When did some brown bears evolve into large, white bears that hunt on Arctic sea ice? Biologists have agreed that polar bears are related to brown bears, but have disagreed about when the polar bears first diverged.

A team of evolutionary biologists have set the time as about 150,000 years ago. They base their estimate on genetic analyses of material drilled from a tooth in the oldest polar bear fossil yet found. The fossil was found on an island in the Svalbard archipelago.

The researchers reconstructed the early bear’s mitochondrial genome and compared it to the mitochondrial genomes of six other specimens. The results show that today’s polar bears are closely related to brown bears in southeastern Alaska. Their findings are reported in the Proceedings of the National Academy of Sciences (PNAS) journal.

Other analyses of the ancient bear’s tooth showed that within 10,000 to 30,000 years of branching off from the brown bear lineage, the polar bears had achieved their current status as the top predator on sea ice and by 100,000 years had spread to the polar basin. Future sequencing of the genome, according to Charlotte Lindqvist, a biologist at the University of Buffalo and a co-author of the PNAS report, could provide clues to how the polar bears evolved so quickly and survived warming events in the last interglacial period.

In an article about the report, Science News quotes biologist Ian Stirling of the University of Alberta, who called the study “the most exciting development in polar bear research in recent years. It conclusively resolves a diversity of opinion about how recently polar bears evolved.” He added 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.

Lindqvist points out that earth was in the last stages of an ice age when the polar bears branched off. She speculates that climate changes might have forced the bear to follow the retreating ice northward to what has become its Arctic habitat.