Polar bear split from brown bear over 4 million years ago

[Science News]  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.