The Arctic's dwindling population of polar bears all descend from a single mama brown bear which lived 20,000 to 50,000 years ago in present-day Ireland, according to a new study.

DNA samples from the great white carnivores - taken from across their entire range in Russia, Canada, Greenland, Norway and Alaska - revealed that every individual's lineage could be traced back to this Irish forebear.

The analysis of genetic material inherited only through females also showed that brown and polar bears mated periodically over the last 100,000 years.

This raises the possibility that such cross-species mingling - thought by some scientists to be an additional threat to polar bears already struggling to cope with climate change - played a positive role in their recent evolution, the researchers said.

'Hybridisation could certainly result in the loss of unique genetic sequences, which could push them toward extinction,' said Beth Shapiro, a professor at Pennsylvania State University and lead researcher for the study.

'But scientists should reconsider conservation efforts focused not just on polar bears but also on hybrids, since hybrids may play an underappreciated role in the survival of certain species.'

Several 'pizzlies' - a cross between a grizzly and a polar bear - have been spotted in recent years as the Arctic species has been pushed outside its familiar habitat by mounting temperatures and melting ice.

The fierce predators use the edge of the ice cap as a staging area to stalk seals, their preferred food.

Global warming has hit the Arctic two or three times harder than other parts of the planet, redesigning the environment in which dozens of terrestrial and marine mammals live.

Based on DNA and fossil evidence, it has long been known that polar bears - Ursus maritimus - branched off from the larger family of brown bears about 150,000 years ago.

But whether the subsequent inter-species mating was incidental or whether it fundamentally shaped the animal's gene pool was unknown.

To delve deeper, an international team of scientists led by Ms Shapiro analysed mitochondrial DNA in 242 ancient and living brown and polar bears.

Mitochondrial DNA are part of cells with their own DNA that are passed exclusively from females to offspring, in this case from mother bear to cub.

'We found that the matrilines of the polar bears coalesce to a relatively recent common ancestor' that once lived along Ireland's Atlantic shore, said Daniel Bradley, a professor at Trinity College in Dublin and a co-author of the study.

Previous research had traced the earliest female brown bear ancestor for modern polar bears back 14,000 years to the Alaskan ABC islands.

The new findings, published in the journal Current Biology, not only change the location but push back the date an additional 6,000 to 36,000 years.

Only by comparing the more recent maternal genetic lineage with core DNA formed by both male and female parents were researchers able to pinpoint the brown bear ancestor.

'The nuclear DNA goes back much farther, and probably emerges from a common ancestor with brown bears earlier than 500,000 years ago,' explained Ms Shapiro in an email.

'It is exactly this difference - between the very recent common ancestor along the maternal line and the much older common ancestor in the other genomic DNA - that makes it possible to infer that all living polar bears are descended from a brown bear that lived more recently,' she said.

In order to discern this pattern, there had to have been occasional mating after the two species split, she added.

Because they evolved in separate environments, neither species could survive long in the other's ecological niche due to different body shapes, metabolism and hunting habits.

Polar bears, for example, are expert swimmers, whereas brown bears are more adept at climbing.

There are currently 20,000 to 25,000 polar bears left in the wild, according to the International Union for the Conservation of Nature (IUCN).