Polar bears much older than assumed
Top image: New research suggests that the polar bear species is much older than previously believed. Photo: Lucie Strub-Klein/UNIS.
New DNA studies suggest that polar bears evolved into a distinct species as many as 4-5 million years ago. Climate change and genetic exchange with brown bears helped create the polar bear as we know it today.
24 July 2012
Press release: Buffalo, N.Y/Oslo
A recent analysis of sequenced polar bear genomes gives new insight into the evolution of the polar bear species. Climate changes and exchange of genetic material with the brown bear species through millions of years have shaped the polar bear species as we know it today.
Scientists at the Natural History Museum at the University of Oslo, the Norwegian Polar Institute and the University Centre in Svalbard (UNIS) have been part of an international study headed by the Pennsylvania State University and the University at Buffalo, USA.
The study indicates that the size of the polar bear population has varied according to the central climatic changes in the last 4-5 million years. The population has increased in cold periods and decreased in the warmer periods.
The study shows that the polar bear started evolving as a separate bear species from the brown bears some 4-5 million years ago, but continued to interbreed with brown bears until much more recently.
DNA from the Svalbard jawbone
According to Charlotte Lindquist, project leader and assistant professor in biology at the University at Buffalo, the cause was climatic changes; in warmer periods when the Arctic drift ice withdrew the two species came in closer contact as their ranges overlapped.
The result of the study is published in the online edition of Proceedings of the National Academy of Sciences of the United States of America and represents the most extensive analysis of the polar bear DNA done to date.
The team behind this study consisted of scientists from 13 institutions from North America, Europe and Asia. They sequenced and analyzed genomes from 28 polar bears. Material from living polar bears was collected in parts in Svalbard by Jon Aars at the Norwegian Polar Institute.
In addition, genetic material from a more than 115,000 year old polar bear jawbone found in Svalbard by UNIS/University of Iceland professor Ólafur Ingólfsson, was used.
– We generated a first-rate set of data, including deep sequence coverage for the entire genomes of a polar bear, three brown bears and a black bear, plus lower coverage of 23 additional polar bears, including a 120-thousand-year-old individual, explains Stephan Schuster, professor in biochemistry and molecular biology at the Pennsylvania State University.
In addition, DNA from the polar bear jawbone found in Svalbard by professor Ingólfsson was sequenced.
The scientists discovered that the polar bear as a separate species is much older than previously believed, in fact much older that the 600,000 years recently suggested in another study, which was based on analysis of only small segments of the polar bear genome.
-The analysis of the whole polar bear genome shows that earlier conclusions were misleading, says Webb Miller, first author of the Pennsylvania State University study.
– Rather than polar bears splitting from brown bears a few hundred thousand years ago, we estimate that the split occurred 4 to 5 million years ago, he says.
– This means that polar bears definitely existed during subsequent warmer periods, says Øystein Wiig, polar bear expert and professor in mammalogy at the Natural History Museum in Oslo.
– However, this does not automatically imply that the polar bear can sustain warmer periods in the future, he says. – The polar bear will be dependent on some sea ice in the Arctic.
The analyses suggest that the old polar bear jawbone from Svalbard is from an extinct sibling group of all polar bears living today. This indicates that Svalbard in earlier times could have been one of several refuge areas for polar bears in periods with very little sea ice in the Arctic. When the sea ice extent has expanded again, the polar bear has once more spread over large areas of the Arctic.
The analyses show us that the size of the current polar bear population is much smaller in size compared to prehistoric times, says Lutz Bachmann, professor in molecular systematics at the Natural History Museum in Oslo.
– In fact, they have lost much of their genetic variation and might therefor be more sensitive to reduced population numbers in the future due to for instance climate changes, he says.
Discrepancies between the estimated age of polar bears in the new study and past studies could be explained by interbreeding between polar bears and brown bears since the species split from each other.
What makes a polar bear a polar bear?
The new analysis uncovered more genetic similarities than previously known between polar bears and ABC brown bears, an isolated group from southeastern Alaska – suggesting that these animals have exchanged genes since becoming separate species.
Polar bears have genetic differences from brown bears that let them survive in an Arctic climate with very different diets, and the new study identified genes that may be responsible for traits such as polar bears’ pigmentation and the high fat content of their milk.
This study received financial support from Penn State University, The College of Arts and Sciences at the University at Buffalo, The U.S. Geological Survey’s Changing Arctic Ecosystem Initiative, The U.S. Fish and Wildlife Service, The Ontario Ministry of Natural Resources in Canada, The Alaska Department of Fish and Game, The National Institutes of Health, The National Science Foundation, UNIS, The Norwegian Polar Institute and the University of Oslo.
Lindqvist, C., Schuster, S.C., Sun, Y., Talbot, S.L., Qi, J., Ratan, A., Tomsho, L.P., Kasson, L., Zeyl, E., Aass, J., Miller, W., Ingolfsson, O., Bachmann, L., Wiig, Ø. 2010. Complete mitochondrial genome of a Pleistocene jawbone unveils the origin of polar bears. Proceedings of the National Academy of Sciences USA, 107, 5053-5057.