The female ancestor of all polar bears was a brown bear that lived in the region of present-day Britain and Ireland 20,000 to 50,000 years ago, researchers say.
Climate changes affecting North Atlantic ice sheets probably gave rise to periodic overlaps in bear habitats that led to hybridization, or interbreeding, causing maternal DNA from brown bears to be introduced into polar bears, scientists say.
The research, led by Beth Shapiro of Penn State and Daniel Bradley of Trinity College Dublin, is expected to help guide future conservation efforts for polar bears, listed as threatened under the Endangered Species Act, a Penn State release said Thursday.
Polar bears are expert swimmers that have adapted to a highly specialized, arctic lifestyle, while brown bears -- including Grizzlies and Kodiaks -- are climbers preferring mountain forests, wilderness regions, and river valleys of Europe, Asia, and North America, the researchers said.
"Despite these differences, we know that the two species have interbred opportunistically and probably on many occasions during the last 100,000 years," Shapiro said.
"Most importantly, previous research has indicated that the brown bear contributed genetic material to the polar bear's mitochondrial lineage -- the maternal part of the genome, or the DNA that is passed exclusively from mothers to offspring."
While the specific population of brown bears that shared its maternal DNA with polar bears has been extinct for roughly 9,000 years, Shapiro says clear genetic evidence shows the two species were in contact long before the brown bear's disappearance from the British Isles.