Unlocking the Secrets of Polar Bears' Extraordinary Evolution
We are a self-centered species, and what follows will seem like a particularly blatant example of it: Take polar bears, a species we are pushing rapidly toward extinction, and study them—quick, before it's too late—to learn how their biological adaptations can help us cope with our own deep-fried, high-fat modern diet.
"For polar bears, profound obesity is a benign state," said one researcher, in the press release for a new study being published today in the journal Cell. "We wanted to understand how they are able to cope with that," said another. "If we learn a bit about the genes that allow them to deal with that, perhaps that will give us tools to modulate human physiology down the line."
Wow, and could I please have a side of bacon with that?
OK, I think the press release was pandering (starting with the headline "Humans May Benefit..."). The scientists, possibly nudged along by the news office, were just succumbing to the myth that most people will care about the study of other species only to the extent that it might somehow make their own lives more comfortable. Let's be honest, though, polar bears are amazing all by themselves, and that's what this new research is really all about.
What interested the scientists was the chance to learn how polar bears have adapted to live all winter in some of the coldest and least hospitable conditions on Earth, without access to drinking water, subsisting almost entirely on a heart-attack diet of seal blubber and yet also swimming ultramarathon distances in the summer.
"How is that even possible?" said Eline Lorenzen in an interview. She's a molecular ecologist at the University of California, Berkeley, and part of a team of researchers extending from Denmark to China who used detailed genetic analysis to understand not only the how but the why and the when of polar bear evolution.
Polar bears are a surprisingly new species. Other large mammals typically separate into new species, at most, once every million or 2 million years, said Lorenzen. Forest and savannah elephants, for instance, went their separate ways 3 million years ago. Humans and chimpanzees last shared a common ancestor perhaps 7 million years ago. But according to two separate genetic techniques used for the new study, polar bears evolved from brown bears just 479,000 to 343,000 years ago.
Moreover, they seem, based on analysis of an ancient polar bear jawbone, to have completed the shift to their present form and behavior 110,000 years ago. Assuming a generation time of just over 11 years, that represents a radical change in appearance, behavior, and physiology in as little as 20,500 generations. In evolutionary terms, that's basically overnight.
How did they come to live like that? The timing of polar bear evolution coincided, according to the study, with a long period of unusually warm weather, which "could have enabled brown bears to colonize northern latitudes that were previously uninhabitable for the species." But when the climate switched back to colder conditions, isolated populations either died or rapidly adapted to "some of the world's harshest climates and most inhospitable conditions."
The most obvious change was the shift to white coloration, for camouflage against snow and ice. But some of the most important adaptations were the invisible ones, according to the new study, mainly involving genes for coping with high fatty-acid intake and cardiovascular function.
Because they prey mainly on the thick blubber of ringed seals, polar bears typically have a total blood cholesterol level of about 381 milliliters per deciliter and a fatty triglyceride level of 292. For a human, that would spell atherosclerosis and a heart attack. But polar bears don't suffer any known problems from this diet, apparently thanks to mutations to a gene known as APOB. In humans, mutations to APOB are associated with cardiovascular calamity. But the mutation in polar bears leads to production of a protein that seems to help clear lipids and cholesterol harmlessly from the blood. In the single sentence in the study relating to human beings, the coauthors note that this finding should encourage researchers to study a broader range of species "in our search for the underlying genetic causes of human cardiovascular diseases."
As for the bears, nothing in the new study can help with the main cause of their current decline: loss of Arctic sea ice from climate change. An estimated 20,000 to 25,000 polar bears survive in the wild, down 30 percent since 1970. "Due to their long generation time and the current greater speed of global warming," the International Union for Conservation of Nature reports, "it seems unlikely that polar bears will be able to adapt to the current warming trend in the Arctic. If climatic trends continue, polar bears may become extirpated from most of their range within 100 years."
Brown or grizzly bears have meanwhile begun to move north as the climate warms, and they occasionally interbreed with polar bears, producing hybrids known as "pizzlies." Thus the polar bear's extraordinary adaptations to life on top of the world may unravel even faster than they first evolved.