DNA Study Finds Deeper Antiquity of Polar Bear Species A fascinating new paper makes a strong case, using new genetic clues, that polar bears have been around a lot longer, and thus endured more climate vagaries, than most previous estimates. The research is described in "Nuclear Genomic Sequences Reveal that Polar Bears Are an Old and Distinct Bear Lineage," a paper being published on Friday in the journal Science. The journal's summary of the article makes the prime point: "Polar bears diverged from their closest relatives about 600,000 years ago, according to a new genetic analysis. The findings suggest the cold adapted species is about five times older than previously thought, and may have had more time to adapt to arctic conditions than recently assumed. You'll see various interpretations. Those concerned about global warming (including at least one study author) are stressing that a longer evolutionary timeline implies the bears" adaptation to climate change in the past was a slow process (meaning the speed of change now poses new threats). Those questioning the vulnerability of this species to warming will point to its successful survival through two previous warm intervals between ice ages as evidence the bear can deal with reduced ice and other big environmental shifts. Finally, there are basic questions about the robustness of the conclusions, which are based on a new line of genetic analysis not previously applied to polar bears. James Gorman of the science staff at The Times captures this complexity well in his news story: I had a few questions for the authors, and you can read some answers are below. I also sought reactions from some polar bear specialists and biologists focused on DNA clues to when the species split from its brown bear kin. Read on for their thoughts, as well. Here's my exchange with Frank Hailer, the lead author and a scientist at the Biodiversity and Climate Research Center in Frankfurt: I'm writing to get some input from you on your incredibly valuable new paper on polar bear evolutionary history and genetics. I'd written on the mitochondrial DNA work but your research clearly is an important advance. I have a couple of questions, one being where the paper supports the conclusion in the related news releases about the polar bear's evolution being a slow process. As in this line is from the Science summary, for instance: This study suggests that past adaptation to a changing climate may have been a slow process. Consequently, polar bears may not have enough time to adjust to these warmer conditions as they have in the past. Where does the paper clarify the pace of genetic change as it relates to past (and future) periods of warming? Our study looks at the evolutionary history of polar bears. Previous studies had suggested, mainly based on mtDNA results [using DNA from the mitochondria within cells], that polar bears should be an example of unusually rapid adaptation to arctic conditions. This was, because mtDNA data suggested that polar bearshad evolved within some 150,000 years from a brown bear population that had colonized arctic habitats. Our study now removes the necessity for unusually rapid adaptability in polar bears, providing some 600,000 years of time for evolutionary adaptation. This figure is much more in line with what evolutionary biologists have found in other mammals, with regard to speciation and adaptation. Also, I'm not sure I see strong support for this concluding sentence: "Although polar bears have persisted through previous warm phases, multiple human-mediated stressors (e.g., habitat conversion, persecution, and accumulation of toxic substances in the food chain) could magnify the impact of current climate change, posing a novel and likely profound threat to polar bear survival." The trends on other sources of stress for polar bears seem to be in encouraging directions, with Russia introducing more control on hunting (illegal and legal) and research on potentially harmful industrial chemicals showing declines. For example, levels of PCB's and other industrial chemicals appear to be declining in studied polar bear populations, as in this new paper. Also, international cooperation on management of polar bear populations is on the rise, and reflected in the 2011 polar bear treaty involving native groups and governments in Russia and the United States. For current status and predictions on population changes in polar bears, see the evaluation by the IUCN specialist group, the world authority on polar bears and their conservation. See also "Will Polar Bears Survive"? On the threat from contaminants, see this quote from "Good News for Norwegian Polar Bears: PCBs Levels Down": Predicting the future? While these figures are encouraging, Bytingsvik notes that the overall levels of OH-PCBs and PCBs in the cubs are still too high. As a comparison, the 2008 concentrations of OH-PCB in cubs was roughly 90-170 times higher than levels that are known to affect thyroid hormones in human babies. "Polar bear cubs can be much more susceptible to the effects of these kinds of environmental contaminants than adults because they are in a vulnerable phase of growth and development," she says. Bytingsvik's research is part of a larger international project to assess the condition of polar bears in the Arctic called BearHealth. The Norwegian leader of the project, Bjørn Munro Jenssen, is Bytingsvik's supervisor. He says her findings are definitely good news. "PCBs are considered to be among the worst environment contaminants, so it's good to see that the levels have gone down," he said. At the same time, we can't forget that animals in the Arctic are exposed to a number of other environmental pollutants that are carried