Ancient DNA suggests polar bears evolved recently

The polar bear probably evolved no more than 150,000 years ago and is most closely related to brown bears that now live in southeastern Alaska, new genetic analyses of a rare fossil suggest.

Ursus maritimus, the polar bear, is a specialized predator that — ignoring the bears that forage for garbage in towns and villages along the Arctic coast — hunts solely on sea ice.

Several previous studies agreed that polar bears are closely related to brown bears but provided widely divergent answers about when polar bears first evolved, with estimates ranging between 70,000 and 1 million years ago, says Charlotte Lindqvist, an evolutionary biologist at the University at Buffalo in New York. Now, genetic analyses of material from a fossil first described two years ago narrow the window when the huge white bears first appeared, Lindqvist and her colleagues report in a paper to be published in the Proceedings of the National Academy of Sciences.

The fossil, a jawbone found on an island in Norway’s Svalbard archipelago, was unearthed from rocks laid down as sediment between 110,000 and 130,000 years ago. Certain features of the jawbone and its teeth, including their size and shape, indicate the remains come from a polar bear and not another type of bear, Lindqvist says.

The Scandinavian remains represent by far the oldest polar bear fossil yet found, she notes.

Using material drilled from one of the fossil’s canine teeth, researchers were able to reconstruct the bear’s mitochondrial genome, the sequence of genetic material passed down only through female ancestors. Then, the scientists compared those results with the mitochondrial genomes of six other specimens, including modern polar bears and brown bears from two regions in Alaska.

Together, the genetic tests show that modern-day polar bears, which live throughout the Arctic and fall within one species, are most closely related to brown bears living on islands in Alaska’s panhandle — the so-called ABC islands of Admiralty, Baranof and Chichagof. The analyses also suggest that polar bears first appeared about 150,000 years ago, not too long before the Svalbard polar bear patrolled the Arctic.

“This is the most exciting development in polar bear research in recent years,” says Ian Stirling, a biologist at the University of Alberta in Edmonton, adding, “It conclusively resolves a diversity of opinion about how recently polar bears evolved.”

About 150,000 years ago, Earth was in the last stages of an ice age and was beginning to warm, and high-latitude ice sheets were melting and breaking up, Lindqvist says. Dramatic environmental changes at that time, probably similar to those that marked the end of the most recent ice age about 10,000 years ago, may have led to the evolution of the polar bear, she and her colleagues speculate.

Then, as temperatures warmed further and Arctic sea ice retreated, polar bears might have been forced to follow the ice northward to scattered refugia like the Svalbard archipelago, which lies just 1,300 kilometers from the North Pole.

Besides providing a rare look at how rapidly the polar bear could evolve into a new species, the new findings might shed light on the pattern and timing of continental ice sheet melting at the end of the penultimate ice age, Stirling says. Also, he notes, the research may provide hints about whether and where modern-day polar bears might survive as Arctic sea ice retreats in today’s warming climate.