April 20, 2012
Polar Bears Older Than Thought, Have Distinct Lineage
Polar bears have been around much longer than previously believed, and likely interbred with brown bears at one point after the two species separated, according to a Germany study published Friday in the journal Science.
The researchers examined mitochondrial DNA, a special "additional genome" that lives in the cell's energy factories and is passed on only from the mother, from polar bears, brown bears and black bears. They found that the brown bear and polar bear ancestral lines have a common ancestor, but split into separate species about 600,000 years ago.
Until now, polar bears had been thought to have branched off relatively recently from brown bears, developing their white coats, webbed paws and other variations over the last 150,000 years or so to adapt to life on Arctic Sea ice.
However, if polar bears were only 150,000 years old, they would have had to evolve many specialized traits in a remarkably short time, the German researchers said.
"I've been long puzzled by the suggestion that the polar bears would have been such a miraculous and rapidly evolving species," said study leader Frank Hailer of the Senckenberg Nature Research Society in Frankfurt, Germany.
"I had this lingering question: Is it really true?” he said during an interview with LiveScience.
Hailer and colleagues examined the polar bears´ nuclear DNA, which comes from both parents and is much larger than the mitochondrial genome. The team compared 9,000 base-pair sequences -- the chemicals that make up the "rungs" of DNA's ladder-like structure -- from the nuclear DNA of 45 polar, brown and black bears.
Using this comparison, the researchers were able to create a family tree, with the assumption that the larger the genetic differences between the species, the farther they were apart in evolutionary time.
The researchers analyzed the bears´ lineage to estimate when the polar bears and brown bears separated.
"We found that polar bears are much older than we previously knew from other studies; their appearance dated to about 600,000 years ago," Hailer said.
"That would make sense around that time for something like a polar bear to evolve, because Arctic habitats were much larger than they are today, so there would have been much larger habitats that would have been suitable for a species like a polar bear."
The researchers theorize that the mitochondrial DNA data might have come from a hybridization event between brown and polar bears 150,000 years ago, during the last warm interglacial period. It was during that time that the sea ice melted and polar bears took to land, where they shared a habitat with brown bears.
This hybridization would have introduced the brown bear mitochondrial DNA into the polar bear population, the researchers said. Moreover, if the brown bear DNA helped the polar bears survive the warm period, perhaps it could have easily spread throughout the population.
It appears "the polar bear population at the time they hybridized with brown bears was very small," Hailer said.
"The impact of hybridization was very large, so the piece of mitochondrial DNA that came from brown bears to polar bears replaced all the original polar bear mitochondrial DNA."