The great white bears that roam Alaska's polar coast and across the Arctic appear to be the descendants of a single female brown bear that lived in Ireland during the heart of the last ice age, according to a new discovery by an international team of scientists.
That all modern polar bears can trace their ancestry through this Irish Eve -- a bruin beauty from a long extinct population that prowled bleak steppes of what is now Ireland and England some 20,000 to 50,000 years ago -- was shown by a detailed analysis of 242 brown bear and polar bear mitochondrial DNA lineages from the past 120,000 years, says a study published this week in the journal Current Biology.
Previous research had found that modern polar bears likely share a close ancestry with the brown bear population now living on the Southeast Alaska islands of Admiralty, Baranof and Chichagof, the so-called "ABC" bears. At one point, evidence seemed to suggest that modern polar bears might have actually descended from this population only about 14,000 years ago.
But the new findings paint a far more complex natural history for the two species, with interbreeding and epic dispersal occurring throughout the ages whenever the movement of ice sheets and shifting of climate forced the animals to abandon habitat or share the same turf.
Not only have polar bears and brown bears probably mated and produced hybrid offspring more often than previously thought, the authors concluded in the paper, "Ancient Hybridization and an Irish Origin for the Modern Polar Bear Matriline," but also, such interbreeding "may be a mechanism by which species deal with marginal habitats during periods of environmental deterioration."
"We found that brown bears and polar bears, which are hybridizing today in the wild, have been hybridizing opportunistically throughout the last 100,000 years and probably longer," added a lead author Beth Shapiro, the Shaffer Associate Professor of Biology at Penn State University and an expert in the analysis of ancient DNA, in a release about the paper. "Generally, this seems to happen when climate changes force the bears to move into each others' habitat.”
Scientists have long known that brown bears had contributed genetic material to the polar bear mitochondrial DNA -- the DNA passed down only by the mother to the offspring. The genes of all polar bears, it seemed, clearly passed through brown bear wombs at some point in the ancient past. This new study pinpoints that ancient mother as Irish.
"Until now, it was unclear just when modern polar bears acquired their mitochondrial genome in its present form," Shapiro said.
The findings offer an intriguing clue into how polar bears might respond as climate change reduces the extent of summer floes in the Arctic -- and why their genetic material ends up mixing so often in the first place. When ice cover shrinks, polar bears are forced to spend more time on land, where they inevitably come into more frequent contact with their brown bear cousins.
The two species may have "vastly different" physical traits and habits, the authors noted -- including body size, skin and coat color, fur type, tooth structure, paw shape, hunting strategies and prey species.
"Polar bears are expert swimmers that have adapted to a highly specialized, arctic lifestyle, while brown bears -- a species that includes grizzlies and Kodiaks -- are climbers that prefer the mountain forests, wilderness regions, and river valleys of Europe, Asia, and North America," they wrote.
But their differences don't keep them apart. Bears will be bears.
"When they come into contact, there seems to be little barrier to them mating," Shapiro said.
In pursuit of polar bear history
Compared to other bear types and species, not much is known about the origins of polar bears, though ultimately, scientists believe, they evolved from brown bears. Yet sorting out the details of polar bear evolution has never been more urgent, given the impact of climate warming on their habitat.
The shrinkage of sea ice during summer -- along with other consequences of Far North climate change such rising air temperatures, thawing permafrost, sea level rise, vegetation shifts, increasing rainfall -- has the potential to devastate animals that have adapted to hunt, mate, raise offspring, den and migrate in a frozen world.
"Among the most threatened of these is the polar bear, for which declining sea ice represents the greatest challenge to its survival," wrote Shapiro and 17 other authors from seven countries.
Over the past couple decades, summer ice extent in polar bear habitat has shrunk to record low levels. The fear that U.S.-managed populations will be driven to extinction prompted the United States to list the bears as threatened and designate 187,000 square miles of North Slope and Arctic Ocean as habitat critical to their survival. The designation is now embroiled in court challenges.
