When did polar bears evolve to form a separate and distinct species from their relatives, the brown and black bears? Researchers from China, Norway, Iceland, Denmark, Singapore, Canada, Mexico and the USA have recently collaborated to address this very question using whole genome sequence analysis. The team compared both mitochondrial and nuclear DNA extracted from modern day polar, brown and black bears with DNA retrieved from an ancient polar bear jawbone (115-130,000 years old) excavated in Svalbard, Norway. Their results suggest that polar bears and brown bears have had largely independent evolutionary histories for the last 4-5 million years, with black bears having split off even further before this. However, a detectable level of gene flow has continued between polar and brown bears since then, suggesting interbreeding events: indeed, brown bear/polar bear hybrids have recently been identified in arctic Canada, where territory ranges of both species overlap.

The team also identifed a few polar bear genes that were hugely changed from those seen in black or brown bears, which may have enabled polar bears to adapt to their harsh arctic lifestyle. Genes involved in skin pigmentation and coat colour patterns were altered, which could have allowed the polar bear to develop the dark black skin that is perfect for absorbing heat and keeping warm. Genes involved in metabolism and weight gain were also modified, potentially leading to improved storage of the large fat reserves necessary to maintain both energy levels and core body temperatures. Finally, changes in genes involved in hair follicle development and function were evident, and may have led to the polar bear developing the highest mean hair density of all ursid bears, a very useful adaptation in an extremely cold environment.

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Miller W, Schuster SC, Welch AJ, Ratan A, Bedoya-Reina OC, Zhao F, Kim HL, Burhans RC, Drautz DI, Wittekindt NE, Tomsho LP, Ibarra-Laclette E, Herrera-Estrella L, Peacock E, Farley S, Sage GK, Rode K, Obbard M, Montiel R, Bachmann L, Ingólfsson O, Aars J, Mailund T, Wiig O, Talbot SL, & Lindqvist C (2012). Polar and brown bear genomes reveal ancient admixture and demographic footprints of past climate change. Proceedings of the National Academy of Sciences of the United States of America, 109 (36) PMID: 22826254