Polar bears evolved as a separate species far earlier than previously thought, according to a new genetic study, which adds to worries about their ability to adapt in a rapidly warming world.
Research published today in the journal Science, shows the Arctic's top predators split off from brown bears, their closest relatives, around 600,000 years ago - five times earlier than scientists had generally assumed.
The finding suggests polar bears took a long time to adapt to their icy world and may therefore struggle to adjust as the Arctic gets warmer and the sea ice melts, depriving them of vital hunting platforms.
Despite being a very different species in terms of body size, skin and coat colour, fur type, tooth structure, and behaviour, previous research had indicated that polar and brown bears diverged only recently in evolutionary terms.
That assumption was based on studying mitochondrial lineage - a small part of the genome, or DNA, that is passed exclusively from mothers to offspring.
But after studying DNA from inside the cell nucleus, using samples from 19 polar and 18 brown bears, Frank Hailer of Germany's Biodiversity and Climate Research Centre and colleagues reached a very different conclusion.
They found both polar and brown bears were much older, as species.
"Previous studies suggested that polar bears would have had to be evolving very rapidly, since they were so young," says Hailer.
"Our study provides a lot more time for polar bears to adapt ... It makes more sense from an evolutionary standpoint that polar bears would be older."
His team's calculations put the moment when the two types of bears diverged in the Pleistocene period, when the climate record shows that global temperatures reached a long-term low.
That could be coincidental but it suggests that the planet's cooling may have triggered the split.
While the latest research implies that past polar bear adaptation was probably a slow process, it also means the animals have been through warming phases before.
"If they go extinct in this phase of warming, we're going to have to ask ourselves what our role in that process was," says Hailer.
"In previous warm phases between the ice ages polar bears were able to survive. The main difference this time is that humans are impacting polar bears as well."
Genetic studies are an important tool in researching the evolutionary history of polar bears, since the animals typically live and die on sea ice. As a result, their bodies sink to the sea floor, where they get ground up by glaciers or remain undiscovered, making fossils scarce.