Polar bear DNA from a partially fossilized jawbone found in Norway shows that the species is relatively young and adapted quickly to the arctic environment.
Researchers in the U.S., Norway and Iceland extracted DNA from a jawbone and tooth found in 2004. The remains are estimated to be between 110,000 and 130,000 years old.
The research analysis, published in the journal Proceedings of the National Academy of Sciences, shows how the polar bear is related to the brown bear and how quickly it responded to changes in climate.
"Our results confirm that the polar bear is an evolutionarily young species that split off from brown bears some 150,000 years ago and evolved extremely rapidly during the late Pleistocene, perhaps adapting to the opening of new habitats and food sources in response to climate changes just before the last interglacial period," said Charlotte Lindqvist of the University of Buffalo, in a statement.
Lindqvist said the study found that although polar bears survived the last warming period, they have since become highly specialized to living on the Arctic ice.
"Climate change now may be occurring at such an accelerated pace that we do not know if polar bears will be able to keep up," she said.
Polar bear fossils rare
Polar bear fossils are rare, in part because the animals live on ice and their remains are either scavenged or fall to the bottom of the ocean.
"They don't get deposited in the sediments like other mammals," said study co-author Oystein Wiig of the University of Oslo's Natural History Museum.
Because so few polar bear fossils have been found, little is known about their evolutionary history.
The partially fossilized remains, or subfossil, of the polar bear jaw and canine tooth were found in the Svalbard archipelago of Norway.
Lindqvist, while working in Olso, extracted DNA from the polar bear subfossil by drilling into the jaw and pulling out powdered remains with intact genetic material.
The researchers were able to sequence DNA from the bear's mitochondria, cellular powerhouses with their own genetic material. Because mitochondrial DNA has a high rate of mutation and is only passed down from mothers to offspring, it's considered very useful in studying evolutionary changes in organisms.
Oldest mammal genome sequenced
"This is, by far, the oldest mammal mitochondrial genome to be sequenced," said Stephan Schuster of Penn State's Center for Comparative Genomics and Bioinformatics.
"It's about twice the age of the oldest mammoth genome that has, to date, been sequenced," he said.
They compared the ancient polar bear genetic data to the mitochondrial genomes of living bears.
"Since the brown bears from Alaska's Admiralty, Baranof and Chichagof Islands are the polar bears' closest relatives, it was crucial to include them in our study in order to more precisely date when polar bears appeared as a distinct species," said Lindqvist.