"Rapid declines in ice extent over the last 50 years have coincided with changes in the distribution, abundance, fecundity, and body size of polar bears," they added. "Fewer sea ice days mean longer open-water periods during summer, forcing polar bears onshore in search of food and leading to potentially dangerous interactions with humans. An increase in (time spent on land) may also be a key driver for opportunistic … breeding with brown bears, with several adult hybrid bears reported in the last five years."
What's in the genes?
The authors gathered mitochondrial DNA from 242 samples of polar bears and brown bears that lived from 120,000 years to modern times in every known geographic range that contained bears. Among them: samples of the previously unstudied brown bears of Pleistocene Ireland and England, extinct for at least 9,000 years.
The results portray a complicated, free-wheeling natural history, with brown bears dispersing from one continent to another, and the two species tossing DNA back and forth in a lusty juggling act that spans the millennia.
"The reconstructed matrilineal history of brown and polar bears has two striking features," the authors concluded. "First, it is punctuated by dramatic and discrete climate-driven dispersal events. Second, opportunistic mating between these two species as their ranges overlapped has left a strong genetic imprint.
"In particular, a likely genetic exchange with extinct Irish brown bears forms the origin of the modern polar bear matriline."
The authors outlined three possible scenarios of how this could have happened.
In the first, the ancient Irish brown bears, Alaska's ABC bears and an extinct line of Beringian bears all descended from a common brown bear ancestor that lived about 100,000 years ago. Then, between 50,000 and 20,000 years ago, the entire polar bear species branched off from the single Irish mother and spread across the Arctic as a completely new species.
What discounts this version, the authors point out, is that there are much older polar bear lines out there. For the Irish bruin to be the mother of all polar bears, the evidence for earlier polar bear populations would have to be incorrect.
In the second scenario, the two very ancient polar bear lines spread across the Arctic beginning about 100,000 years ago. They interbred with Irish and ABC brown bears between 75,000 and 50,000 years ago, introducing polar bear DNA into those brown bear populations. Then the Irish mother made her move -- and her descendents made over the polar bear world as their own.
The authors explain:
These polar bears hybridized opportunistically with small, coastal populations of brown bears, leading in at least two instances to fixation of the polar bear matriline in brown bear populations (Ireland and the ABC islands). More recently, additional hybridization occurred in Ireland, followed by fixation of the Irish brown bear matriline in polar bears.
The third scenario has polar bears and brown bears diverging much longer ago, more than 125,000 years ago, with our Irish brown bear mother and an unknown polar boar mating at the key moment.
Regardless of which scenario occurred, the modern polar bears all share this same Irish female ancestor, whose genes spread white sow to white sow across the entire Arctic.
"If polar bears were already widely distributed at this time, this suggests a complete replacement of the previous mitochondrial lineage within a remarkably short time frame," the authors wrote.
Since hybridization played such an important role in polar bear history at moments of key environmental change, the authors said, expect the process to occur again.
"Today, the arctic climate is again changing rapidly, and the habitat of brown and polar bears is once again beginning to overlap, providing the opportunity for the two species to hybridize," they wrote.
"Hybridization is known to occur more frequently in regions where population density is low, or where species are near the edge of their ecological range, and in some circumstances may provide the means to replace damaged alleles or to transfer novel traits between species, providing a fitness advantage to hybrid offspring."
Given the new findings, the authors of the paper suggest that governments ought to consider protecting hybrids of the two species -- "because they may play an underappreciated role in the survival of species."
At the same time, they don't recommend other changes in the bears' status.
"Our results suggest that, although the genetic mixing observed in bears today may be an important component of the long-term evolution of the polar bear, brown and polar bears have remained evolutionarily distinct lineages over geological time, suggesting that they are likely to remain as such in the future."
Contact Doug O'Harra at doug(at)alaskadispatch.com