The Arctic is not the most hospitable place on earth. Some Arctic animals, such as reindeer, have different genetic adaptations that allow them to thrive there, and one of the region’s top predators is no exception. Scientists work together when polar bears (Ursus maritimus) may have evolved some of the genes that distinguish them from brown bears (Ursus arctos). New genomic analysis shows they may have changed as recently as 70,000 years ago. The findings are detailed in a study published September 15 in the journal BMC Genomics.
Polar bears are very closely related to brown bears, but have numerous important adaptations that help them survive in extreme Arctic conditions. She have two layers of fur to keep them warm and dry. The first is a large fluffy layer that sits right next to their skin. The other layer is made of longer hairs called protect hair which act as a raincoat. Their characteristic bright white shade of fur helps camouflage them. Polar bears can also digest large amounts of cholesterol from blubber without damaging their hearts. This helps them thrive as they eat seals and even some whale species such as belugas.
[Related: Jackrabbit’s color-changing fur may prepare them for climate change.]
Scientists believe that polar bears and brown bears separated relatively recently in evolutionary terms – within roughly the last one million years. How and when polar bears adapted to the Arctic is still up for debate.
In this studya team analyzed the genomes of 119 modern polar bears, 135 modern brown bears and two fossilized polar bears. One of the fossils was the Polar pygmy jawbone from the Norwegian archipelago of Spitsbergen dating to between 130,000 and 100,000 years ago. The other fossil was a juvenile polar bear skull nicknamed Bruno that was found in the Beaufort Sea in Alaska. Despite the name, Bruno was a female bear who lived somewhere between 100,000 and 70,000 years ago and its genome has helped scientists delve into this divergence between brown bear and polar bear.
“We found some variants that may have been selected over the past 70,000 years (i.e. they did not appear in the fossil polar bears),” study co-author and evolutionary biologist from the University of Copenhagen. Michael Westbury tells Popular science. “It was always assumed that when polar bears diverged from brown bears, they must have quickly adapted to the Arctic in one rapid evolutionary change. However, our results suggest that this may not have been the case, and that adaptation to the Arctic was a more gradual process.”
The team compared these genomes to determine when seven key genes for Arctic adaptations were selected. That’s what they found for four of the genes (ABCC6, AIM1, COL5A3 and POLR1A), all polar bear genomes had the same DNA variant present. This is called a fixed allelethe only variant present for a specific gene within a population.
However, the brown bear genomes had multiple alleles. This suggests that these genes had already been selected in an ancient polar bear ancestor, and that polar bears had adapted for life in the Arctic earlier in their evolution.
However, three of the genes, named 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 related to cardiovascular functions. APOB and TTN are related to metabolism. APOB and LYST relate to pigmentation.
[Related: What an ancient jawbone reveals about polar bear evolution.]
The team believes that this genes may be associated with the adaptations that became necessary for polar bear survival in more recent history, perhaps toward the end of the last ice age. It is also unclear whether other arctic animals have similar adjustments to these genes that affect their coat color, heart health and metabolism.
“I assume copies of these genes are found in most animals, but it may be the specific polar bear variants that allowed them to live in the Arctic,” says Westbury. “We did not look at whether other Arctic animals had the same variants, but that would be interesting for further research.”
