By Mark Waghorn via SWNS
Polar bears mated with grizzlies around 125,000 years ago, according to new research.
The finding is based on the skull of an ancient juvenile unearthed on Alaska's northern coast.
Scientists mapped its complete genome by obtaining DNA from a fragment of powdered bone.
The creature carried genes from its brown cousins. It's been nicknamed Bruno - even though later analysis identified it as female.
Interbreeding happened during a warm interglacial period between the last two ice ages.
It left a surprising amount of polar bear ancestry in all modern brown bears. The phenomenon has implications for climate change.
The same is happening in the Arctic today. The resulting hybrids are known as 'pizzlies' or 'grolars'.
Over the last decade, sightings have increased. It could fuel the transmission of diseases - and change ecosystems forever. Crossbreeds found in Alaska and Canada are not genetic anomalies.
In the islands off of Southeast Alaska, bears resemble grizzlies but contain polar bear DNA - indicating decades of sporadic interbreeding,
First author Dr. Ming-Shan Wang, of the University of California, Santa Cruz (UCSC), said: "The availability of Bruno's paleogenome has made it possible to detect an ancient admixture event that impacted all living brown bears."
Admixture, the genetic exchange between differentiated populations, is common in the history of species.
The polar-grizzly cocktail is far from the only recent animal hybrid. The coywolf - a coyote-dog-wolf amalgamation - and a lynx-bobcat mix have been popping up along the northern Atlantic coast.
The more scientists analyze species’ genomes, the more they realize animals we label as 'pure breeds' actually share DNA.
That includes us. About three percent of our DNA is from Neanderthals.
The analysis found Bruno belonged to a group ancestral to living polar bears. At some point, her lineage crossed with primitive brown bears - and had sex with each other.
As a result, polar bears account for as much as ten percent of the genomes of brown bears today.
Co-author Professor Beth Shapiro, also from UCSC, said: "We never would have seen this without Bruno's genome because all living brown bears have that admixture as part of their genomes."
Despite striking differences in appearance, behavior and habitats, the species are closely related - and can readily 'hook up' when their ranges overlap.
Disappearing sea ice is forcing polar bears onto Arctic coastal areas - while brown bears expand their range northward.
Previous studies of ancient DNA have shown admixture has occurred in populations of brown bears at least four times between around 15,000 and 25,000 years ago. In all cases, gene flow was from polar bears.
Explained Prof Shapiro: "The admixed individuals, if they survive, do so as brown bears, perhaps because they have difficulty hunting successfully on the sea ice if they are not completely white.
"Polar bears have always been a small population with not much genetic diversity."
The international team found some evidence of possible gene flow from brown bears into Bruno's lineage.
But the absence of admixture in polar bears today supports the idea brown bear ancestry reduces fitness for life as a polar bear.
Polar bears diverged from brown bears about 500,000 years ago. They evolved into highly specialized hunters of marine mammals on the Arctic sea ice.
Brown bears, in contrast, are generalists ranging widely across North America, Europe, and Asia.
Bruno lived during a time of changing climate after the peak of a warm interglacial period when temperatures and sea levels were considerably higher than they are now.
Similar conditions can be expected in the future as a result of rapid climate change driven by the burning of fossil fuels and other human activities.
As Arctic sea ice declines, many polar bear populations are already struggling to survive.
Co-author Dr. Ian Stirling, a polar bear biologist with Environment and Climate Change Canada, said: "If the rapid, unnatural, and severe human-caused warming of the Arctic we are documenting today continues unabated, it is uncertain whether polar bears will have a sea ice habitat to return to and survive genetically."
Textbooks say brown bears aren't supposed to inhabit the same environments. Polar bears are marine mammals. Grizzlies are terrestrial.
But as the Arctic warms, sea ice is shrinking and the tundra is expanding. Disparate populations are meeting, fornicating - and creating a new breed that's capable of reproducing.
Prof Shapiro said: "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."
Climatic shifts that have brought polar bears and brown bears together in the past include glacial periods when sea ice was more extensive.
They mixed in southeast Alaska, Japan, Russia - and even Ireland. Brown bears acquired additional polar bear genes on top of the ancient admixture revealed by Bruno's genome. Brown bears are now extinct in Ireland.
As for what brown bears might have gained from their polar bear ancestry, scientists can only speculate.
Prof Shapiro said: "It is possible brown bears got something cool from polar bears, but we can't say for sure at this point."
The researchers stumbled on Bruno's skull by chance. They were walking the Beaufort Sea coastline in 2009 surveying for recent erosion. It was resting just above the high tide line.
Co-author Dr. Pamela Groves, of the University of Alaska Fairbanks, said: "We weren't even looking for bones, as typically we find ancient bones a hundred miles inland where they have been stored in permafrost along sleepy rivers.
Polar bears spend most of their lives at sea. Finding any remains is extremely rare. Bruno's ancient skull is the only one ever recorded - and the single prehistoric polar bear bone from North America.
Prof Leslie Rissler, program director at the US National Science Foundation which funded the research, said: "Understanding how past changes in climate drove interactions between organisms is critical to predicting how current changes will create new admixtures, increase disease transmission or impact natural resources or society."
The study was published in the journal Nature Ecology & Evolution.
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