Filed by KOSU News in Science.
July 23, 2011

Nearly 12 percent of Americans claim some Irish ancestry. Even President Obama has a little Irish in him. But we’ve got nothing on polar bears.

According to a study in the journal Current Biology, every polar bear alive today can trace its ancestry to one mama bear that lived in Ireland during the last Ice Age. And what’s more, she wasn’t even a polar bear: She was a brown bear.

Study do-author Beth Shapiro, an associate professor of biology at Penn State University, tells Guy Raz, host of weekends on All Things Considered, that her team studied DNA extracted from ancient bear bones.

“In every cell, there are two different sources of DNA. There’s the mitochondrial DNA, which you inherit from your mother,” she says, “and there’s the nuclear DNA, which is a mix of the DNA that you get from your mother and your father.”

When Shapiro looked at the nuclear DNA, she found that brown bears and polar bears began to evolve separately from each other around a million years ago. But the mitochondrial DNA, the kind you inherit from your mother, told a very different story.

“They diverged only about 20- or 30,000 years ago,” she says. “And that difference is intriguing, and it means there’s something weird going on in the history of polar bears.

Shapiro says what probably happened is that the Ice Age brought polar bears and brown bears back together, as encroaching ice drove both kinds of bears to the very edges of their habitats. “And when they overlapped, they were able to breed,” she says.

But when the ice receded, the brown bears went back to being brown bears, and the polar bears went back to their icy habitat and bred with each other to produce more polar bears — just with a little extra DNA.

That hybridization is starting to happen again today, but for a different reason. As the Arctic ice melts, polar bears are coming farther south to search for food, Shapiro says, and they’re beginning to crossbreed with brown bears, producing hybrids known either as “pizzly” or “grolar” bears.

That could be the end of polar bears as we know them, Shapiro says.

“They can’t return to the ice and act like polar bears because that habitat isn’t going to exist anymore,” she says. “If the only way that polar bears can reproduce is by hybridizing with brown bears, then we’re not going to have any more polar bears.” [Copyright 2011 National Public Radio]