The polar bear is a much older species than previously thought and probably emerged as the Arctic's top land predator when a cold-adapted bear diverged from an ancestral brown bear about 600,000 years ago, a study has found.
The findings suggest that the evolution of the world's largest land carnivore was a much slower process than originally believed, which indicates that the polar bear may be more vulnerable to rapid climate change in the Arctic than previously suggested.
A year ago, scientists produced DNA evidence to suggest that polar bears evolved from ancient brown bears about 150,000 years ago, which surprised many experts because it was a relatively short time for such a big, specialised mammal to have evolved.
This led some to speculate that the polar bear may be able to adapt to the far faster climate changes now taking place in the Arctic, where the sea ice on which the animals hunt has disappeared at a dramatic rate over the past 40 years. However, this earlier study was based on mitochondrial DNA which accounts for less than 1 per cent of an animal's genetic material and is only inherited through the maternal line.
The latest study, published in the journal Science, is based on the much larger amounts of DNA within the animal's chromosomes, which contains a more complete genetic history of the species.
When scientists compared chromosomal DNA of 19 polar bears to the similar DNA of other bear species, they found a clear relationship with the brown bear, which sometimes has overlapping territories with the polar bear, especially during warmer periods.
The scientists estimate that the last common ancestor of polar bears and brown bears lived between 338,000 and 934,000 years ago. The most likely date, however, was about 600,000 years ago, when coincidentally there was a marked global cooling resulting in one of the most pronounced ice ages, said Frank Hailer of the Biodiversity and Climate Research Centre in Frankfurt.
"It was the first dramatic cooling period of the ice ages. Our data on the polar bear lineage may be coincidental but it fits in with the time period when the climate was very cold," he said.
In recent years there have been several reports of encounters between male polar bears and the smaller brown bear resulting in fertile offspring. One hybrid was the result of a hybrid polar-brown bear mating with a pure-bred polar bear.
Dr Hailer said the earlier study on mitochrondrial DNA could be explained by something similar happening about 150,000 years ago, when female brown bears mated with male polar bears resulting in fertile female offspring that "back-crossed" with pure-bred male polar bears, carrying their mitochondrial DNA with them.
"It shows that polar bears and brown bears hybridised about 150,000 years ago and then backcrossed into the main polar bear population. The mitochondrial DNA of polar bears today stems from this period," Dr Hailer said.