Polar bears diverged from their closest relatives 600,000 years ago, far earlier than previously thought, suggesting more challenges in the face of climate change, scientists said Thursday.

Previous genetic analysis of polar bears had determined the species was only about 150,000 years old. But in fact, it took them five times longer for the polar bear to adapt to arctic conditions, according to the study by Frank Hailer and colleagues.

In turn, the bears may not have enough time to adjust to a rapidly changing climate, the study suggested.

The earlier studies had focused mainly on mitochondrial or mtDNA, which only accounts for a small portion of the entire genome and is passed from a mother to her offspring. They had concluded that polar bears were a recently evolved type of northern brown bear.

But Hailer's study, published in Friday's edition of the journal Science, examined data from many independently inherited regions of the nuclear genome that showed that both polar and brown bears are much older, genetically distinct species of their own right.

The species' earlier origin "implies that polar bears as a species have experienced multiple glacial cycles and had considerable time to adapt to arctic conditions," the study said.

"However, the low genetic diversity in polar bears suggests that changes in the environment, such as warm phases, caused population bottlenecks."

It warned that changes in habitat, hunting, toxic substances and other "stressors" caused by humans "could magnify the impact of current climate change, posing a novel and likely profound threat to polar bear survival."