Shapiros Nature Ecology The study also focused on what may have happened to other polar bear genomes during periods of low ice – in this case about 120,000 or 125,000 years ago, when Arctic ice levels, according to Shapiro, were similar to today’s. But here she looked at the relationship between polar bears and brown bears.
Her team constructed a phylogenetic tree – a bit like an evolutionary map showing how the bears differed from a common ancestor over time – using Bruno’s genome and those of living polar bears, brown bears and a black bear. (Shapiro was able to use one of Laidre’s Southeast Greenlandic polar bear genomes in his analyzes, although the time difference between its life and Bruno’s is enormous. The sample pool, she says, “lacks 100,000 years of evolution.”)
From this and other analyzes, the researchers got some evidence that about 20,000 years before Bruno was born, brown bears and polar bears interfered to generate hybrid offspring. The researchers assumed that polar bears could have found their way on the coast during this hot period. The bodies of the marine mammals they hunted could have attracted brown bears – leading to mating opportunities. As a potential result of this ancient cross, Shapiro says, up to 10 percent of the genome of the modern brown bear comes from the polar bear’s ancestry.
Finding out how and when polar bears and brown bears mingled, specialized further, or diverged is a difficult task given the limited fossil history and the complexity of evolution. “Evolution is a messy process,” said Andrew Derocher, a polar bear researcher at the University of Alberta who was not associated with the studies. He compares the process of evolutionary species formation to a “massive flock of vines creeping up the base of a tree” that crosses and wraps itself. “Eventually, some of these vines can get their own trajectory, and that’s what our species is,” he says. “But in this process, they can cross over, they can reconnect and merge, and it’s certainly impossible to separate it from each other because they’re so interconnected.”
Yet these two studies are linked, says Laidre, “in the sense: Where did polar bears last when sea ice was low, and how?” The research can provide some insight into how bears in the past – and today’s southeastern Greenlandic bears – have survived in warmer climates with less ice.
But how genetic changes manifest themselves in physical form, and how these changes may have helped bears survive past warming events, are still open questions, the researchers say. And these research findings should not make us feel that the problem of Arctic warming has been solved, or that today’s bears can easily adapt to rapidly shrinking levels of sea ice. “It seems that global warming is happening too fast,” Lindqvist says. She wonders if the polar bears “can keep up”.
After all, polar bears are dependent on seals as their food source – and these seals are dependent on sea ice. “There are parts of the Arctic that used to be excellent seal habitats and excellent polar bear habitats,” Derocher says. “But there is no sea ice there anymore. And as a result, there are virtually no bears. There are very few seals and the ecosystem is basically unraveled.”
So what can really help? “Global response to climate change,” says Laidre. “That is it.”