All polar bears alive today are descended from a single female brown bear that most likely hailed not from Alaska — as widely presumed — but from Ireland, scientists said.

The discovery, reported online last week in the journal Current Biology, suggests that polar bears and various kinds of brown bears probably encountered one another many times over the last 100,000 years or so as climate change forced them into other species’ 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; they have a carnivore’s fearsome set of teeth, and they 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 brown bears living on islands off Alaska’s coast.

Although members of the two species can, and have, met and mated, 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 to encroach northward into polar bears’ Arctic refuge.

So imagine study leader Ceiridwen Edwards’s 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.

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.

When the researchers compared the Irish mitochondrial DNA with genetic samples of bears that lived in Asia, Europe, North Africa and North America over the last 120,000 years, they found 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, glacial ice would have extended into Ireland, allowing polar bears to roam into brown bear territory and enabling cross-species hybridization. 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.

— Amina Khan, Los Angeles Times