Evolution

Polar Bear, Polar Bear

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Science  19 Mar 2010:
Vol. 327, Issue 5972, pp. 1430-1431
DOI: 10.1126/science.327.5972.1430-d
CREDIT: ØYSTEIN WIIG, UNIVERSITY OF OSLO

Polar bears are adapted to living in one of the harshest environments on earth, their range being determined by the extent of Arctic polar sea ice. They arose from the brown bear lineage and are most closely related to a group of genetically distinct brown bears that inhabit the Admiralty, Baranof, and Chichagof Islands of Alaska's Alexander Archipelago (known as the ABC brown bears). Time estimates for the brown bear–polar bear divergence vary considerably. Stratigraphy and dating of a rare polar bear fossil jaw bone, found on the Svalbard Archipelago in Norway, suggest that it is 130,000 to 110,000 years old and reveal that polar bears were a distinct species at this time. Lindqvist et al. extracted DNA from this fossil and sequenced its mitochondrial genome (mtDNA). Comparison of the ancient polar bear mtDNA sequence with a number of mtDNA genomes from modern polar bears and brown bears revealed that the ancient polar bear lies almost directly at the branching point of polar bears and ABC brown bears, indicating that this bear was very close to the common ancestor of both. Time estimates for the divergence suggest a range of 177,000 to 131,000 years ago. Furthermore, stable isotope analysis of carbon and nitrogen in the fossil indicated that the ancient polar bear was a marine predator at the top of the Arctic marine food chain, like its modern counterpart (and unlike brown bears). Thus, polar bears appear to have adapted very rapidly to their Arctic sea ice environment, perhaps within 10,000 to 30,000 years after the split from the brown bear precursor.

Proc. Natl. Acad. Sci. U.S.A. 10.1073/pnas.0914266107 (2010).

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