Polar bears, long thought to have branched off relatively recently from brown bears, developing their white coats, webbed paws and other adaptations over the last 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 on 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. Because of climate change, and threats from shipping, hunting and pollution, the polar bear is listed as ”œvulnerable,” one level below endangered, by the International Union for Conservation of Nature.
The report comes to no conclusion about how sensitive the bears are to the current loss of the sea ice that they live on, and the evolutionary tale it presents can be read in different ways.
The findings challenge the idea that the bears adapted very quickly, but confirm that they have made it through warming periods and loss of sea ice before. It may have been touch and go for the bears, however, because the authors find evidence of evolutionary bottlenecks, probably during warm periods, when only small populations survived, even though warming was occurring much more slowly than it is now.
Another researcher, Beth Shapiro of Pennsylvania State University, suggested in a recent paper that interbreeding might have occurred in periods of environmental stress. In the journal Current Biology in 2011, Dr. Shapiro and a team of scientists reported that polar bears and extinct Irish brown bears interbred about 130,000 years ago, and that the brown bear mitochondrial DNA from that mating has spread to all polar bears over time. Dr. Hailer said that ice loss now could be far more threatening to polar bears than in the past because it is happening faster than ever before, and because the bears also face hunting and pollution.