Every polar bear alive today shares a common maternal ancestor, and it isn't even a bear from the same species. Their mitochondrial DNA reveals a 100,000 year story of interbreeding and hybridization...and the story is far from over.
Mitochondrial DNA is passed along exclusively from mothers to their offspring. Because this DNA never mixes with genetic material from the father, it remains pretty much completely unchanged over the generations. By studying the mitochondrial DNA of the living members of a given species, it's possible to work backwards and figure out when their most recent common maternal ancestor - in other words, the individual that supplied that same piece of mitochondrial DNA to all her descendants - must have lived. (For a more complete overview of what mitochondrial DNA is and why it's important, check out this earlier post.)
Researchers have now been able to do that with polar bears, and the results are surprising. The female ancestor of all polar bears isn't a polar bear at all, but instead a brown bear, a species that includes Grizzlies and Kodiaks. This ancestor lived 20,000 to 50,000 years ago during one of the last major Ice Ages. It probably was located on the glaciers right around what is now Ireland and Britain.
The differences between the two bear species isn't small - they're different sizes, their fur and teeth have different structures, their skin and coat colors aren't anything alike, and that's not even getting into their behavior. Polar bears are natural swimmers that live in the glaciers and tundra of the Arctic, while brown bears live out in the mountains, forests, and river valleys of North America and Europe.
So what happened all those thousands of years ago? To find out, we spoke with Penn State biologist Beth Shapiro and University of Dublin geneticist Daniel Bradley, the two authors of the study. Shapiro explained to us just how they figured out what, when, and where the maternal ancestor of all polar bears must have been:
We compare the DNA sequences isolated from the different bones. We build evolutionary trees that describe how each of the individuals is related to every other individual. These trees showed that all living polar bears have a common maternal ancestor that lived only 20,000 to 50,000 years ago, when that ancestor diverged from a brown bear. This was surprising, because polar bears existed prior to 50,000 years ago: there area fossils that are 100,000 years old, and they are morphologically and behaviorally quite different from each other. The only way to explain what we observed with the mitochondrial DNA is that, sometime after the two bear species split from each other, mating between the two already-different species happened. That mating resulted in the mitochondrial genome from the Irish brown bears being captured by polar bears. It then spread throughout the entire polar bear population, leaving the distribution we see today. We could tell this happened in Ireland because we know where, in space, each bone came from that we have DNA sequence information.
The particular Irish brown bear population with which these polar bears interbred all those years ago most likely died out about 9,000 years ago. Daniel Bradley elaborated on how they determined the maternal ancestor's Irish origins:
The closest sequences we found to modern polar bears are those we uncovered from Irish bears. We estimate that these were brown bears because stable isotope analysis indicates they had a terrestrial diet, unlike polar bears which are strongly adapted to a marine ecology.
They explained that it's not clear whether this interbreeding actually bestowed an evolutionary advantage. Bradley suggests this may have been an adaptive response to climate change, but it might well have just have been happenstance, the byproduct of the Ice Age forcing the bears' natural habitats to temporarily overlap. We do now know that hybridization among large mammals is surprisingly common - we see a lot of productive interbreeding with various species related to the cow, there's evidence of hybridization among the mammoth species, and even ancient humans probably crossbred with Neanderthals.
While this particular period of interbreeding happened tens of thousands of years ago, polar bears and brown bears have been interbreeding for far, far longer than that, going back as much as 500,000 years. It appears that whenever climate change changed their habitats enough to bring the species into contact, they started mating without hesitation. During this particular period, the Ice Age would have rendered Ireland pretty much uninhabitable due to glaciation, which forced the native brown bears to survive out on the ice shelf. That brought them into contact with polar bears, and the rest is mitochondrial history.
So what does this mean for the future of these species, particularly since we're now going through a period of climatic upheaval that is bringing the two species back into contact? Shapiro explained to us that the bears will likely again turn to hybridization to survive, but that isn't necessarily a good thing in the long run, particularly for the more vulnerable polar bears:
Clearly, they are hybridizing today, as their ranges begin to overlap and they happen to bump into each other. But, as long as polar bear habitat remains, polar bears will be able to mate with polar bears, and will continue to thrive in their very specific habitat. If the polar bear habitat disappears, so will the polar bear. If the only way they can survive is to hybridize with brown bears, then all the genetic adaptations that are specific to the polar bear risk being lost.
What both researchers stressed to us is the often forgotten importance of hybridization and interbreeding in our biological history. Polar bears and brown bears, it seems, are particularly dynamic species and well-suited to crossbreeding in order to survive times of pronounced warming and cooling. The only hope, perhaps, is that things don't get so extreme that one species breeds the other out of existence.
Read the original paper at Current Biology.