An international team of researchers has revealed that polar bears evolved some 600 thousand years ago, five times earlier than previously thought.
The new findings, published in the journal Science, are the result of an analysis of information from the nuclear genome of polar and brown bears, and shed new light on conservation issues regarding this endangered arctic specialist.
Previous studies had suggested that the ancestor of polar bears was a brown bear that lived some 150 thousand years ago (late Pleistocene). That research was based on DNA from the mitochondria – organelles often called the ‘powerhouses of the cell’.
The team led by researchers from the German Biodiversity and Climate Research Centre (BiK-F) now took an in-depth look at the genetic information contained in the cell nucleus.
“Instead of the traditional approach of looking at mitochondrial DNA we studied many pieces of nuclear DNA that are each independently inherited,” explained Dr. Frank Hailer of BiK-F, a lead author of the study. “We characterized those pieces, or genetic markers, in multiple polar and brown bear individuals.”
This genetic survey was well worth the effort – the information obtained from nuclear DNA indicates that polar bears actually evolved in the mid Pleistocene, some 600 thousand years ago. This provides much more time for the polar bear ancestors to colonize 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 furcovered feet now seem less surprising.
“In fact, the polar bear genome harbors a lot of distinct genetic information, which makes a lot of sense, given all the unique adaptations in polar bears,” Dr. Hailer said.
The team explains the findings by past events of hybridization between polar and brown bears – 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 hybridized with polar bears at some point in the late Pleistocene. It appears that much of the nuclear genome remained unaffected by hybridization, so polar bears retained their genetic distinctiveness.
“Each part of the genome tells its own story,” said Dr. Axel Janke of BiK-F, a co-author on the study. “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.”
“Inferring a species’ evolutionary history based on mitochondrial DNA alone is like solving a puzzle with only a few of the many available pieces,” the researcher added. “You need to study many genetic markers (loci) to put together the full picture.”