There is a surprising amount of polar bear ancestry in the genomes of all living brown bears after an analysis of ancient DNA from a 100,000 year old polar bear.
The study was published in the journal Nature Ecology & Evolution. In 2009, a juvenile polar bear's skull was found on the coast of the Beaufort Sea. The bear they nicknamed Bruno was later found to be a female.
The availability of Bruno's paleogenomics has made it possible to detect an ancient event that impacted all living brown bears.
According to the team's analyses, Bruno belonged to a polar bear population that was ancestral to living polar bears. She said that after around 125,000 years ago, the polar bear and brown bear families crossed paths and hybridized.
Up to 10% of the genomes of brown bears are related to polar bears. Bruno's genome is part of all living brown bears' genomes.
When their ranges overlap, polar bears and brown bears can easily hybridize. Climate warming and the disappearance of sea ice have caused reports of hybrid bears to increase.
Some populations of brown bears have had at least four different instances of admixture between 15,000 and 25,000 years ago, according to previous studies. The direction of the flow of genes was from polar bears to brown bears.
If they survive, the admixed individuals will become brown bears because they can't hunt on the sea ice if they are not completely white. The polar bears have a small population.
There was some evidence of brown bears in Bruno's family, but the lack of brown bears in today's polar bears supports the idea that brown bears are less fit for life as a polar bear. After diverging from brown bears about half a million years ago, polar bears evolved into hunters of marine mammals. There are brown bears in North America, Europe, and Asia.
After the peak of a warm interglacial period when temperatures and sea levels were much higher than they are now, Bruno lived. Climate change caused by the burning of fossil fuels can cause similar conditions in the future. Many polar bear populations are struggling to survive.
If the rapid, unnatural, and severe human-caused warming of theArctic continues, it is uncertain whether polar bears will have a sea ice habitat to return to and survive genetically.
As the climate changes and these species overlap and encounter each other again in the wild, we shouldn't be surprised. Gene flow can happen between what we think of as different species.
When sea ice was more extensive, it allowed polar bears to mix with brown bears in southeast Alaska and even Ireland. The brown bears in these locations acquired more polar bear genes than Bruno's genome revealed.
Scientists don't know what brown bears gained from their polar bear ancestry. It's possible that brown bears got something cool from polar bears, but we can't say for sure at the moment.
It was a coincidence that Bruno's skull was found. The skull of a person was found just above the high tide line by a group of people walking along the coast.
"We weren't even looking for bones, as typically we find ancient bones a hundred miles inland where they have been stored in permafrost along sleepy rivers," saidGroves. It is extremely rare to find the remains of a polar bear. Bruno is the only ancient polar bear skull that has ever been found.
The program director of the U.S. National Science Foundation said that understanding how past changes in climate drove interactions between organisms is critical to predicting how current changes will impact natural resources or society.
The University of Cambridge, UC Santa Cruz, and the University of Washington, Seattle have authors.
More information: Beth Shapiro, A polar bear paleogenome reveals extensive ancient gene flow from polar bears into brown bears, Nature Ecology & Evolution (2022). DOI: 10.1038/s41559-022-01753-8. www.nature.com/articles/s41559-022-01753-8 Journal information: Nature Ecology & Evolution
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