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 (including grizzly bears). They found that polar bears mixed in with brown bears quite a bit over the millennia.
Polar bear fossils are rare and many of those 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 the teeth of this ancient bear and compared it to 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’re so closely related,” said Charlotte Lindqvist, a biologist at the University at Buffalo and lead author of the article, in a phone call with Gizmodo. “Obviously they had quite an intertwined evolutionary history, where they mixed their DNA throughout their history after splitting into separate lineages.”
Polar bears and brown bears diverged as species between 1.3 and 1.6 million years ago, Lindqvist said. Although the two species look very different today, they crossed paths after their divergencea process that has seen more brown bear genes circulating in polar bears, according to recent research.
In other words, modern polar bears are genetically mixed with brown bears. In fact, previous research suggests that all polar bears alive today descended from a group of brown bears who lived in Ireland and mated with “pure” polar bears during the Pleistocene era.
“We see a dominant signal of gene flow entering polar bears, which then suggests that polar bears as a species inherited DNA from brown bears,” Lindqvist said. “Since these are such different species – polar bears being arctic specialists and brown bears being more generalists – you might wonder what kind of impact this might have on the polar bear as a than species.”
It’s impossible to say what these ancient polar bears might have looked like without more fossil evidence. Because most animals live and die on ice caps (which have become smaller and disappeared in recent years), most ancient polar bear bones probably lie at 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. These interactions increase the likelihood of crossover.
In theory, the two species could “merge” in the same way as modern times. humans subsumed Neanderthals in their populations, Lindqvist said, although the bears’ breeding seasons don’t overlap much and interbreeding is more likely to be a chance encounter than a widespread phenomenon. Such events will not save the animals, as habitat loss and other climate change issues will likely thwart any form of adaptation resulting from 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 is entirely up to humans to decide to what extent the habitat of large predators remains intact. But by learning more about where these bear species came from and how they interacted in the past, we can make some guesses about where the two species went.
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