In a new study in PNAS, researchers at Penn State and the University at Buffalo say their analysis of polar bear DNA has led them to extend to origin of the species back to five million years instead of 600,000 years, reports The New York Times' James Gorman. Previous studies relied on fossils and bones to determine how far back polar bears go, but given the animals' natural habitat, "death usually means burial at sea and skeletons lost to time," Gorman says. Recent studies of polar bears' mitochondrial DNA and snippets of nuclear DNA suggested they diverged from brown bears between 130,000 years ago and 600,000 years ago. But mitochondrial DNA only presents the maternal line, and the nuclear DNA pieces weren't big enough to give a whole picture, Gorman says.
In the new study, the researchers looked at the full genomes of polar bears, brown bears, and black bears and their data suggests that polar bears split from brown bears about five million years ago, Gorman adds. They also found "evidence of intermittent interbreeding between brown and polar bears from the time of the split until the present — most likely during periods of warming, when brown bears moved north and polar bears were forced onto land," he says. "In most brown bears the percentage of genetic material traceable to polar bears is 2 percent. But on the Alexander Archipelago, a 300-mile-long string of islands off the southeast coast of Alaska, the brown bears have 5 to 10 percent polar bear DNA, suggesting more frequent interbreeding."