8/9/2012 7:23:00 PM

New Insights on Polar Bear Evolution Don‰Ûªt Lessen Threats from Global Warming

A flurry of news articles are circulating in response to a recent paper that concluded that precursors to the modern polar bear separated from brown bear ancestors as early as four or five million years ago, instead of less than a million years ago as previously suspected.

Polar bear lit by the glow of sunrise

If this new interpretation is true, it means that ancestors of the polar bear survived warm periods of the distant past. But what's known and unknown about polar bear evolution and the polar bear's future in a human-induced warming world?

Here's a snapshot view:

  • Although a new study of polar bear and brown bear genomes suggests that the modern polar bear's ancestors may have split from a common ancestor with brown bears four to five million years ago, the polar bear's early evolutionary history is still unresolved
  • Scientists do not know what the polar bear's ancestors may have been like during ancient warm periods
  • A fossil from 120,000 years ago proves that polar bears at that point were essentially like modern bears—but no earlier fossils have been found, so when polar bears developed adaptations to cold and ice is still unknown
  • MODERN polar bears depend on sea ice for access to their prey and will disappear without it
  • Regardless of what polar bears may have been like during earlier warm periods, the current arctic warming will eventually exceed anything modern polar bears and their ancestors have experienced unless we take action to reduce greenhouse gas emissions

And a more in-depth look:

In a recent paper published in the Proceedings of the National Academy of Sciences (PNAS), Webb Miller and a host of other genetics experts concluded that precursors to the modern polar bear separated from brown bear ancestors as early as four or five million years ago instead of less than a million years ago as has been previously reported. The methods used by these authors, including examination of the whole polar bear genome, may offer a more in-depth look at polar bear evolution than previously has been possible.  They don't however, as some have suggested, reduce the threat polar bears face from our increasingly warming world. Regardless of how long polar bears may have been around, the current human-caused warming is different from anything polar bears may previously have faced.

 Polar bear family

This PNAS paper dates the separation of polar bear from a common ancestor with the brown bear to the Pliocene epoch, the geologic period between 5.3 to 2.6 million years ago, when global mean temperatures were thought to be two to three degrees C warmer than present.

The much cooler Pleistocene epoch, which was characterized by a more persistent arctic sea ice cover and the cyclical advance and retreat of continental ice sheets, followed the Pliocene.  Previous interpretations suggested it was the late Pleistocene when ancestors of modern polar bears may first have moved onto the ice.  A 4+ million year old separation time would mean polar bears not only parted company with their brown bear cousins during a warmer period than previously hypothesized, they also, according to a recent paper by Melles and others in Science, would have passed through multiple warm periods before arriving where they are today. 

The PNAS paper suggests, as did another recent paper, that the evolutionary path of the polar bear has included episodes of hybridization throughout its evolutionary history.  The authors concluded these may have coincided with warm periods when polar bears were forced ashore and overlapped more frequently with coastal brown/grizzly bears.  In some parts of their range polar bears and grizzly bears always have occurred in adjacent habitats. A warmer environment with less sea ice could increase the frequency of interspecies contact, but serendipity also could explain occasional interbreeding. Because we do not know when, during that far distant past, early polar bear ancestors began to live on the ice like modern polar bears, any explanation of hybridization is at most educated speculation. Ideally we will one day have enough information to move from speculation to formation of testable hypotheses about early evolutionary stages.

Just as the earliest humans that left their common ancestor with the great apes were quite different than we are today, if the polar bear's precursors did depart the brown bear lineage during the warm environment of 4-5 million years ago, they probably would have been nothing like modern polar bears. 

The prevailing hypotheses regarding polar bear evolution is that hunting on the sea ice began to develop when a group of ancestral terrestrial bears ventured offshore to take advantage of an untapped ecological niche—the sea ice habitat and the fat-rich seals that occupy it. The reason ancestors of the polar bear may have diverged from the brown bear line during an ancient warm period, as suggested in this new paper, is unclear, but it probably was not driven by a move to the sea ice. 

Mom and cubs waiting for freze-up

Summer sea ice may have been absent from the Arctic during much of the Pliocene and winter-time sea ice cover may have been only 10% of what it is now (Haywood and Valdes 2004, Robinson 2009). The oldest known fossil similar to modern polar bears is approximately 120,000 years old. Without fossils to help us understand earlier periods, we can only speculate when the specialized features that define today's polar bear came about and how they relate to observed genetic patterns.

