DNA Analysis Shows Polar Bears Have Adapted Quickly in the Past

Genetic analysis of an ancient polar bear fossil has formally dated the species’ birth to 150,000 years ago, shortly before an Ice Age thaw produced a climate comparable to what’s expected in a globally warmed future.

“They’ve certainly experienced climate changes before,” said Charlotte Lindqvist, a biologist at the State University of New York at Buffalo and co-author of the analysis, published March 1 in the Proceedings of the National Academy of Sciences. “The big question is whether they’re going to be able to survive in the future.”

Polar bears have become an icon of climate change concerns, with environmentalists and many researchers predicting their imminent doom. The bears spend their summers hunting seals on fast-dwindling Arctic sea ice. As the ice melts, the bears starve.

According to a 2007 United States Geological Survey review, two-thirds of all polar bears will likely vanish [pdf] by the mid-21st century. An international consortium of arctic researchers has said that “the survival of polar bears as a species is difficult to envisage” [pdf] if summer sea ice is lost. That may happen within several decades.

However, critics of those conclusions say polar bears may prove more adaptable than expected.

In 2007, University of Iceland geologist Ólafur Ingólfsson, co-author on the new study, found a fossilized polar bear jawbone on the Arctic Ocean island of Svalbard. He estimated its age at between 110,000 and 130,000 years. Until then, the species was thought to be about 90,000 years old. The new estimate meant they’d survived the Eemian, a period of globally high temperatures that started 130,000 years ago, ending the next-to-last Ice Age, and lasted for 15,000 years until the last Ice Age began. Earth scientists consider the Eemian a preview of climate changes expected in the next few centuries.

In the recent study, Ingólfsson and Lindqvist compared DNA extracted from the fossil’s mitochondria — cell structures that float outside the nucleus and have their own genetic material — with mitochondrial DNA from modern polar bears and brown bears, their closest relative. They used the amount of genetic change that occurred over the last 130,000 years to calculate the genetic mutation rate and then extrapolated the same rate back in time. The result suggests the two species split around 150,000 years ago.

Structural evidence and dietary mineral traces from the jawbone show that 20,000 years after the split, the animal was already as big as a modern polar bear, ate a similar diet rich in marine mammals, and lived in similar regions. The new findings imply these adaptations took place in a relatively short time.

If the species changed so dramatically once, perhaps it might change so dramatically again. But Lindqvist warned against drawing premature conclusions.

“In evolutionary terms, they adapted in a short period of time to the specialized species they are today. But I’m talking tens of thousands of years, not decades,” she said.

Rather than physically changing, she expects polar bears to gather in the last few hospitable places on Earth. Such regions likely provided refuge during the Eemian, said Lindqvist.

Image:Steve Hillenbrand/USFWS