As warming temperatures drive polar bears south, they're starting to mix with brown bears much the way they did thousands of years ago.

GUY RAZ, host: A survey in 2008 showed that close to 12 percent of Americans claim Irish ancestry, and at least half of our presidents have. Even President Obama has a little Irish in him. But it turns out Americans have nothing on polar bears. A new study says all living polar bears can trace their ancestry to Ireland, specifically one brown bear who lived there around 20 to 30,000 years ago during the last Ice Age.

Beth Shapiro is an associate professor of biology at Penn State and one of the authors of the polar bear study. She joins me now from the studios of member station WPSU in State College. Beth, welcome to the program.

BETH SHAPIRO: Thank you very much.

RAZ: How did polar bears end up in Ireland, because I thought there were no - I've been to Ireland. I have never seen a bear there.

SHAPIRO: Well, brown bears used to live in Ireland. They went extinct eight or 9,000 years ago. But it was before that that these bears happen to interact with polar bears.

And you have traced all polar bears to one brown bear in Ireland?

All living polar bears.

RAZ: All living polar bears.

SHAPIRO: We have to keep that in mind.

RAZ: OK.

SHAPIRO: Before these living polar bears had a common ancestor in Ireland, there were polar bears - and this is an important part of the story. So we did this by looking at the DNA sequences that we could retrieve from the bones of these bears that used to live in Ireland as well as all across the Northern Hemisphere. See, in every cell, there are two different sources of DNA. There's the mitochondrial DNA, which you inherit from your mother...

RAZ: Right.

SHAPIRO: ...and there's the nuclear DNA, which is a mix of the DNA that you get from your mother and father. If we look at that mixed DNA, the nuclear DNA, for brown bears and for polar bears and we ask when they started to divert to become different species, we can see that that happened somewhere around half a million to maybe a couple of million years ago. But if we look at this mitochondrial DNA, they diverged only about 20 or 30,000 years ago. And that difference is intriguing, and it means that there's something weird going on in the history of polar bears.

RAZ: So how did they actually crossbreed? I mean, presumably, they're different habitats, right? Brown bears don't live on ice and polar bears do.

SHAPIRO: Well, around the time that we think this hybridization or crossbreeding happened, there was a lot of ice on the landscape. It was during or just before the peak of the most recent ice age. There was a lot of ice that was forming over Ireland, and that was pushing the brown bears that lived there in caves toward the center of the island toward the edges of that habitat. But at the same time, there was quite a lot of ice forming over the British-Irish Sea, and that was forcing polar bears toward the edges of their habitat. And when they overlapped, they were able to breed.

RAZ: So the exact reverse is happening now. As the ice melts, polar bears move further south, and they start to interact with brown bears.

SHAPIRO: Yes. And in the last five years, there have been several instances of hunters finding what they're calling hybrid brown bear and polar bear.

RAZ: Mm-hmm. They call them the tizzly bear...

SHAPIRO: Yes.

RAZ: ...which is a stupid name, but that's - they've actually found some of these.

SHAPIRO: It's a terrible name. And sometimes, they're called grolar bears as well. I don't think there's been a decision made quite yet on them.

RAZ: What does this mean for people who are worried about the future of polar bears? Because, of course, this is a - this is an endangered species, right?

SHAPIRO: It doesn't mean much in terms of changing the way that we think about polar bears. I mean, yes, they hybridized in past. But what's important about that is when they had hybridized, there was still plenty of habitat available for both of these species to go back to. And they could mate with polar bears and become polar bears, or mate with brown bears and maintain those characteristics that are important to being a brown bear.

Today, the problem is that they can't return to the ice and act like polar bears because that habitat isn't going to exist anymore. If the only way that polar bears can reproduce is by hybridizing with brown bears, then we are not going to have any more polar bears.

RAZ: That's evolutionary biologist Beth Shapiro. Her study on the ancestry of polar bears is out this month in the journal Current Biology. Beth Shapiro, thank you so much.

SHAPIRO: Thank you. Transcript provided by NPR, Copyright NPR.