A female grizzly bear who lived in Ireland less than 50,000 years ago was an ancestor of all modern polar bears, suggesting "grolar" hybrids were an important part of polar bear history.
The unexpected DNA evidence suggests that interbreeding between polar bears and brown bears — usually known as grizzly bears in North America — is not unusual at times when climate change caused the range of the two species to overlap, such as around the beginning of the last ice age.
"It's something that's part of the history of the polar bear," said Daniel Bradley, a genetics researcher at Trinity College Dublin who co-authored the study published Thursday in the journal Current Biology.
Brown bears vs. polar bears
For the most part, polar bear and brown bear genes are quite different, suggesting they became separate species long ago. They have a different skull shape, fur colour, teeth and diet. Grizzlies tend to forage on land for food ranging from roots to deer to berries. Polar bears mainly eat seals and other foods from the ocean.
Researcher Daniel Bradley noted that brown bears are very diverse and that genetic differences among different brown bear lineages can be much greater than the differences between brown bears and polar bears.
Grizzly-polar bear hybrids are rare, but a number have been seen in Canada in recent years. A bear in the Northwest Territories in April 2010 is believed to be the first second-generation hybrid — the offspring of a female grizzly-polar bear hybrid and a male grizzly bear — ever found in the wild.
Bradley and his colleagues wrote that their results suggest that hybrid bears should be protected, as both polar bears and grizzly bears currently are.
"They may play an underappreciated role in the survival of species," the paper said.
Previous DNA studies had suggested that polar bears became a separate species about 800,000 to 150,000 years ago, and were most closely related to brown bears in the Admiralty, Baranof and Chichagof Islands near Alaska.
Bradley and his colleagues uncovered the evidence about the bears' ancestry by looking at mitochondrial DNA extracted from the bones of museum specimens of Irish brown bears, which became extinct around 3,000 years ago. They compared it to mitochondrial DNA from other brown bears around the world as well as two ancient polar bear specimens from over 100,000 years ago.
"What was interesting was that the sequences from our brown bears were closer to modern polar bears than those ancient polar bears were," he said in a phone interview Thursday.
What is mitochondrial DNA?
Most of the DNA in an organism comes from both parents and is found in the nucleus. It is known as nuclear DNA.
However, special parts of the cell called mitochondria also contain DNA, which is only passed on through the mother. Because there can be hundreds of mitochondria in each cell, it is much easier to extract and analyze mitochondrial DNA from ancient specimens than it is to extract nuclear DNA.
In fact, the results, which Bradley said were unexpected, suggest that all polar bears share a female Irish brown bear ancestor from within the last 50,000 to 20,000 years — not that long ago on the evolutionary time-scale.
Based on the similarity between modern polar bear DNA and that of the Irish brown bear around the last ice age (between 100,000 and 10,000 years ago), they may have interbred multiple times.
About 22,000 years ago, an ice sheet covering most of Britain and Ireland was at its largest. That would have provided habitat for polar bears and may have forced brown bears into lowlands near the sea, where they could have encountered and mated with polar bears, the researchers suggested.
Bradley said he's not sure how polar bears would have arrived in Ireland, but they may have crossed over the ice from Scandinavia.
The large differences between the mitochondrial DNA of ancient and modern polar bears is due to the fact that many polar bear lineages have died out, Bradley said. Modern polar bears are all very closely related to one another and may belong to a lineage that is different from that of the ancient specimens in the study.
The research team involved in the study included scientists from Ireland, the U.S., Belgium, England, Spain, Denmark, Russia, Scotland and Sweden.