By Jonathan Amos
Science reporter, BBC News, San Francisco
What may be the oldest known remains of a polar bear have been uncovered on the Svalbard archipelago in the Arctic.
The jawbone is about 23cm long
The jawbone was pulled from sediments that suggest the specimen is perhaps 110,000 or 130,000 years old.
Professor Olafur Ingolfsson from the University of Iceland says tests show it was an adult, possibly a female.
The find is a surprise because polar bears are a relatively new species, with one study claiming they evolved less than 100,000 years ago.
If the Svalbard jawbone's status is confirmed, and further discoveries can show the iconic Arctic beasts have a deeper evolutionary heritage, then the outlook for the animals may be more positive than some believe.
"We have this specimen that confirms the polar bear was a morphologically distinct species at least 100,000 years ago, and this basically means that the polar bear has already survived one interglacial period," explained Professor Ingolfsson.
"And what's interesting about that is that the Eeemian - the last interglacial - was much warmer than the Holocene (the present).
POLAR BEAR (URSUS MARITIMUS)
Largest of five living bear species of Ursus genus
Brown bear (U. arctos) is nearest evolutionary cousin
Two species able to produce fertile hybrid offspring
Highly specialised predator of seals - but will take other prey
Global population of polar bears may number 20-25,000
Most recent IUCN Red List status: Vulnerable
Previous oldest recovered remains are about 70,000 years old
"This is telling us that despite the ongoing warming in the Arctic today, maybe we don't have to be quite so worried about the polar bear. That would be very encouraging."
The jawbone's discovery is being presented here in San Francisco at the American Geophysical Union (AGU) Fall Meeting.
The specimen was found at Poolepynten on Prins Karls Forland, a narrow strip of land on the far west of the archipelago.
The sediments there are well-described, and record at least two glaciations sandwiched with marine sequences. In other words, they record periods when Poolepynten was alternately covered by ice and water.
These periods are understood in good detail by Professor Ingolfsson's team, so although direct dating at the dig site gives an age range for the bone of 80-140,000 years ago, the group is confident the specimen can be placed at the upper end of this scale.
The 23cm-long bone itself retains some critical details that have helped identify it.
"It is very well-preserved," Professor Ingolfsson told BBC News.
"We can measure various parameters, such as the cheek-teeth row-length, and the size of the hole made by the third molar - which is very characteristic of polar bears. We've compared all this, both to fossil and recent materials, and there's no question it's a polar bear." They speculate it was a female bear.
Researchers have studied the DNA of modern polar bears to try to gauge when the Arctic animals separated from brown bears, their nearest evolutionary cousins.
Different models have variously put the radiation as near as 70,000 years ago and as distant as 1-1.5 million years ago. One of the problems has been in finding the ancient specimens to put alongside, and constrain, these genetic estimates.
Until recently, one of the oldest polar bear specimens was thought to be British - a 70,000-year-old animal found at Kew Bridge in London.
The presumption was that the creature lived at a lower latitude during a period when ice sheets were more extensive.
But scientists are now confident the Kew animal was in fact a brown bear.
"It's a huge bear; it's a runner - a hunting bear," said Andy Currant, a palaeontologist from London's Natural History Museum. "It's got some of the features of a polar bear, but it's undoubtedly a brown bear.
"With something like polar bears, to make an identification you've got to have a skull or a lower jaw - they've got very reduced teeth, rather surprisingly, and you've got to see that. So I was interested to learn that [Ingolfsson's group] has that."
Building up a more detailed picture of the ancient history of polar bears will be challenging, though. The animals spend much of their lives out on the ice, and when they die their remains are likely to be scavenged by other creatures or fall to the bottom of the Arctic Ocean. Finds will continue to be extremely rare.
Concern over the bears' future status centres on the observations of shrinking ice in what is a rapidly warming Arctic. The ice provides a platform from which to hunt ringed, and other, seals. If the ice is diminished and the bears cannot adapt quickly, many of them may be squeezed out of their ecological niche.
The sediments record the passing of ice and water
Professor Ingolfsson is hopeful the bears will cope - and believes the palaeo-record will offer some reassurance.
"The polar bear is basically a brown bear that decided some time ago that it would be easier to feed on seals on the ice. So long as there are seals, there are going to be polar bears. I think the threat to the polar bears is much more to do with pollution, the build up of heavy metals in the Arctic.
"This is just how I interpret it. But this is science - when you have little data, you have lots of freedom."
The team, which includes Professor Oystein Wiig from the University of Oslo, Norway, will develop its research on the Svalbard specimen by trying to extract DNA.