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Polar bears' ancient roots pushed way back

Genetic blueprints suggest the animals arose millions of years ago

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3:01pm, July 23, 2012
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The polar bear has been around for a surprisingly long time. A new analysis of its DNA suggests that Ursus maritimus split from the brown bear between 4 million and 5 million years ago — around the same time when, some scientists believe, the Arctic’s thick sea ice first formed.

With such old origins, the creature must have weathered extreme shifts in climate, researchers report online July 23 in the Proceedings of the National Academy of Sciences. Simulations of how the DNA changed over time suggest that polar bear populations rose and fell with the temperature. After thriving during cooler times between 800,000 and 600,000 years ago, the bears seem to have suffered a genetic bottleneck and crashed after a warmer period that started about 420,000 years ago.

Whether the Arctic icon will survive today’s rapid warming remains an open question. “Even if this species has for sure experienced dramatic climatic changes before, that does not mean it’s safe today,” says Charlotte Lindqvist, an evolutionary biologist at the University at Buffalo in New York.

Lindqvist and colleagues had previously dated the emergence of the polar bear to no more than 150,000 years ago (SN: 3/27/10, p. 14). But that research relied on a single fossil containing mitochondrial DNA, genetic material inherited from the mother. In April, the origin was pushed back to 600,000 years ago by Frank Hailer, an evolutionary biologist at the Biodiversity and Climate Research Centre in Frankfurt (SN: 5/19/12, p. 12). His team examined more than 9,000 letters in nuclear DNA (derived from both parents) of modern bears.

The new study provides an even clearer picture, says Lindqvist, because it looks at the entire genetic blueprints of 23 modern polar bears.  Lindqvist and colleagues spelled out more than 2.5 billion letters in the creatures’ nuclear DNA and identified more than 13 million points of comparison.

But the date in the new study was calibrated using the rate at which mutations appear in the DNA of primates, Hailer says. “They’re assuming that the genetic clock in bears ticks at the same pace as that in primates.”

Even as it establishes an ancient pedigree for the polar bear, the new research blurs the lines between the species and its darker cousins. Isolated brown bears living in Alaska’s Admiralty, Baranof and Chichagof islands share between 5 and 10 percent of their DNA with polar bears, Lindqvist’s team found. A previous study found that ancient brown bears in Ireland also shared much mitochondrial DNA with their comrades to the north. 

So although polar bears and brown bears separated long ago, they seem to get together every now and then for a fling — perhaps when the weather warms, sending polar bears southward and brown bears northward. And that interbreeding may in itself be a threat.

“If you get more and more hybridization, you may lose whatever makes polar bears special,” says Ceiridwen Edwards, a geneticist at the University of Oxford in England.

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