A scan of polar bear DNA has shown that polar bears came into existence far earlier than previously assumed. Working with fossils alone, scientists had thought that polar bears, after intermingling with brown bears, made their first snowy white appearance anywhere between 60,000 to 600,000 years ago. But a genetic analysis now shows that they first emerged considerably earlier — as much as 4-5 million years ago.
The study, which was conducted by Penn State University and the University at Buffalo, indicates that polar bears are not a recent spinoff of brown bears, and are instead a species that ebbed and flowed over the millennia according to changing climatic conditions — growing in population during cool periods, and shrinking in warmer times.
And fascinatingly, it was during these warmer periods that polar bears would intermingle with their brown cousins. Writing in the New York Times, James Gormans explains:
The progress of species formation, at least in this case, is a bit like a long, ambivalent divorce in which the two parties separate but occasionally fall back into bed even after the official decree.
Dr. [Charlotte] Lindqvist, said the DNA showed evidence of intermittent interbreeding between brown and polar bears from the time of the split until the present - most likely during periods of warming, when brown bears moved north and polar bears were forced onto land.
In most brown bears the percentage of genetic material traceable to polar bears is 2 percent. But on the Alexander Archipelago, a 300-mile-long string of islands off the southeast coast of Alaska, the brown bears have 5 to 10 percent polar bear DNA, suggesting more frequent interbreeding.
The microscope of whole-genome studies and comparisons also gives glimpses into the adaptation of polar bears to the Arctic environment, including genes for high fat content in milk and for blubber accumulation.
Quite obviously, this study has implications for our understanding of what will happen to polar bears given the current period of climate change. It's tempting to suggest that today's polar bears, like their ancient predecessors, will survive this current warming trend. But as the authors of the study warn, the situation is considerably different this time. Not only is climate change happening at a much more rapid pace, but the diversity of bear bears in warmer areas has been drastically diminished. Consequently, polar bears may be in a bit of trouble as they look for potential mates down south.
Check out the entire study in PNAS.