Surprise discovery: All living brown bears share polar bear ancestry, UCSC scientists find

UC Santa Crus scientists studied the skull of Bruno, a female juvenile polar bear that lived 100,000 years ago
UC Santa Crus scientists studied the skull of Bruno, a female juvenile polar bear that lived 100,000 years ago, which led to new understandings of ancient polar bear and brown bear hybridization.
(Pam Grove / Via UC Santa Cruz)

Bruno, a 100,000-year-old polar bear, is at the root of a breakthrough understanding. UC Santa Cruz scientists Beth Shapiro and Ming-Shan Wang led the study that shows that interbreeding of the ancestors of polar bears and brown bears leaves an imprint on brown bears to this day. 

Thanks to a lucky discovery of an ancient polar bear skull in Alaska in 2009, a team of researchers led by UC Santa Cruz scientists has made quite a discovery: Polar bear ancestry accounts for 10% of the genomes of living brown bears today. How and when did the mixing happen?

Researchers cite interbreeding between the ancestors of polar bears and brown bears about 125,000 years ago.

Fundamental to the finding: The 100,000-year-old polar bear, a female juvenile named Bruno, is the only recorded ancient polar bear skull ever found and the only ancient polar bear bone found in North America.

Beth Shapiro, a UCSC professor in ecology and evolutionary biology and a leading researcher in ancient DNA, was an author of the study published last week in the journal Nature Ecology & Evolution. She told Lookout that although scientists were previously aware of the mixing of the bears as far back as 30,000 years ago, Bruno allowed them to learn that not only had they interbred far earlier, but that significant exchanges in DNA occurred. Ming-Shan Wang of UCSC’s Paleogenomics Lab also led the study with Shapiro.

At the time of the interbreeding about 125,000 years ago, the polar bears were navigating a warm interglacial period in the Pleistocene Epoch, and that movement led to an overlapping of their habitats — similar to what scientists are observing today.

Shapiro said that having this insight allows scientists to better understand how species interact with each other, how changes in climate might have played a part in the phenomenon, and how it could help improve decisions around preservation today.

In addition to Wang and Shapiro, study authors include Alisa Vershinina, Megan Supple, Joshua Kapp, Russell Corbett-Detig, Sarah Crump and Richard Green at UC Santa Cruz; Gemma Murray of the University of Cambridge; Kristin Laidre at the University of Washington; and Love Dalén at the Swedish Museum of Natural History in Stockholm.

This interview was edited for clarity.

Lookout: What were scientists able to learn from studying Bruno’s genome?

Beth Shapiro: So our previous work has shown that this happened during the last ice age off the southeast coast of Alaska and Ireland, and in some islands off the northeast of Siberia. But those were all, somewhere between 15 and 30,000 years ago — the last peak of the last cold part of the ice age, when polar bear habitat was pushed further south because the ice was pushed further south. So this bear, Bruno, was much older than that. And it gave us an opportunity to ask whether or not this admixture between these two lineages also happened during the previous warm interval, about 125,00 years ago.

So we extracted DNA from the bones, and we discovered a few things. The first is that Bruno is a girl. We discussed changing the name, but I think Bruno is a perfectly acceptable name for a female polar bear.

And the other thing that we learned from Bruno’s genome is that there was an admixture event between brown bears and polar bears, before Bruno’s lineage was alive, and that this admixture event pushed a ton of DNA into brown bears. So all living brown bears share a common ancestor more recently than when Bruno was alive, and for this reason, we never would have been able to see this ancient episode of interbreeding and gene flow because all living brown bears have genes in their DNA from this admixture event. And as much as 10% of the genomes of all living brown bears have this in their DNA.

The other interesting thing that we saw was that some of the DNA actually did move from brown bears into polar bears — perhaps because the polar bear population was really small at the time — but that it’s gone by now. So this really does suggest that it’s not good — it’s maladaptive for polar bears to have any brown bear DNA.

UCSC scientists studied the skull of Bruno--which led to new understandings of polar and brown bear hybridization.
(Pam Grove / Via UC Santa Cruz)

Lookout: Scientists don’t know what brown bears inherited from the polar bear genes. What makes it difficult to determine the specific genes that were passed along to brown bears?

Shapiro: It’s tiny, little broken-down bits of DNA that are just spread across the whole genome. And we could probably identify what those tiny little pieces of DNA are if we develop some sophisticated approach to be able to do it. But we also don’t really know what every part of the genome does. It would be hard to say what the functional consequences of any of it would be. It would just require a lot more data to figure it out and a way better understanding of how genes map to functions and physical characteristics and behavioral characteristics.

Lookout: Why is this study important and how does this help further our understanding of climate and the genome?

Shapiro: We’ve been working on a whole bunch of other bears, bear populations — I work on ancient DNA and we work on mammoths and bison and extinct horses and all sorts of things — and with DNA that we’re able to get from fossils. We’re starting to get a much better idea of how species and populations and ecosystems evolved, how things like ice ages and changing distributions of habitats can impact the evolutionary trajectory of the species that we share the planet with. Gives us the past, which is by its very nature different from anything that exists today and so it’s ripe for discovery. When we get DNA and genome sequences from animals that were alive 100,000 years ago, we can learn a lot about how the world has changed over that time.

We’re able to get genomic data from animals and plants that used to be alive, then we get a snapshot of what the world was like when they were alive, and how things changed between then and now. And when we’re thinking about what we might do to protect and preserve species today, we can think of the past as a sort of completed evolutionary experiment. Hopefully, when we learn about the past, and how populations and species and entire ecosystems were impacted by things like past rapid warming events, or the last ice age, or the first appearance of people on a landscape, then hopefully we can use what we learned from the past to make better, more informed decisions about how to protect and preserve species in the present.

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