LONDON (Reuters Life!) - Scientists have found that the female ancestor of all polar bears was a brown bear which lived in present-day Britain and Ireland during the last ice age -- 20,000 to 50,000 years ago.
Changes in climate affecting the North Atlantic ice sheet probably gave rise to periodic overlaps in bear habitats and these overlaps then led to interbreeding between bears, causing maternal DNA from brown bears to be introduced into polar bears, according to a study by British and American researchers.
The findings, published online in the journal Current Biology, should help guide future conservation efforts for polar bears, which are listed as threatened under the U.S. Endangered Species Act.
Polar and brown bears are very different species in terms of body size, skin and coat color, fur type, tooth structure, and many other physical features, the researchers explained.
And in terms of behavior, they're also quite distinct -- polar bears are expert swimmers that have adapted to an Arctic lifestyle, while brown bears are climbers that prefer mountain forests and the river valleys of Europe, Asia and North America.
"Despite these differences, we know that the two species have interbred opportunistically and probably on many occasions during the last 100,000 years," said Beth Shapiro of Penn State University, who led the study.
"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.
"But, until now, it was unclear just when modern polar bears acquired their mitochondrial genome in its present form."
After performing genetic analyses of 242 brown bear and polar bear mitochondrial lineages sampled throughout the last 120,000 years and across many geographic ranges, Shapiro's team found that the modern polar bear's mitochondrial DNA probably underwent fixation -- a drastic reduction in genetic variation and a transition to a state where an entire gene pool includes only one form of a particular gene.
This fixation probably occurred during or just before the peak of the last ice age, they said, possibly as early as 50,000 years ago near present-day Ireland.
"The odd thing is that although polar bears and brown bears have been around for a long time, and are clearly different, these Irish brown bear genes have swept through polar bears so quickly," said Mark Thomas of University College London's department of genetics, evolution and environment, who worked with Shapiro.
"It just goes to show that ancestry and origins are not as simple as we might think."
Reporting by Kate Kelland, editing by Paul Casciato