“ScienceDaily (Apr. 20, 2012) — A study appearing in the current issue of the journal Science reveals that polar bears evolved as early as some 600,000 years ago. An international team led by researchers from the German Biodiversity and Climate Research Centre (BiK-F) shows the largest arctic carnivore to be five times older than previously recognized. The new findings on the evolutionary history of polar bears 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.
Polar bears are uniquely specialized for life in the arctic. This fact is undisputed, and supported by a range of morphological, physiological and behavioural evidence. However, conducting research on the evolutionary history of polar bears is difficult. The arctic giant spends most of its life on sea ice, and typically also dies there. Its remains sink to the sea floor, where they get ground up by glaciers, or remain undiscovered. Fossil remains of polar bears are therefore scarce. Because the genetic information contained in each organism carries a lot of information about the past, researchers can study the history of the species by looking at the genes of today's polar bears.
Analysis of the genetic information in the cell nucleus
Recent studies had suggested that the ancestor of polar bears was a brown bear that lived some 150,000 years ago, in the late Pleistocene. That research was based on DNA from the mitochondria - organelles often called the 'powerhouses of the cell'. Researchers from the German Biodiversity and Climate Research Centre (BiK-F), together with scientists from Spain, Sweden and the USA, now took an in-depth look at the genetic information contained in the cell nucleus. Frank Hailer, BiK-F, lead author of the study explains: "Instead of the traditional approach of looking at mitochondrial DNA we studied many pieces of nuclear DNA that are each independently inherited. We characterized those pieces, or genetic markers, in multiple polar and brown bear individuals."
Polar bears had much more time for adaptation and speciation than previously assumed
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 600,000 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 fur-covered feet now seem less surprising. "In fact, the polar bear genome harbours a lot of distinct genetic information," says Hailer, "which makes a lot of sense, given all the unique adaptations in polar bears."
Maternally inherited (mitochondrial) DNA was showing a biased picture
Previous studies of mitochondrial DNA had indicated that polar bears are much younger as a species. The authors of the new paper in "Science" explain this apparent discrepancy with 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…”