Researchers have extracted DNA from a 115,000-year-old polar bear jawbone and used it to analyze the genetic relationship between these Arctic predators and their brown bear cousins (grizzlies included). They found that polar bears intermixed with brown bears quite a bit over the millennia.
Polar bear fossils are rare, and many of those that are found are relatively young. But scientists got lucky just over a decade ago, when a 130,000-to-115,000-year-old polar bear jawbone was discovered in Svalbard.
The recent team produced a new, more complete genome extracted from that ancient bear’s teeth, and compared it with the genomes of 64 modern polar and brown bears. Their research is published this week in the Proceedings of the National Academy of Sciences.
“It’s hard to look at polar bears and polar bear evolution without also looking at brown bear evolution and brown bears, because they are so closely related,” said Charlotte Lindqvist, a biologist at the University at Buffalo and lead author of the paper, in a phone call with Gizmodo. “Clearly, they have had a fairly intertwined evolutionary history, where they have been mixing their DNA throughout their history after they split into separate lineages.”
Polar bears and brown bears diverged as species between 1.3 million and 1.6 million years ago, Lindqvist said. Though the two species look quite different today, they interbred after their divergence, a process that saw more genes from brown bears flow into polar bears, according to the recent research.
In other words, modern polar bears are genetically admixed with brown bears. In fact, previous research suggests that all living polar bears today descended from a group of brown bears that lived in Ireland and mated with “pure” polar bears during the Pleistocene Epoch.
“We’re seeing a dominant signal of gene flow going into polar bears, which then suggests that polar bears as a species have inherited DNA from brown bears,” Lindqvist said. “Since they’re such different species—the polar bears being Arctic specialists and the brown bears being more generalist—you can ask what kind of impact might that have on the polar bear as a species.”
It’s impossible to say what those ancient polar bears might have been like without more fossil evidence. Because most of the animals live and die on ice sheets (which have gotten smaller and disappeared altogether in recent years), most ancient polar bear bones are probably sitting on the bottom of the Arctic Ocean.
In the future, the researchers state in the paper, the two species will come into increased contact as sea ice melts due to climate change. Those interactions increase the likelihood of interbreeding.
In theory, the two species could “melt together” in a similar way to how early modern humans subsumed Neanderthals into their populations, Lindqvist said, though the bears’ breeding seasons don’t overlap much, and the interbreeding is more likely to be in chance encounters than a widespread phenomenon. Such events won’t save the animals, as habitat loss and other climate change-related problems will likely counter any sort of adaptations resulting from the species’ interbreeding. “The pace of environmental change is so rapid,” Lindqvist added. “The question is: can they keep up?”
Sex won’t save the polar bear; it’s entirely up to humans how much of the apex predators’ habitat remains intact. But by learning more about where these bear species came from and how they’ve interacted in the past, we can make some guesses about where the two species are going.