Hardy polar bears have survived past global warming

POLAR bears have patrolled the planet's icy regions for millions of years longer than previously thought - riding out several episodes of global warming in that time. While this suggests their future might not be so bleak, it is no guarantee they will survive the melting occurring in the polar regions today.

Charlotte Lindqvist of the University at Buffalo, New York, and an international team of researchers have just completed the most comprehensive analysis yet of the polar bear genome. The team looked at DNA from 23 living polar bears and a 110,000-year-old polar bear jawbone. Aided by comparisons with the genomes of brown and black bears, they found that polar bears first emerged as a separate species between 4 and 5 million years ago. Previous studies had suggested the species didn't diverge from brown bears until much later - perhaps just 600,000 years ago.

The analysis also showed that polar and brown bears have intermittently interbred since their initial divergence, and that populations of polar bears have see-sawed over the past million years in tandem with the climate. Polar bear numbers dropped during warm periods but bounced back with prolonged ice ages - most noticeably during a period of cooling between 800,000 and 600,000 years ago (Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.1210506109).

The population looks set to fall again as melting pack ice forces polar bears back to the land-based habitats of brown bears, where interbreeding has recently been observed. Should extensive interbreeding occur, both polar and brown bear populations could decline with the emergence of a new hybrid species.

"Even if polar bears have experienced warming periods in the past, it is no guarantee that they'll survive this time," says Lindqvist. "It seems likely there were still pockets of suitable polar bear habitat available during past warm periods, but the question is whether that will hold for the future."

Lindqvist says that the analysis reveals that polar bear DNA has lost diversity as populations have dwindled, drifted apart and become genetically isolated, suggesting today's bears have less resilience to the environmental change, habitat loss, pollution and diseases they now face.

Earlier this year, Frank Hailer of the Biodiversity and Climate Research Centre in Frankfurt, Germany, and colleagues estimated that polar bears diverged from brown bears 600,000 years ago - a result that itself pushed back the evolutionary record of polar bears by about 450,000 years (Science, DOI: 10.1126/science.1216424).

However, Hailer says that the new results broadly tally with his team's findings, which also suggested that polar bears may have evolved much earlier than the 600,000 year mark.

"It's a good new, educated guess based on a lot of exciting data, but the actual numbers and assumptions on which the earlier split rely remain open to debate," Hailer says.

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