northward on the wind or by ocean currents. On top of that, there's climate change. This creates big challenges for many species. Read on for reactions from other scientists not involved with the work. This is a deeper dive for those eager to explore the details in the science. The responses are unedited. Andrew Derocher, a bear biologist at the University of Alberta who's also affiliated with the environmental group Polar Bears International: This is certainly an interesting paper that presents yet another picture of polar bear evolution. As the paper notes, it is fairly consistent with some of the earlier works on this by Bjorn Kurten from 1964 who suggested a quarter million years or so for the separation (Kurtén B. (1964) The evolution of the polar bear, Ursus maritimus Phipps. Acta Zool Fenn, 1-30.). This scenario in this latest Science paper was also postulated by the recent work on the Irish ancestry (Edwards et al.) - they raised the possibility that polar bears were a much older species and the Ireland link was a more recent event. There are dates in the genetics literature on polar bears that go back to a separation of 1.17 million years ago. The issue of bottlenecks is certainly relevant given that polar bears show so little genetic diversity and as they note, this could be a problem if polar bears are greatly reduced in abundance as the sea ice retreats northward with global warming. We are seeing the effects in the southern polar bear populations very clearly now and our analyses suggest that we'll see the western Hudson Bay bears extirpated in 2-3 decades or less. I don't think there is much in this that changes my view of what polar bears have done and what the future holds for the species. Polar bears haven't seen what the ice models are predicting if we don't deal with the warming patterns and sea ice loss. What I can almost guarantee is that this is far from the last word on the evolution of polar bears: the story changes all the time with new material, new analyses, and new interpretations. I think it is wonderful that groups have picked up the issue for serious investigation. For years polar bears were used as an example of quantum evolution and it is clear that they are a very different species from brown bears. I've worked on both species and while the "genetics" say these 2 species are very closely related, behaviourally and ecologically, they are vastly different. The nonsense of polar bears adapting to land isn't playing out in the field: lots of bears in very poor condition the last couple of years in western Hudson Bay. The cub production in Hudson Bay is only about 20-40% of historic levels from the 1980s-90s (this is based on the recent aerial survey results from Hudson Bay that was covered some in the media recently). You can't keep a population if you don't have adequate reproduction. I'm not sure there is much more to say on this. Interesting but it doesn't shed any light on the future (except that the bears have limited genetic diversity compared to brown bears - but we knew this already - so the potential to "adapt" is limited from a genetic perspective). Linda J. Gormezano, a vertebrate zoologist at the American Museum of Natural History (whose fascinating field work in Hudson Bay was described by me awhile back) Hailer et al provide additional, very compelling evidence for an older origin of the polar bear. Their data also indicate that the point when it was previously thought that the polar bear line started may have been one of possibly multiple hybridization events with grizzly bears instead. The original matrilineal line for polar bears ends during this period leading them to suggest that it was replaced by an introgression of female grizzly DNA. That is, reproduction occurred between male polar bears and female grizzly bears, then the offspring backcrossed into the polar bear population. They also find evidence of possible gene flow in the opposite direction (polar bear DNA into the grizzly population) suggesting different scenarios of who was mating with who. They suggest that the fragmentation of the brown bear genetic lines and subsequent hybridization with polar bears were caused by the onset of the Eemian (125 ka), last major warming period, and the "adaptive introgression" that resulted may have helped both polar and grizzly bears survive in their changing environments. Some evidence of this may be the well-known behavioral (as well as genetic) similarities between the two species - such as grizzlies able to hunt seals on the ice and polar bears able to hunt and capture caribou on land. As the authors of this paper note, declines in the polar bear population likely recurred through previous warming periods, leading to bottlenecks, including the one that likely preceded the hybridization event with grizzly bears. We are already seeing climate related effects on the western Hudson Bay population (reduced survival, reproduction, etc.) that are predicted to lead to a drastic population decline. Interestingly, we are also seeing more and more grizzly bears moving into western Hudson Bay in recent years overlapping polar bears on land where ~30 years ago they (grizzlies) were almost completely absent (Rockwell et al. Also, as warming temperatures limit seal hunting opportunities on the ice, adult male polar bears will likely fare the best, securing the best sites from smaller bears (females, subadults). This may set the stage for a repeat of the more likely hybridization scenario described by the authors where male polar bears resort to mating with female grizzly immigrants as eligible females from their own species disappear. As far as I know there are no documented interactions between the two species in western