Polar bears rapidly evolved their white fur and Arctic lifestyle as they adapted to an ice age environment, new genetic evidence suggests.

But despite surviving a return to warmer conditions, they may not be able to keep up with the accelerated pace of man-made climate change, according to scientists.

The evolution of polar bears has presented a puzzle because their ancient fossils are so rare.

Usually, fossils are preserved in sediments. But because polar bears live on ice, their dead remains tend to fall to the bottom of the ocean or are removed by scavengers.

In 2004, scientists discovered a rare and well-preserved fossilised jawbone and canine tooth in Norway's Svalbard archipaelago.

Analysis of DNA extracted from the specimens has now revealed important clues about the origins of polar bears.

Study leader Dr Charlotte Lindqvist, from the University at Buffalo, in New York State, US, said: "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."

The late Pleistocene period was marked by extensive glaciation, with ice sheets covering much of the northern hemisphere. Many large animals became extinct during this time, including woolly mammoths, sabre-toothed cats and ground sloths, as did Neanderthal humans.

As the Pleistocene gave way to the Holocene about 12,000 years ago, the world became warmer and the ice retreated. Polar bears continued to survive, confining themselves to the icy Arctic where they live to this day, by hunting seals. The fossil specimens from Norway date to around 110,000 to 130,000 years old.

DNA from their mitochondria - the energy generating power-plants in cells - was compared with that of a population of Alaskan brown bears believed to be the closest living relatives of polar bears. Analysis of the DNA helped to pinpoint when polar bears first appeared as a distinct species.