When it comes to bears, the polar species seems to be the new kid on the block. A rare fossil jaw found in Norway's Svalbard archipelago is helping researchers confirm that polar bears evolved from brown bears only about 150,000 years ago.

Polar bears live much of the year on the Arctic sea ice and have become something of a symbol of the threat of global warming, which is melting that ice.

"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," Charlotte Lindqvist, an assistant professor in the University of Buffalo's department of biological sciences, said in a statement.

Lindqvist and colleagues were able to analyze mitochondrial DNA from the fossil and report their findings in Tuesday's edition of Proceedings of the National Academy of Science. DNA is the coding that directs the development of individual life forms and the mitochondrial version comes from the cells that generate energy.

While scientists had known polar bears were descended from brown bears, there was debate over when the division occurred. By comparing the DNA from the fossil, which was estimated at 110,000-to-130,000 years old, to DNA from modern polar bears and brown bears they were able to calculate when the types separated.

Polar bear fossils are rare since their remains are usually scavenged by other animals or sink to the bottom of the ocean. The specimen tested was found in 2004 by an Icelandic geologist.