Polar bears' "mitochondrial Eve," the female from whom all of today's polar bears are descended, was not a polar bear at all.
On July 7 researchers published their findings about the species history of polar bears in Current Biology. One of their discoveries was that the mitochondrial DNA in the cells of every polar bear's body originated in the cells of a female brown bear that lived in the neighborhood of Britain and Ireland during the era when Neandertals' numbers began go dwindle.
Mitochondrial DNA is passed on directly from mother to offspring—unmixed with any male input, unlike the DNA found in the cell nucleus—which means researchers can use it to trace the maternal line back through history.
By comparing the sequences in the mitochondrial DNA of 242 bears, ranging from living animals to fossils (living over a period of time spanning 120,000 years), from all across the globe, researchers could discover when and where modern polar bears' mitochondrial lineage originated. This photograph shows one of the Irish caves where bear fossils were found.
In the past 100,000 years, matings among brown and polar bears have successfully produced hybrid offspring, the analysis revealed. One of these matings occurred 20,000 to 50,000 years ago, and the entire modern population of polar bears is descended from the female half. The brown bears that shared the mitochondrial lineage of this polar bear ancestress have since died out, as have polar bears whose mitochondrial DNA stemmed from a different source.
The interspecies hybridization that begat today's polar bears was possible because of climate fluctuations. Although brown bears need habitats with a warmer climate than their polar cousins, shifts in temperature could push the two populations into proximity by rendering their natural habitats unlivable. For example, an increase in temperature would melt Arctic ice, leading polar bears to seek land-based homes farther south. A cooler period, on the other hand, would make parts of Ireland uninhabitable for the brown bears while facilitating the spread of glaciers, driving brown bears to new habitats such as on the ice shelves formed by the glacier in the Irish Sea basin.
"The bottom line is that the two species bumped up against one another for extended periods of time on different occasions, sharing both habitats and genes," Beth Shapiro, a biologist at Pennsylvania State University, said in a prepared statement. Shapiro, along with Daniel Bradley of Trinity College Dublin, led a team of scientists from both coasts of the U.S. and across Europe.
Brown and polar bear hybrids still exist today. Although brown and polar bears have been known to mate and produce offspring when living together in captivity, wild hybrids are more rare. In fact, the first wild hybrid sighting did not occur until 2006, when an Idahoan hunter shot one in Canada. In the past five years, however, more wild hybrids have been spotted.
Some conservationists would like hybrids to join polar bears on lists of endangered species (polar bears are currently classified as "threatened" under the Endangered Species Act). As for polar bears, their ability to survive climate change may get a boost from this research. Understanding how polar bears coped with changing ice distributions thousands of years ago could help scientists predict how they will respond to future change.
Bears are no strangers to travel, having spread from Europe across the globe over the course of more than 100,000 years. This video illustrates their diffusion.