by Amina Khan
All polar bears alive today are descended from a female brown bear that most likely hailed not from Alaska, as widely presumed, but from Ireland, scientists said.
The discovery, reported online Thursday 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 they are superb swimmers. The smaller brown bear, which includes grizzly and Kodiak bears, lives on land in warmer climes and eats plants and small animals.
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 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 the University of Oxford.
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