Despite being separate species, polar bears and brown bears interbred during periods of global warming in the past - and appear to be doing the same today.
An analysis of newly sequenced polar bear genomes, led by the Pennsylvania State University and the University at Buffalo, shows that the polar bear population has grown during periods of cooling and shrunk in warmer times over the past million years.
And it also suggests that while polar bears evolved into a distinct species some four or five million years ago, they may have interbred with brown bears until much more recently.
The reason may well be changes in Earth's climate, with the retreat of glaciers bringing the two species into greater contact as their ranges overlapped, says the team.
"Maybe we're seeing a hint that in really warm times, polar bears changed their life style and came into contact, and indeed interbred, with brown bears," says Stephan Schuster of Penn State.
The team generated deep sequence coverage for the entire genomes of a polar bear, three brown bears and a black bear, as well as lower coverage of 23 more polar bears, including a 120,000-year-old individual. "Very few vertebrate species have such comprehensive genomic resources available," says Schuster.
And, working from this data, the scientists discovered that polar bears are a much more ancient species than previously thought, dating back several million years, rather than the 600,000 years suggested by a recent study that looked only at small segments of DNA.
"We showed, based on a consideration of the entire DNA sequence, that earlier inferences were entirely misleading," says Webb Miller of Penn State.
It means that polar bears definitely persisted through warming periods during Earth's history. However, since then, smaller numbers have meant that they've lost a lot of their past genetic diversity, and are very likely more sensitive to climate change threats today.
The analysis uncovered new genetic similarities between polar bears and ABC brown bears, an isolated group from southeastern Alaska - suggesting that they've exchanged genes since becoming separate species.
Indeed, hybridization between brown bears and polar bears has been observed recently in Arctic Canada.