found in Britain were in caves in Inchnadamph in Sutherland.
The bear's skull was found in 1927 and is held in the collections of the National Museum of Scotland.
An almost complete skeleton of another bear was recovered after years of work from the same Scottish site and later confirmed as that of a male brown bear.
The first pieces were discovered in 1995 by cavers exploring a network of caves.
But it was only in 2008 that Edinburgh-based caving club, Grampian Speleological Group, reached some of the final fragments.
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