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The polar bear (Ursus maritimus) is the largest and heaviest land carnivore in the world. Specially adapted to living in the Arctic, it has also become the poster animal of climate change, as the polar ice caps have slowly melted in the past century. But the polar bear may not have even been white in colour to begin with.
A new study has calculated that around 70,000 years ago, polar bears began to diverge from their closest relative, brown bears (Ursus arctos) by developing key features that we know them by today — huge behemoths with thick white fur and fully adapted to a life on the sea ice, complete with a lipid-rich diet.
“The divergence of polar bears from brown bears (Ursus arctos) and their adaptation to their Arctic lifestyle is a well-known example of rapid evolution. Previous research investigating whole genomes uncovered twelve key genes that are highly differentiated between polar and brown bears, show signatures of selection in the polar bear lineage, and are associated with polar bear adaptation to the Arctic environment,” the study noted.
So how did the polar bear develop its trademark characteristics?
The scientists analysed the genomes of 119 polar bears, 135 brown bears, as well as two fossilised polar bears.
The latter included a jawbone found on Svalbard, an archipelago administered by Norway in the Arctic Ocean. It was found to be between 130,000 and 100,000 years old.
The second fossil, a skull, belonged a juvenile polar bear and was found the Beaufort Sea near Alaska. This fossil was between 100,000 and 70,000 years old.
The team found that three genes associated with polar bear adaptation to the Arctic—APOB, LYST, and TTN—developed very recently in polar bears, just 70,000 years ago.
These genes are associated with cardiovascular function, metabolism and pigmentation.
“This could suggest that the variants we discuss here may not have been essential in the early adaptation of polar bears, but may have been driven by increased selective pressures during the later stages of the last glacial period,” the scientists wrote.
Study co-author and University of Copenhagen evolutionary biologist Michael Westbury told the portal Popular Science that this means the polar bear may have only gradually adapted to the Arctic environment after diverging from brown bears by developing such specialised characteristics such as very thick fur, which is white to provide camouflage as well as a body structure that can process a lipid-rich diet.
Late Pleistocene polar bear genomes reveal the timing of allele fixation in key genes associated with Arctic adaptation was published in the journal BMC Genomics on September 16, 2024.