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Polar Bear Fossil Sequencing Yields Evolutionary History

In the complete sequencing of the mitochondrial genome of a rare ancient polar bear fossil, scientists have gained an understanding of the animal’s evolutionary history and its ability to adapt to changing environments. "Our results confirm that the polar bear is an evolutionarily young species that split off from brown bears some 150,000 years ago and evolved extremely rapidly during the late Pleistocene, perhaps adapting to the opening of new habitats and food sources in response to climate changes just before the last interglacial period," said Dr. Charlotte Lindqvist, research assistant professor in the Department of Biological Sciences at the University of Buffalo and co-lead author of the report. "We have found that polar bears actually survived the interglacial warming period, which was generally warmer than the current one," Dr. Lindqvist said, "but it's possible that Svalbard [the region in Norway in which the fossil was found] might have served as a refugium for bears, providing them with a habitat where they could survive. However, climate change now may be occurring at such an accelerated pace that we do not know if polar bears will be able to keep up. The polar bear may be more evolutionarily constrained because it is today very specialized; morphologically, physiologically, and behaviorally well-adapted to living on the edge of the Arctic ice, subsisting on a few species of seals." At an estimated 110,000 to 130,000 years old, "this is, by far, the oldest mammal mitochondrial genome to be sequenced," said Dr. Stephan Schuster, from Penn State's Center for Comparative Genomics and Bioinformatics and co-lead author of the report. "It's about twice the age of the oldest mammoth genome that has, to date, been sequenced."

"Very few polar bear fossils have been found, leading to widely varying estimates of exactly when and how polar bears evolved,” said Dr. Oystein Wiig, polar bear expert and the article’s senior author, from at the University of Oslo's Natural History Museum. "Because polar bears live on the ice, their dead remains fall to the bottom of the ocean or get scavenged. They don’t get deposited in the sediments like other mammals."

In 2004, an Icelandic geologist found a rare, well-preserved fossil jawbone and canine tooth in the Svalbard archipelago of Norway. This specimen was subsequently sent to Dr. Wiig for analysis.

The article reporting the polar bear sequencing was published in the March 2 issue of PNAS. [Press release] [PNAS article]