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Today’s Polar Bears Started Out Brown and Irish [Video]

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Polar bearPolar bears’ "mitochondrial Eve," the female from whom all of today’s polar bears are descended, was not a polar bear at all.

On July 7 researchers published their findings about the species history of polar bears in Current Biology. One of their discoveries was that the mitochondrial DNA in the cells of every polar bear’s body originated in the cells of a female brown bear that lived in the neighborhood of Britain and Ireland during the era when Neandertals’ numbers began go dwindle.

Mitochondrial DNA is passed on directly from mother to offspring—unmixed with any male input, unlike the DNA found in the cell nucleus—which means researchers can use it to trace the maternal line back through history.

By comparing the sequences in the mitochondrial DNA of 242 bears, ranging from living animals to fossils (living over a period of time spanning 120,000 years), from all across the globe, researchers could discover when and where modern polar bears’ mitochondrial lineage originated. This photograph shows one of the Irish caves where bear fossils were found.

Cave in Ireland, home to bear fossilsIn the past 100,000 years, matings among brown and polar bears have successfully produced hybrid offspring, the analysis revealed. One of these matings occurred 20,000 to 50,000 years ago, and the entire modern population of polar bears is descended from the female half. The brown bears that shared the mitochondrial lineage of this polar bear ancestress have since died out, as have polar bears whose mitochondrial DNA stemmed from a different source.

The interspecies hybridization that begat today’s polar bears was possible because of climate fluctuations. Although brown bears need habitats with a warmer climate than their polar cousins, shifts in temperature could push the two populations into proximity by rendering their natural habitats unlivable. For example, an increase in temperature would melt Arctic ice, leading polar bears to seek land-based homes farther south. A cooler period, on the other hand, would make parts of Ireland uninhabitable for the brown bears while facilitating the spread of glaciers, driving brown bears to new habitats such as on the ice shelves formed by the glacier in the Irish Sea basin.

"The bottom line is that the two species bumped up against one another for extended periods of time on different occasions, sharing both habitats and genes," Beth Shapiro, a biologist at Pennsylvania State University, said in a prepared statement. Shapiro, along with Daniel Bradley of Trinity College Dublin, led a team of scientists from both coasts of the U.S. and across Europe.

Stuffed polar-brown bear hybrid on display in Natural History Museum at TringBrown and polar bear hybrids still exist today. Although brown and polar bears have been known to mate and produce offspring when living together in captivity, wild hybrids are more rare. In fact, the first wild hybrid sighting did not occur until 2006, when an Idahoan hunter shot one in Canada. In the past five years, however, more wild hybrids have been spotted.

Some conservationists would like hybrids to join polar bears on lists of endangered species (polar bears are currently classified as "threatened" under the Endangered Species Act). As for polar bears, their ability to survive climate change may get a boost from this research. Understanding how polar bears coped with changing ice distributions thousands of years ago could help scientists predict how they will respond to future change.

Bears are no strangers to travel, having spread from Europe across the globe over the course of more than 100,000 years. This video illustrates their diffusion.




Photo credits: Daniel J. Cox /, Mike Simms, Wikimedia Commons / Messybeast

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  1. 1. Oaky 6:51 pm 07/8/2011

    Just like our forebears.

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  2. 2. doug l 9:37 am 07/9/2011

    Interesting news though it’s worth keeping in mind that the world back then was a vastly different place, with vast regions whose characteristics today are limited to glacial refugias. Bears before the rise of humans would have been the lords of the coastlines and ice edges and not merely predators of the forested mountainsides.

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  3. 3. priddseren 5:53 pm 07/9/2011

    It would seem Global Warming and cooling is entirely natural. Scientist today would have us believe Polar bears are doomed with melting ice caps, yet here is evidence that polar bears survived past ice cap melts.

    Perhaps, humans should just leave these animals alone. It is more likely human intervention will hasten or even cause the extinction of these animals.

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  4. 4. Simanonok 4:50 pm 07/11/2011

    "It would seem Global Warming and cooling is entirely natural"

    Where have you been? Nobody disputes this, and the evidence of past Ice Ages documents gradual changes over thousands of years. What you seem to not understand is that human activities have put so much carbon dioxide into the atmosphere so rapidly as to threaten to alter Earth’s climate over very short time spans, in tens of years rather than thousands.

    However, you are correct in that "humans should just leave these animals alone" and the best way to accomplish that is to stop psychopathic hunters from shooting them.

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  5. 5. GotchaLookin 4:57 pm 07/11/2011

    "… during the era when Neandertals’ numbers began go dwindle."

    Go dwindle? Is that the opposite to "Neanderthal go bragh?" Och, da’ Bears! What they would say, if they could only type … (grin)

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  6. 6. briseboy 5:31 pm 07/13/2011

    Some fun fax:

    Shave a polar bear, and he’s black.

    Their fur is optical fiber – hollow, clear, directing light to the skin to warm. The yellowing is related to organisms enjoying the interior of the fibers.

    As chubby as polar bears get and look, they stop when chased by those mighty white hunters mentioned by Simanonok, because of getting too hot, certainly not tired. They have been measured swimming, coolly, over 200 miles. They are, after all, classified as marine mammals.

    The bear has what seems to be the ultimate olfactory organ of all mammals. Although the wolf has shown an ability to smell some substances at a millionth the quantity that we can, the bear is far better at the sense – both polar and ursus arctos (that’s bear, bear in Latin, Greek).

    More predatory attacks have occurred by Ursus Americanus (can’t call ‘em black because I have seen blonde, cinnamon, chocolate, milk chocolate, whitish, grayish – and I haven’t shaved any recently) than by the griz or polar bears. But outside of the polar bear, bears avoid humans when possible.

    The polar bear knows well that his nose’s negritude can give him away, so has been known to cover the nose with a paw while sneaking up on those on the menu.

    Although this is SA and concerned with science, I hope you all will respect and act to protect this animal whom the ancients recognized as evaluating, courageous, and better, (as the saying goes) encompassing discretion.

    - We are late learning that attributes we have arrogated to our own species are far more widespread than previously realized. Cognition and emotion are associated with mammalian brain anatomy, and whatever few unique abilities we have, others also have capacities, perhaps unknowable to us, unique to them.

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