This paper examined a jaw bone of a fossil polar bear found to the far east of Norway. The polar bear is known to be closely related to the brown bear as they have evolved within this group. The polar bear is most closely related to one particular group of brown bear than any other which consists of a genetically unique clade of brown bear populations. These bears live exclusively on the Admiralty, Baranof, and Chichagof (ABC) islands of south-eastern Alaska’s Alexander Archipelago. In past studies various divergence times of brown and polar bears have been given and range from 200 thousand years ago (kya) to over 1000 kya. However, these dates are based on limited sampling and a lack of data prompting further research by Lindqvist et al. (2010).
The fossil which was used in this research consisted of one jawbone. The measurements and traits the specimen possessed consisted with measurements of a polar bear. Fossil remains of polar bears are extremely rare as the animals live and die on the sea ice. The presence of scavengers also creates problems for specimens to remain in-tact. Therefore dating of this specimen could yield important information regarding the evolution of polar bears.
A canine tooth from the fossil specimen was removed and analysed by accelerator mass spectrometry and provided a date of at least 45 kya. When this was combined with estimates based on the sediments and stratigraphy of the area and overall date of 130-110 kya was provided for the jawbone. This made the specimen the oldest polar bear fossil every found, making it an extremely important find. It also confirms that the polar bear was a district species at least 110 kya, before any genetic data is produced. Also, isotope analysis suggests that specimen had a similar diet to present day polar bears providing some insight into the ecology of the area at that time.
By examining the mtDNA not only was the species confirmed as a polar bear but an insight into the evolution of the polar bear was also examined. By using the sequences of other polar, extant and modern, and brown bear the lineage of these bears could be investigated. It was found that the jawbone specimen lies directly at branching point between polar and the ABC brown bears, therefore specimen existed closely to the most recent common ancestor of the two types of bear. When the divergence ages were examined it was found that the brown/polar bear lineage split around 500 kya. When the ABC and polar bear split was investigated an estimate average age of 152 kya was given, with all polar bears splitting around 134 kya. These dates suggest that polar bears adapted rapidly both morphologically and physiologically to their current ecology. These bears are therefore a good example of evolutionary opportunism.
As this study showed it is possible to sequence this polar bear fossil’s mtDNA, it may be possible in the future to sequence the full genome of the specimen. Combining this with more genomes from present-day polar bears could provide an insight not just into the evolution of the polar bear but how the bears rapidly evolved to survive the last interglacial period. This may provide a more accurate outlook on how polar bears will cope with the predicted changes of their current habitat.
Full citation of article:
Lindqvist, C., S. C. Schuster, et al. (2010). “Complete mitochondrial genome of a Pleistocene jawbone unveils the origin of polar bear.” Proceedings of the National Academy of Sciences 107(11): 5053-5057. Web link to article: here.