Hudson Bay yet, but given the new information on how polar bears evolved and may have interacted with grizzlies, it will be especially interesting to see how this all plays out as climate induced changes in distribution and behavior cause the two species to interact more frequently. Steven C. Amstrup, senior scientist at Polar Bears International and scientist emeritus, United States Geological Survey. This is a very interesting study. One thing I would say about this new study is that it is likely not the final word. Early genetics work suggested polar bears diverged from brown bears about 250K [250,000] years ago. This corresponded well with what people thought the fossil record was telling them-mainly based on the work of Bjorn Kurten. Subsequently there have been other works suggesting that polar bears may have split off as long ago as 1 million years and as recently as about 150K. Polar bears are very poorly represented in the fossil record, so what we know about their evolution is largely dependent on modern molecular genetics. And as that science advances, geneticists keep trying new approaches and discovering new estimates. I believe it may be some time before geneticists settle on a time of separation on which they all can agree. Second, the important points from a polar bear perspective are: 1. Regardless of the date of their divergence from brown bears, it has been mostly cooler throughout their evolutionary history than it is now. That is, most of the period of polar bear existence has been during continental glaciations rather than inter-glacials. And 2. even if they did diverge 600K years ago, they have only been through 2 periods that were noticeably warmer than the present time. These warming periods, however, did not approach the anticipated warming of this century. People will undoubtedly speculate about what this paper says regarding the prognosis for polar bears in a continually warming world. The short answer is that it really does not change the view we have long held. We currently are in the Holocene interglacial warming period. Our global mean temperature should now be declining as a result of cyclical changes in the energy reaching the earth from the sun (these changes which follow the Milankovic cycle account for changes in the path the earth takes around the sun, and changes in the tilt of the earth on its axis). The Eemian interglacial period (around 130K-120K years ago) was the last interglacial before our current one. The global mean temperature at that time is thought to have reached about 1°C warmer than the Holocene warming. About 400K years ago, the Holsteinian interglacial, also saw global mean temperatures about 1°C warmer. There were less significant interglacials in between the Eemian and Holsteinian, but they did not reach the level of warmth during these two major interglacials. So, if this new estimate of the age of polar bear divergence from brown bears is right, polar bears have seen 2 periods warmer than the recent past. If the divergence occurred 150K years ago, polar bears have seen one warmer period. The important point is that if we continue on our current greenhouse-warming path, it will be far warmer in 50 years than at anytime in the existence of polar bears. That is, by the latter half of this century, polar bears will be off the charts of anything they have experienced during their evolutionary history. And whereas the past interglacials were natural cycles, the current warming is not. Recall that the earth now should be cooling slowly as a result declining solar energy, but clearly it is not. Climate models suggest that at 1 degree warmer, there still was significant year-round sea ice available for polar bears to occupy, so it is not surprising they survived the low-ice phase of the cycle. Ultimately, a continually warming world will have no sea ice habitat for polar bears, and that is the rub they face. In short, whether polar bears survived one or two warming periods of the past, they never have faced the kind of warming we will see in the next century. The world is guaranteed to warm as long as greenhouse gas concentrations (GHG) in the atmosphere rise. There are uncertainties in the rate at which it might warm, and the natural variation in the climate system means it will not be a smooth warming. BUT, there is ultimate certainty that the earth can only warm as GHG concentrations rise. AND, models are pretty consistent that by the latter part of this century we will be living in a climate very different from that in which polar bears (and humans) have flourished. From the standpoint of doing something about global warming, the uncertainties are irrelevant unless we don't care what kind of world we leave for future generations-of humans and polar bears. Charlotte Lindqvist, an evolutionary biologist at the University of Buffalo whose previous research included analysis of the mitochondrial DNA from a fossil polar bear jaw was discussed on Dot Earth (see James Gorman's article, linked above, for more from here): The 150,000-year window is based on mitochondrial genome data, and since nuclear data show a different phylogenetic pattern and confirm a sister relationships between the two species, it is no surprise that an estimated split between these two species appear to be older. However, these date estimates will strongly depend on the data and sample input and on the models and calibrations used. I am not at all convinced that the approach taken in this paper gives us good evidence for a suggested time frame. I will agree to what both Steve and Andrew have pointed out: this is definitely not the last word. Click here for more scientific discussion of the polar bears" fate in a warming climate.