All brown bears today have some polar bear ancestry due to genetic mixing more than 100,000 years ago.
Researchers obtained ancient DNA from the skull of a young polar bear that was found in 2009 on the coast of the Beaufort Sea in Arctic Alaska.
They nicknamed the bear Bruno, although DNA analysis later showed it to be a female.
Analysis of the ancient DNA from the 100,000-year-old polar bear revealed that extensive hybridisation between polar bears and brown bears occurred during the last warm interglacial period in the Pleistocene.
This left a surprising amount of polar bear ancestry in the genomes of all living brown bears, researchers say.
Corresponding author Beth Shapiro, professor of ecology and evolutionary biology at UC Santa Cruz said the team’s genomic analyses show that Bruno belonged to a polar bear population that was ancestral to living polar bears.
She added that at some point, probably after around 125,000 years ago, the polar bear lineage leading to Bruno and the brown bear lineage leading to all living brown bears crossed paths and hybridised.
As a result of this, polar bear ancestry accounts for as much as 10% of the genomes of brown bears living today.
Prof Shapiro said: “We never would have seen this without Bruno’s genome, because all living brown bears have that admixture as part of their genomes.”
While polar bears and brown bears are distinct species with differences in appearance, behaviour, and habitats, they are closely related and can readily cross-breed when their ranges overlap.
Experts say reports of hybrids have increased in recent years as the climate warms and disappearing sea ice forces polar bears onto Arctic coastal areas, while brown bears expand their range northward.
Previous studies of ancient DNA have shown that admixture has occurred in certain populations of brown bears at least four different times between around 15,000 and 25,000 years ago.
In all cases, the direction of gene flow was from polar bears into brown bears.
While the new study found some evidence of possible gene flow from brown bears into Bruno’s lineage, the absence of admixture in polar bears today supports the idea that brown bear ancestry reduces a bear’s fitness for life as a polar bear.
After diverging from brown bears about 500,000 years ago, polar bears evolved into highly specialised hunters of marine mammals on the Arctic sea ice.
Prof Shapiro added: “We shouldn’t be surprised to see admixture happening again today as the climate changes and these species are overlapping and encountering each other again in the wild.
“Climate change allows gene flow to occur between what we think of as different species.”
The findings are published in Nature Ecology & Evolution.