For polar bears, being tubby is a way of life. Fat can make up 50% of their body weight; the blubber-laden seals they eat make bacon look downright healthy. Now, a new, extensive comparison of the genomes of polar bears and their closest relative, the brown bear, has revealed how polar bears survive such unhealthy diets.
The work also suggests that the bears evolved these changes relatively quickly, likely because they had to adapt to extreme conditions that forced them to switch to a diet that would be toxic to other mammals. “It’s a schoolbook example of evolution,” says Eske Willerslev, an evolutionary geneticist at the University of Copenhagen who helped the lead the research.
Brown bears—some of which are called grizzlies—and polar bears are closely related and are even able to interbreed. In the past few years, researchers have used genetic information to sort out this relationship  and to understand how polar bears thrive in the frigid Arctic, feeding primarily on seals and other marine life captured from holes in the ice. This work has included sequencing the animals’ genomes, which has indicated that polar bears are truly a distinct species that at times lived apart from brown bears and at times intermingled and interbred with them. But researchers disagree about when the polar bear began to split off from brown bears, with estimates ranging from about 600,000 years to as much as 5 million years ago.
In the latest sequencing effort, Willerslev and researchers from Denmark, China, and the United States analyzed the genomes of 80 polar bears from Greenland and 10 brown bears from North America and Europe. “[It’s] the most comprehensive genomic data set to date, as far as bears are concerned,” says Frank Hailer, an evolutionary biologist from Goethe University Frankfurt in Germany.
Drawing on that data, Willerslev and his colleagues conclude that polar bears split off from brown bears between 343,000 and 479,000 years ago . Although little more than a blink in time from an evolutionary perspective, that was long enough for key genetic differences to evolve, they note in a report today in Cell.
The most distinctive polar bear genes include many related to fat processing and to the development of the heart and circulatory system. Indeed, nine of the 16 most distinctive genes are ones that in humans are associated with heart disease, Willerslev says. One that stood out was a gene called APOB, which helps transfer fat from blood into cells.
In brown bears, the sequence of this gene varies from one bear to another, but all the polar bears surveyed have an identical version, with the exact same genetic code at nine variable spots in the gene, about half of which should change the function of the APOB protein.
That all polar bears have the same version indicates that it is very beneficial, perhaps enabling the animals to eat lots of fat without developing artery-clogging plaques that can plague humans who eat high-fat diets, says study co-author Eline Lorenzen, a molecular ecologist at the University of California (UC), Berkeley. An independent chemical analysis of a 120,000-year-old fossil polar bear jawbone  showed that the species was already dependent on marine prey by then—a big switch from the brown bear’s chiefly vegetarian fare—indicating that these genetic changes occurred in just a few hundred thousand years. “That’s very surprising for such a large mammal,” she says.
Given the number of genomes studied and the sophisticated analysis used, the date for when the species diverges is “the best estimate of what we’ve gotten so far, and it makes sense,” says Beth Shapiro, an evolutionary biologist at UC Santa Cruz, whose earlier work also suggested these bears split less than a million years ago. But Charlotte Lindqvist, an evolutionary biologist from the University at Buffalo in New York who has proposed a much earlier date for the origin of the polar bear, is not completely convinced. “The evolutionary history of these animals has probably been very complex,” with the two species separating and interbreeding multiple times, she points out. “I don’t really see that [new study] resolves anything.”
Both she and Hailer contend that researchers need to get genomes from a wider distribution of bears, particularly brown bears. “It might be that some conclusions will be altered when the genetic diversity within these bears can be characterized more thoroughly,” Hailer says.