NEW BEARS ON THE BLOCK: What brought polar bears into existence some 150,000 years ago? Researchers are digging into DNA preserved in a rare bone to try to find out. Image: WIKIMEDIA COMMONS/ANSGAR WALK
Climactic changes might currently be threatening the survival of polar bears (Ursus maritimus), but similar shifts appear to have played an important part in bringing the species into existence in the not too distant past.
Researchers announced today that they have sequenced the mitochondrial genome of an ancient polar bear. The genetic traces they found in the bear's 110,000- to 130,000-year-old jawbone reveal that the species likely split from brown bears (U. arctos) just 150,000 years ago, at a time when specializing in arctic living quickly became an advantage rather than a liability.
The species' rapid evolution sheds new light on one of the emblematic species of climate change. Although the breed has become a popular flagship species for the issue, scientists knew little about how past climate affected the bear's evolutionary success.
The new genetic portrait gives a firmer frame to the polar bear's relatively rapid adaptations in the past. "From a conservation perspective it does tell us the polar bear has been through warming periods before," says Lisette Waits, a professor in the Department of Fish and Wildlife Resources at the University of Idaho, who was not involved with the research.
The bear's mitochondrial genome is the oldest one yet reported from a mammal—nearly twice as old as the mammoth genome. The results were published in the March 1 online edition of Proceedings of the National Academy of Sciences.
Parsing the polar bear
To arrive at the new portrait of polar bear evolution, the researchers used high throughput sequencing (a speedy DNA sequencing technique) to compare the genetic information gleaned from the ancient polar bear jawbone with that of six mitochondrial genomes of two modern polar bears and four brown bears from various habitats. They found that the polar bear whose jawbone was sequenced "existed very close to the most recent common ancestor of polar bears and brown bears." Previous estimates of polar bear evolution had placed the split any where from 70,000 years ago to more than one million years ago.
A mitochondrial genome does not give away physiological secrets like the more complete nuclear genome (such as the 4,000-year-old one recently completed for an ancient human). So the genetic fragments cannot tell us the color or thickness of this particular animal's fur. But it does lend some perspective to the genetic pedigree of the species and individual.
Judging from comparisons of modern bear genetics, it is estimated that the common ancestors of truly modern polar bears lived about 45,000 years ago, surviving what may have been a population bottleneck resulting from changing climate patterns.
Discovering the jawbone (from the bear's left mandible) was an accomplishment in itself. "Just finding such an ancient fossil was pretty magnificent," says Charlotte Lindqvist, a research assistant professor at the State University of New York at Buffalo and lead author of the study. As the researchers pointed out, because polar bears live primarily on sea ice, their remains often end up in the ocean. But this rare relic was found in 2004 on a small spit of land on the Norway's Svalbard Island.
Having been preserved in large part by the cold, dry climate, the fragment, which includes a canine tooth, still held some genetic remnants. "That was pretty astonishing as well," Lindqvist says. She and her colleagues were also able to ascertain a bit about the bear's eating habits. "From isotope analysis, we were able to get a good idea of its feeding ecology, and it fits very well at the top of the feeding chain of marine mammals" like polar bears today, she notes.
And even without the genetic revelations, the jawbone itself gives a few hints about the individual. "We can say it was probably a male adult polar bear of a size which can be found in modern polar bears and had a feeding ecology similar to today," Lindqvist says.
"I'm really impressed with their accomplishment," says Waits, who had previously published estimates about the genetic origins of polar bears that align closely with the new findings.
The news of these findings comes on the heels of a February report that grizzly bears have been encroaching onto polar bear territory in Canada's Wapusk National Park in Manitoba. The observation has concerned many conservationists, who worry about the grizzlies competing with—or even attacking—polar bears. But competitors though they may be, grizzly bears (being of the brown bear subspecies Ursus arctos horribilis) are such close relatives that the two species can produce fertile offspring.
The arrival of grizzlies into polar bear country opens up a fresh set of questions about the polar bear's genetic future in the midst of new discoveries about their past. "We know from earlier observation that polar bears and brown bears can hybridize, so there's a whole new potential for change in speciation or change in the polar bear gene pool or change in the brown bear gene pool," Waits says.