Tuesday, July 12, 2011

Polar bears came from Ireland


Perhaps polar bears roar in Gaelic.

An international team, led by Penn state scientist Beth Shapiro, has traced the ancestry of modern polar bears to a single female who likely lived somewhere in Ireland.

And this ursine Eve was probably brown, not white.

According to the research, some 20,000 to 50,000 years ago, ancient polar-like bears opportunistically interbred with the brown bears native to the British Isles. This may have allowed the polar ancestors to survive better during especially warm or cool periods in the earth's history, when the the habitats of the two species was most likely to be pushed together.

Polar bears and brown bears are very different species in many ways — size, diet, habitat, teeth, you name it. In fact, polar bears are the largest land carnivore on earth, whereas the brown bear's subspecies can range from relatively small, like the grizzlies of North America, to as large as polar bears with Yukon browns.

But interbreeding with local subspecies can provide a competitive advantage for both groups when they are brought into contact. Each species will pick up genes from the other, providing genetic diversity.

Strictly speaking, it was already known that brown bears and polar bears have interbred several times in the distant past. What wasn't known was the comparatively precise date range of the most important interbreeding, and the location in which it occurred.

"The bottom line is that the two species bumped up against one another for extended periods of time on different occasions, sharing both habitats and genes," Shapiro said in a press release.

The team traced this ancestry using mitochondrial DNA, a simple, circular form of DNA found inside the energy-producing organelles of every animal (indeed, every eukaryotic) cell. This DNA is passed only from mothers to children, and is less likely to undergo random mutations. This makes it useful in determining accurate lineages of species over thousands of generations, but means only female lineages are looked at.

It's not that the female ancestor is the oldest of all ancestors of modern polars, only that she would have been the only female ancestor of every modern polar bear.

Shapiro said that the next step is to turn the attention of research to chromosomal DNA. The more familiar variety in the nucleus of animal cells could tell us more about the frequency of interbreeding, what environmental changes spurred it to happen, and much more.

Institutions and scientists in California, the UK, Belgium, Spain, Denmark, Russia, Sweden, and of course, Ireland, also contributed to the research.