Polar bears may have trod the planet for millions of years, according to a new genetic analysis. That suggests the white-coated, massive bears have weathered previous natural climate changes, and may predate the Arctic ice that is their preferred—and only—habitat today, which is why the species future remains uncertain presently.
"There's no guarantee that they'll survive this time," says geneticist Webb Miller of Pennsylvania State University, an author of the study published July 23 in Proceedings of the National Academy of Sciences. After all, Miller notes, the species currently lacks much genetic diversity to help it adapt to changing conditions as well as facing unprecedented threats such as heavy metal pollution accumulating in the Arctic. By better understanding how old the species is, the scientists hope to better understand what might be done to allow the polar bear to cope with onrushing climate change and other existential challenges.
By analyzing the genomes of 28 bears—polar bears, including a roughly 120,000-year-old specimen from Norway's Svalbard archipelago, as well as modern brown bears and black bears—the scientists in effect read back in time to a common ancestor at least four million years ago. That finding conflicts with a genetic analysis published in Science earlier this year that suggested the species was only 600,000 years old or so, which the team behind the new research suggests may result from misreading past interbreeding events with closely related brown bears.
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In fact, the key problem here may be that technically the polar bear may not be a species at all. "If one defines that two species separate as when they cannot produce viable offspring, then perhaps brown bears and polar bears aren't yet separate species," Miller admits.
What makes a polar bear a polar bear? There's the white coat and black skin as well as less visible differences like a thicker layer of subcutaneous fat layers and richer milk. These and other unique features of adaptation to the harsh conditions of Arctic life have convinced biologists that the polar bears represent a unique type of animal—a species of its own. After all, the brown bear that is its closest living relative lacks all of these adaptations, looks different, eats different food and would not fare well in the harsh conditions out on the Arctic ice.
Yet, brown bears and polar bears, when they meet, can mate, as evidenced both by the genetic record and observations in the wild. Because polar bears have been spending more time off the ice in recent years, they appear to have begun to interbreed with adjacent brown bear populations, and some of these hybrids are into their second generations. If the basic definition of a species is a group of organisms capable of mating and producing fertile offspring only within their own group, the polar bear and brown bear fail to qualify.
Such interbreeding between bear species makes genetic analysis that much more difficult. After all, if the species interbreed even once, that single event can appear to determine the point at which the two species diverged if a scientist happens to analyze only the portion of the genome influenced by that mating event. And the mixing of genetic material has been going on for a long time between polar and brown bears, making disentangling their genetic history that much harder.
In the case of the polar bear, the differing estimates also rely, on the one hand, on fossil information about when the panda bear became an independent species and, on the other, the mutation rate of primates, including humans. By calibrating what scientists call the molecular clock—the hypothesis that mutation occurs at a predictable rate—to the panda separation 12 million years ago, one group of researchers suggests the polar bear's appearance as a species is a relatively recent phenomenon. By suggesting that bears undergo mutation at roughly the same rate as humans, who also may have interbred with closely related hominins like the Neandertals, the other group finds a much older lineage. "One can already suspect that the mutation rate of carnivores, especially bears, will be most likely different from that of primates," argues bioinformaticist Axel Janke of the Biodiversity and Climate Research Center in Germany, one of the researchers behind the study published in Science.
Regardless of how old the polar bear is as a species, or whether it's a species at all, the purpose of such studies is to gain a better understanding of the great white bear's ability to survive in the Arctic, which is now rapidly transforming as a result of accelerated global warming. Understanding how the bears weathered past climate changes or sea ice–free conditions might help identify what could be done to help the unique animal survive.
The new analysis suggests that the key may be refuges with suitable environmental conditions. For example, the polar bear specimen from roughly 120,000 years ago survived in Svalbard during a warm interglacial period because that Arctic archipelago remained more frozen than other areas. "It is possible that Svalbard may have provided one such important refuge during warming periods, in which small polar bear populations survived and from which founder populations expanded during cooler periods," argues biologist Charlotte Lundqvist of the University at Buffalo, The State University of New York, who is a co-author of the new study. That could also explain modern polar bear's relative lack of genetic diversity, which appears to have been declining for 500,000 years. The new genetic analysis found less difference between polar bears living on opposite sides of the Arctic than between Asians and Europeans.
But places like the ancient icy oasis of Svalbard, should they endure relatively unchanged through human-induced global warming, may not be enough to save the polar bear this time. After all, the bear faces the impacts of human pollution, hunting and increased activity in the Arctic, such as shipping and oil drilling as well as the changes in its sea ice habitat. "There is no species that deserves careful study at this time more than the polar bear," Miller says. "I want my great-grandchildren to be able to see them."