Shapiro’s Nature Ecology study also focused on what may have happened to other polar bear genomes during periods of low ice—in this case, around 120,000 or 125,000 years ago when, according to Shapiro, Arctic ice levels were similar to the present day’s. But here, she looked at the relationship between polar bears and brown bears.
Her team constructed a phylogenetic tree—sort of like an evolutionary map showing how the bears diverged from a common ancestor over time—using Bruno’s genome and those of currently living polar bears, brown bears, and a black bear. (Shapiro was able to utilize one of Laidre’s Southeast Greenland polar bear genomes in her analyses, although the time gap between its life and Bruno’s is enormous. The sample pool, she says, is “missing 100,000 years of evolution.”)
From this and other analyses, the scientists gained some evidence that about 20,000 years before Bruno was born, brown bears and polar bears mixed to generate hybrid offspring. The scientists hypothesized that during this warm period, polar bears might have made their way on shore. The carcasses of the marine mammals they hunted could have attracted brown bears—leading to mating opportunities. As a potential result of this ancient interbreeding, Shapiro says, up to 10 percent of the genome of the modern brown bear comes from polar bear ancestry.
Figuring out how and when polar bears and brown bears commingled, further specialized, or diverged is a difficult task, given the limited fossil record and complexities of evolution. “Evolution is a messy process,” says Andrew Derocher, a polar bear researcher at the University of Alberta who was unaffiliated with the studies. He likens the process of evolutionary speciation to a “massive bunch of vines that are…