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Polar bears older than previously thought

A large male polar bear returns to feed on a fin whale carcass. On land, where bears cannot hunt for seals, food is scarce and polar bears mainly depend on washed up marine mammals for food. Holmiabukta Bay, Northwestern Svalbard, Norway.
[Photo © Florian Schulz - www.visionsofthewild.com]

Polar bears diverged from their closest relatives about 600,000 years ago, according to a new genetic study published in the April 20 issue of the journal Science.

The findings suggest the cold adapted species is about five times older than previously thought, and may have had more time to adapt to arctic conditions than recently assumed. Previous studies of polar bears focused mainly on mitochondrial or mtDNA, which is passed on from mother to offspring and only comprises a very small portion of the entire genome.

Because each part of the genome can tell its own story, using solely mtDNA to reconstruct a speciesí evolutionary history is like reading only a few pages of a book -- scientists are prone to miss large chunks of information.

In this study, Frank Hailer from the Biodiversity and Climate Research Center (BiK-F) and colleagues set out to test whether the nuclear genome told the same story as mtDNA, which indicated that polar bears are essentially a recently evolved type of northern brown bear.

Their results showed the opposite: data from many independently inherited regions of the nuclear genome revealed that both polar and brown bears are much older, as species, than previously suggested. At the time the bears diverged, the Pleistocene climate record shows that global temperatures reached a long-term low.

This could be purely coincidental, the authors argue, but these results suggest that the drastic climatic cooling events during the Pleistocene period were associated with the evolutionary origin of the polar bear. As human activities continue to speed up the rate of climate change, the arctic could reach higher temperatures more rapidly than in previous warm phases.

This study suggests that past adaptation to a changing climate may have been a slow process. Consequently, polar bears may not have enough time to adjust to these warmer conditions as they have in the past.