The Arctic is not the most welcoming place on Earth. Some Arctic animals like reindeer have some genetic adaptations to help them thrive there, and one of the region’s top predators is no exception. Scientists are teaming up when polar bears (Ursus maritimus) may have evolved some of the genes that separate them from brown bears (Ursus arctos). New genomic analysis reveals they may have diverged as early as 70,000 years ago. The findings are detailed in a study published Sept. 15 in the journal BMC Genomics.
Polar bears are closely related to brown bears, but have several key adaptations that help them survive in extreme arctic conditions. They have two layers of fur to help them stay warm and dry. The first is a large downy layer that sits right next to their skin. The next layer consists of longer hairs called guard hairs that act like a raincoat. The bright white signature shade of their fur helps them camouflage. Polar bears can also digest high amounts of cholesterol from the blue without damaging their hearts. This helps them thrive as they eat seals and even some species of whales like belugas.
[Related: Jackrabbit’s color-changing fur may prepare them for climate change.]
Scientists believe that polar bears and brown bears changed quite recently in evolutionary terms – roughly within the last one million years. How and when polar bears adapted to the Arctic is still debated.
In this study, a team analyzed the genomes of 119 modern polar bears, 135 modern brown bears and two fossilized polar bears. One of the fossils was the Poolepynten jawbone from the Norwegian archipelago of Svalbard dating between 130,000 and 100,000 years ago. The other fossil was the skull of a juvenile polar bear nicknamed Bruno that was found in Alaska’s Beaufort Sea. Despite the name, Bruno was a female bear that lived somewhere between 100,000 and 70,000 years ago, and its genome has helped scientists narrow down this brown-gold divergence.
“We found some variants that may have been selected for in the last 70,000 years (ie were not in fossil polar bears),” says study co-author and University of Copenhagen evolutionary biologist Michael Westbury. Popular science. “It was always assumed that when polar bears split from brown bears, they must have quickly adapted to the Arctic in a rapid evolutionary change. However, our results suggest that this may not have been the case, and adaptation to the Arctic was a more gradual process.”
The team compared these genomes to determine when seven genes key to Arctic adaptations had been selected. They found that for four of the genes (ABCC6, AIM1, COL5A3 and POLR1A), all polar bear genomes had the same DNA variant. This is called a fixed allele, which is the only variant present for a specific gene within a population.
However, the brown bear genome had multiple alleles. This suggests that these genes were already selected for in some ancient polar bear ancestor, and polar bears had adaptations for life in the Arctic earlier in their evolution.
However, three of the genes – called APOB, LYST and TTN – contained alleles that were fixed in modern polar bear genomes, but not in ancient ones. APOB, LYST and TTN genes are associated with cardiovascular functions. APOB and TTN are related to metabolism. APOB and LYST are associated with pigmentation.
[Related: What an ancient jawbone reveals about polar bear evolution.]
The team believes these genes may be related to adaptations that became necessary for polar bears to survive in more recent history, perhaps toward the end of the last ice age. It is also unclear whether other Arctic animals have similar adaptations in these genes that affect their fur color, heart health and metabolism.
“I assume that copies of these genes are found in most animals, but it may be the polar bear-specific variants that allowed them to live in the Arctic,” says Westbury. “We didn’t investigate whether any other Arctic animals had the same variants, but that would be interesting for a follow-up study.”