Polar bears have seen hard times

Two genetic studies extend the Arctic icon’s lineage way back

2012 SCIENCE NEWS TOP 25: 13

The polar bear, furry face of wildlife at risk from climate change, now looks as if it may have been around long enough to have survived past warm spells. Just how long remains to be seen, but two genetic studies published this year push the species’ origins back beyond the start of the most recent ice age.

Sergey Uryadnikov/Shutterstock

That’s a big rewrite of polar bear history that has people thinking about the bears’ future; even if the bears survived several past warm periods, that’s no guarantee they will survive this one.

Until the genetic analyses this year, polar bears’ history as a species was thought to be short. A fossil jawbone and tooth from Norway, the oldest well-documented polar bear remnants, date back only 110,000 to 130,000 years. And preliminary genetic studies by Charlotte Lindqvist of the University at Buffalo and her colleagues traced the female polar bear line back only about 150,000 years to a junction with the brown bear lineage.

If polar bears had really evolved as a species that quickly, it would be “a miracle of rapid adaptation,” says evolutionary biologist Frank Hailer of the Biodiversity and Climate Research Centre in Frankfurt.

But this year, two independent teams (one including Hailer, the other with Lindqvist) got the first good look at DNA from the nuclei of polar bear cells. Lindqvist’s previous estimate had come from DNA in mitochondria, which traces only the maternal lineage.

Hailer and his team pushed the split from brown bears back to about 600,000 years ago (SN: 5/19/12, p. 12), and Lindqvist et al to a much more ancient 4 million to 5 million years (SN: 8/25/12, p. 15). Dating back 600,000 years would mean the species survived a warm spell around 400,000 years ago, says paleoclimatologist Gifford Miller of the University of Colorado Boulder. The older estimate would put the bears into a different world that may not have had winter sea ice.

The two teams have many ideas about why their dates differ. But they agree that a greater age does not mean polar bears, now listed as threatened, will be just fine as their specialized sea ice habitat continues to melt.

For one thing, today’s polar bears are not those of yesteryear. The early ancestors in the polar bear lineage may not have been quite the specialized sea ice predators of today. And as Hailer points out, “The studies focus on the past.” With human-caused warming, the planet could heat faster and further than the bears have experienced before.

This is also the first time polar bears have shared a planet with billions of humans during a warm-up. Add in the stresses of hunting and pollution, which thanks to atmospheric circulation patterns tend to accumulate in the Arctic, and the balance might tip against the bears. 

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Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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