New research from an international team of researchers shows that polar bears evolved as early as 600,000 years ago. Writing in the journal Science, the team reveal that the largest arctic carnivore is five times older than scientists previously thought. The findings have implications for efforts to conserve this endangered arctic species.
The study, carried out by scientists from Germany, Spain, Sweden and the United States, is based on an analysis of information from the nuclear genome of polar and brown bears.
Finding out about polar bears' evolutionary history is notoriously tricky as they tend to spend most of their life on sea ice, and typically they tend to die there too. Polar bear remains therefore sink to the sea floor where they get ground up by glaciers or remain undiscovered. Therefore any fossil remains of polar bears are rare.
Previous studies have suggested that a brown bear that lived some 150,000 years ago was the ancestor of the polar bear. That research was based on DNA from the mitochondria, organelles often described as the 'powerhouses of the cell'.
Taking a different approach, the team took an in-depth look at the genetic information contained in the cell nucleus. Study lead author Frank Hailer from the German Biodiversity and Climate Research Centre (BiK-F) explains: 'Instead of the traditional approach of looking at mitochondrial DNA we studied many pieces of nuclear DNA that are each independently inherited. We characterised those pieces, or genetic markers, in multiple polar and brown bear individuals.'
The data obtained from nuclear DNA indicate that polar bears actually evolved in the mid Pleistocene, some 600,000 years ago. This provides much more time for the polar bear ancestors to colonise and adapt to the harsh conditions of the arctic. Based on studies of mitochondrial DNA, polar bears had earlier been considered an example of surprisingly rapid adaptation of a mammal to colder climates. The polar bear's specific adaptations, including its black skin, white fur and fur-covered feet, now seem less surprising.
Hailer comments: 'In fact, the polar bear genome harbours a lot of distinct genetic information, which makes a lot of sense, given all the unique adaptations in polar bears.'
Previous studies of mitochondrial DNA had indicated that polar bears are much younger as a species. This apparent discrepancy with past events of hybridisation between polar and brown bears can be explained by a process recently observed in the Canadian arctic: after their initial speciation, polar bears and brown bears came into contact again, maybe due to past climatic fluctuations. The mitochondrial DNA found in polar bears today was probably inherited from a brown bear female that hybridised with polar bears at some point in the late Pleistocene. It appears that much of the nuclear genome remained unaffected by hybridisation, so polar bears retained their genetic distinctiveness.
'Each part of the genome tells its own story. In our study we analysed nuclear DNA that is inherited from both parents. It provides a more detailed and accurate picture of the evolutionary history of a species than mitochondrial DNA that is inherited only from the mother,' comments one of the senior study authors Axel Janke, also from the BiK-F. 'Inferring a species' evolutionary history based on mitochondrial DNA alone is like solving a puzzle with only a few of the many available pieces. You need to study many genetic markers (loci) to put together the full picture.'
For more information, please visit: Senckenberg Research Institute and Natural History Museum.
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