Polar bears are not descended from brown bears as previously thought; instead the two lines shared a common ancestor 600,000 years ago, German scientists say. It had been believed polar bears branched off from brown bears 150,000 years ago and began to develop their white coats, webbed paws and other adaptations to cope with life in the arctic. However, researchers at the Biodiversity and Climate Research Center in Frankfurt compared DNA samples from 19 polar bears, 18 brown bears and 7 black bears and determined the various lines split from a common ancestor 600,000 years ago, The New York Times reported Friday. While challenging the idea polar bears adapted very quickly, the findings confirm they\'ve survived previous warming periods and loss of sea ice similar to conditions today that have led the International Union for Conservation of Nature to list the animals as \"vulnerable,\" one level below endangered. However, the researchers said, the DNA evidence shows periods of evolutionary bottlenecks, probably during warm periods, when only small populations survived. Some DNA findings, which had previously suggested a more recent origin for polar bears, may in fact be evidence of interbreeding between polar and brown bears long after both species evolved, perhaps when the polar bears were driven to land because of sea ice loss, researchers said.