All polar bears alive today are descended from a single female brown bear who most likely hailed not from Alaska -- as widely presumed -- but from Ireland, scientists announced in July.
The discovery, reported online in the journal Current Biology, suggests that polar bears and various species of brown bears probably encountered each other many times over the last 100,000 years or so as climate change forced them into each other's territory. On some occasions, those meetings produced hybrid offspring whose genetic signature lives on in polar bears today.
The findings were made by analyzing the mitochondrial DNA extracted from 242 bear lineages. Some of them were polar bears and some were brown bears. Some lived recently and others have been dead since the late Pleistocene, which ended nearly 12,000 years ago.
Polar bears and brown bears are uniquely suited to their habitats. Polar bears have white coats to help them blend in and sneak up on prey, a carnivore's fearsome set of teeth, and are superb swimmers. The smaller brown bear lives on land in warmer climes and eats plants and small animals of all sorts.
Based on fossil evidence and genetic analysis, scientists had thought that polar bears' closest relatives were the brown bears living on islands off the coast of Alaska.
Although members of the two species can, and have, met and mated -- as evidenced by the occasional "grolar bear" hybrid popping up in the Canadian Arctic -- those couplings are extremely rare and thought to be brought on by global warming, as melting glaciers force polar bears into the brown bears' habitat, and brown bears encroach northward into polar bears' Arctic refuge.
So imagine study leader Ceiridwen Edwards' surprise when she analyzed mitochondrial DNA in the bones of extinct brown bears collected from Irish caves and discovered that it most closely resembled the DNA of modern polar bears.
Unlike nuclear DNA, mitochondrial DNA is passed down essentially unchanged from mother to child and provides a clear record of maternal lineage. Using mitochondrial DNA, scientists had already determined that all living polar bears could trace their roots to a single "Eve."
But to think that she was an Irish brown bear?
"I thought maybe I'd made a mistake," said Edwards, an archaeological geneticist at University of Oxford.
To rule out the possibility that the bones recovered from the Irish caves belonged to polar bears, not brown bears, she and her colleagues analyzed isotopes of carbon and nitrogen in the bones and found that the remains belonged to an animal with a land-based diet, not one feeding on marine life.
Then the researchers teamed with Beth Shapiro, an associate professor of biology at Pennsylvania State University, who compared the Irish mitochondrial DNA to genetic samples of bears that lived in Asia, Europe, North Africa and North America over the last 120,000 years.
Shapiro's bear family tree showed a number of strange patterns. For instance, Irish brown bears that lived right around the peak of the glacial period, between 38,000 and 10,000 years ago, shared their mitochondrial DNA with polar bears -- more so than the brown bears living on islands off the coast of Alaska.
The researchers think that during colder times, the glacial ice sheet would have extended all the way south into Ireland, allowing polar bears to roam into brown bear territory and making cross-species hybridization possible. One of the resulting female cubs probably went on to become a polar bear matriarch, and the descendents of all other matriarch lines died off.
The study showed that, rather than being an aberration, these hybridizing events may have happened multiple times over the course of polar bears' history and might be a more common part of the evolutionary process than previously thought. The researchers even point out that it may be time to provide protections to such hybrids, the same as those given to "purebred" species.
"I think it's really cool," said Graham Slater, an evolutionary biologist at the University of California, Los Angeles, who was not involved in the study. "These ancient DNA studies are really exciting because they give you a window into the dynamics of how these animals moved around, and who they were interbreeding with, that you just don't get with only the living individuals."