The research, published Monday in the Proceedings of the National Academy of Science, also says the white bear's populations have swung widely in response to changing weather over the millennia — offering hints of how they might respond to climate change today.
"Bears in the past have gone through a lot of extreme changes in climate," said Penn State University biologist Webb Miller, one of the paper's lead authors. "That's not to say they'll make it through this one."
Miller's team used a new method of genomic analysis to compare DNA sequences from modern polar bears with brown and black bears. There were also DNA fragments from a 120,000-year-old polar bear tooth.
"The methods we used are quite recently developed," Miller said. "Being able to look at a few complete genome sequences and make inferences about species history, this is a brand-new research area."
Their first finding was that characteristics marking polar bears — such as white fur and their ability to store fat — stem from genetic markers that are between four million and five million years old.
That completely upends previous theories, which have placed the origin of the bears anywhere between 60,000 and 600,000 years ago. Miller himself wrote a paper three years ago suggesting the correct figure was about 150,000 years.
"Wow, we were really wrong," he said. "Not even close.
"I expect that polar bears have been white for millions of years."
The study also suggested that polar bear numbers have swung dramatically and roughly coincide with climatic changes that increased or reduced the amount of Arctic sea ice the bears use as a hunting platform. Warm eras of less ice supported fewer bears and colder climates produced more.
Evidence for that shows up in DNA strands common to both polar bears and other bears, suggesting that the absence of their preferred habitat forced polar bears onto land, which led to hybridization. As well, modern polar bear populations have less genetic diversity than other bears, suggesting they come from smaller root stocks.
Miller said just because polar bears have made it through previous planetary warm phases doesn't mean current warming isn't a threat.
For one thing, today's climate change comes on top of a slow, steady decline in their numbers that's been going on for hundreds of thousands of years. Modern bears also have to deal with pressures their ancient ancestors didn't — from hunting to the drift of contaminants such as mercury into the Arctic.
As well, their low genetic diversity makes polar bears more vulnerable to threats such as new bacteria entering the Arctic.
"If you've lost your flexibility for responding to new dangers, this is potentially very bad."
Miller said the next step will be to sequence DNA from more bears as well as from other northern species such as Arctic fox or muskox. That will allow scientists to refine the polar bear conclusions and understand how they fit in to what else has happened in the remote past.
Understanding that, said Miller, will allow humans to anticipate how today's warming climate could affect the Arctic.
"Now that we know how to do this, we could crank through lots of species and really see the bigger picture. As the ice packs shrunk and grew and things got warmer and colder, how did the species respond?
"It could give us some data for projecting where we're headed."
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