Polar bears, long thought to have branched off relatively recently from brown bears, developing their white coats, webbed paws and other adaptations over the past 150,000 years or so to cope with life on Arctic Sea ice, are not descended from brown bears, scientists report. Instead, according to a research team that looked at DNA samples from the two species and from black bears, the brown bear and polar bear ancestral lines have a common ancestor and split about 600,000 years ago.
The report, published online Thursday in the journal Science, is the latest attempt to understand the surprisingly murky origins of one of the most familiar animals on earth, and a potent environmental symbol because it is losing the sea ice it depends on to a warming climate. The report comes to no conclusion about how sensitive the bears are to the current loss of the sea ice that they live on. The findings challenge the idea that the bears adapted very quickly but confirm that they have made it through loss of sea ice before. It may have been touch and go for the bears, however, because the authors find evidence of evolutionary bottle- necks, when only small populations survived, even though warming was occurring much more slowly than it is now.
The researchers, including Axel Janke and Frank Hailer of the Biodiversity and Climate Research Center in Frankfurt, compared DNA samples from 19 polar bears, 18 brown bears and seven black bears. What they found, said Hailer, was that polar bears "are older and much more genetically unique" than had been thought. Other studies in the past few years suggested that the species was "a very recent offshoot from brown bears," he said, dating to about 150,000 years ago. That calculation was based on DNA outside the cell nucleus known as mitochondrial DNA, which is passed on only through females, and so gives an incomplete picture of evolution. Hailer and colleagues looked at 14 stretches of nuclear DNA. This is the genetic material that comes from both parents and combines at conception to form a blueprint for a new individual.
Charlotte Lindqvist, at the University at Buffalo, who was not involved in the study, was the lead author of a paper in 2010 in Proceedings of the National Academy of Sciences that looked at mitochondrial DNA and homed in on the 150,000-year time frame for polar-bear origin, with the species splitting off from brown bears. Hailer suggests that this was evidence not of the origin of the bears, but of interbreeding between polar and brown bears.
Lindqvist said the new study "demonstrates that the two species do indeed represent separate lineages." She said that comparisons of the full genome in both species are needed to nail down the timing of polar bear evolution.
NEW YORK TIMES