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.

She said in an e-mail that the new study “demonstrates that the two species do indeed represent separate lineages.” But she questioned whether the evidence was sufficient to provide a firm date for polar bear origins. Comparisons of the full genome in both species are needed, she said, to nail down the timing of polar bear evolution.

For animals so well known, polar bears have been something of a puzzle in terms of their origins. Part of the reason is that they live mostly on sea ice, so fossils preserved on land are rare.

So some questions have had to wait for modern techniques for reading genetic material that have made the DNA of living species as useful as any fossil bed for tracing evolution.

But mysteries remain, some more puzzling than ever. Why does the mitochondrial DNA suggest a much more recent origin for polar bears? Dr. Hailer suggests that it is evidence not of the origin of the bears, but of interbreeding between polar and brown bears long after they evolved, perhaps when the polar bears were driven to land because of sea ice loss.

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.