We can't say that presumed hybridizations occurred when ancestors of polar bears were forced off the ice, nor can we say with confidence when these ancestors began to look and hunt like modern polar bears. But, it does seem reasonable to hypothesize the lifestyle we see in modern polar bears began during the colder environment of the Pleistocene.

Regardless of what we learn from continuing studies of polar bear genes, it is important to remember that hybridization does not offer salvation for polar bears facing loss of their present sea ice habitat. The current rate of global warming is so rapid polar bears will starve out long before their genes may be swamped by those of modern brown bears. And our concern, as polar bear conservationists, is not whether some polar bear genes "live on" in a hybrid terrestrial animal. Rather, it is whether the great white bear we now know and its unique habits and habitats can persist. 

Even in the unlikely event polar bears did jump off of the brown bear lineage and instantly become specialized sea-ice predators, knowing they survived through warm cycles of the past does not reduce their future risks. In those early days, there were no other human stressors in the polar bear's environment. There was no hunting, no oil and gas development or mining, and no shipping traffic. With other threats absent, polar bears, if indeed they were around during warm periods of the distant past, would have faced only natural challenges to survival in small pockets of available habitat that may have persisted. This is a much different scenario than polar bears now face. 

Even more important than the absence of other human stressors is that current warming is not part of natural oscillations in solar energy and feedbacks that warmed the world in the past and then allowed it to cool again. Continued increases in atmospheric greenhouse gas levels guarantee the world can only continue to warm. The world will not warm to some new level and stabilize, and it will not cool down as in past climate cycles. Natural oscillations in weather and climate will continue in the future, but they will occur over a higher and rising baseline. All polar bear habitat ultimately will disappear, and without mitigating greenhouse gas rise it will not return as it did after past warm events.  This is a very different challenge than polar bears ever have faced. 

It will be scientifically interesting to see whether future studies corroborate a Pliocene separation between ancestors of modern polar bears and brown bears. Regardless, we must not allow changing and uncertain interpretations of the past to distract our attention from absolute certainties about the future. We don't know exactly how hot the globe can be and still retain enough sea ice to support some polar bears somewhere, but we do know that without greenhouse gas mitigation, we will exceed whatever that heat level is. And, even if some polar bears could persist in warmth equivalent to that of the Pliocene, that level of heat would create other concerns. Glacial melt during the Pliocene raised sea levels approximately 80 feet above present (Robinson 2009)-high enough to displace nearly ½ of the world's current human population. If we get to the point where policy makers are trying to figure out where all those displaced people may find new homes, few will be thinking about the welfare of polar bears. 

We know that humans are the cause of current global warming. And we know we have the ability to stop it in time to save polar bears. Doing so will benefit the rest of life on Earth, including humans. I think we all should stay focused on this cause. 

Miller, W., S. C. Shuster, A. J. Welch, A. Ratan, et. Al.  2012. Polar and brown bear genomes reveal ancient admixture and demographic footprints of past climate change. PNAS, July 23, 2012. www.pnas.org/cgi/doi/10.1073/pnas.1210506109

Melles, M.,  J. Brigham-Grette, P.S. Minyuk, N. R. Nowaczyk. Et. Al. 2.8 Million Years of Arctic Climate Change from Lake El'gygytgyn, NE Russia. Published Online June 21 2012. Science 20 July 2012: Vol. 337 no. 6092 pp. 315-320. DOI: 10.1126/science.1222135. 

Haywood, A. M., P. J. Valdes.  Modelling Pliocene warmth: contribution of atmosphere, oceans and cryosphere.Earth and Planetary Science Letters 218 (2004) 363-377.

Robinson, M. M.  2009. New quantitative evidence of extreme warmth in the Pliocene Arctic. Stratigraphy, vol. 6, no. 4, pp. 265-275.

Amstrup, S.C.  2003.  Polar bear, Ursus maritimus.  Chapter 27 In Wild mammals of North America: biology, management, and conservation. Edited by G.A. Feldhamer, B.C. Thompson, and J.A. Chapman. John Hopkins University Press, Baltimore. pp. 587"Ô610.

Photos copyright Daniel J. Cox/Natural Exposures.

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