The international study, led by the Pennsylvania State University and the University at Buffalo, found evidence that the size of the polar bear population fluctuated with key climatic events over the past million years, growing during periods of cooling and shrinking in warmer times.
The research also suggests that while polar bears evolved into a distinct species as many as 4-5 million years ago, the animals may have interbred with brown bears until much more recently.
These intimate relations may be tied to changes in the Earth's climate, with the retreat of glaciers bringing the two species into greater contact as their ranges overlapped, said Charlotte Lindqvist, the study's senior author and an assistant professor of biological sciences at UB.
"Maybe we're seeing a hint that in really warm times, polar bears changed their life style and came into contact, and indeed interbred, with brown bears," said Stephan Schuster, co-lead author, a professor of biochemistry and molecular biology at Penn State, and a research scientist at Nanyang Technological University in Singapore.
The findings will be published online in the early edition of the Proceedings of the National Academy of Sciences on July 23. The study is the most extensive analysis to date of polar bear DNA, scientists say. The research team, representing 13 institutions in the U.S., Canada, Europe and Asia, as well as Mexico's Laboratorio Nacional de Genomica para la Biodiversidad (Langebio), sequenced and analyzed the nuclear genomes of 28 different bears, with many DNA samples provided by the U.S. Geological Survey and the Norwegian Polar Institute.
"We generated a first-rate set of data, including deep sequence coverage for the entire genomes of a polar bear, three brown bears and a black bear, plus lower coverage of 23 additional polar bears, including a 120,000-year-old individual; very few vertebrate species have such comprehensive genomic resources available," Schuster said. Using this vast amount of data, the scientists discovered that polar bears are actually an older species than previously thought -- indeed, far more ancient than suggested by a recent study that placed the species' age at 600,000 years old. That analysis looked only at small segments of DNA.
"We showed, based on a consideration of the entire DNA sequence, that earlier inferences were entirely misleading," said study co-lead author Webb Miller, a Penn State professor of biology and computer science and engineering. "Rather than polar bears splitting from brown bears a few hundred thousand years ago, we estimate that the split occurred 4-5 million years ago."
"This means polar bears definitely persisted through warming periods during Earth's history," UB’s Lindqvist said. She cautions, however, that the species' endurance over several million years doesn't guarantee its future survival.
To model historical populations of the polar bear, the scientists used computer simulations to analyze a deeply sequenced polar bear genome.
"This is the first time we can see, from their genes, how the population history of polar bears tracked Earth's climate history," Lindqvist said. "We see an increase in polar bears at the end of the Early Pleistocene as the Earth became much colder, and a continuous decline in the size of the population during warmer times.
“We also found, perhaps unsurprisingly, that polar bears occur in much smaller numbers today than during prehistory," Lindqvist continued. "They have indeed lost a lot of their past genetic diversity, and because of this, they are very likely more sensitive to climate change threats today."
Discrepancies between the estimated age of polar bears in the new study and past studies could be explained by interbreeding between polar bears and brown bears since the species split from each other.
The new analysis uncovered more genetic similarities than previously known between polar bears and ABC brown bears, an isolated group from southeastern Alaska -- suggesting that these animals have exchanged genes since becoming separate species.
"The ABC brown bears' mitochondrial sequences are much more like polar bears' than like other brown bears'," Miller said. "This made us wonder what other parts of their genomes are 'polar-bear-like.' We mapped such regions, which constitute 5 to 10 percent of their total DNA, onto the genomes of two ABC brown bears. As such, brown/polar bear hybridization, which has been observed recently in Arctic Canada, has undoubtedly contributed to shaping the modern polar bear's evolutionary story."
This intermingling between species is just one interesting finding emerging from the enormous trove of data that the PNAS study produced. Another question that the research is beginning to address: What makes a polar bear a polar bear?
Polar bears have genetic differences from brown bears that let them survive in an Arctic climate with very different diets, and the new study identified genes that may be responsible for traits such as polar bears' pigmentation and the high fat content of their milk.
This study received financial support from Penn State University, the College of Arts and Sciences at the University at Buffalo, U.S. Geological Survey's Changing Arctic Ecosystem Initiative, U.S. Fish and Wildlife Service, Ontario Ministry of Natural Resources in Canada, Alaska Department of Fish and Game, the National Institutes of Health and the National Science Foundation.CONTACTS
Barbara Kennedy (PIO at Penn State): firstname.lastname@example.org, (+1) 814-863-4682
Charlotte Lindqvist | Newswise Science News
Researchers simplify tiny structures' construction drip by drip
12.11.2018 | Princeton University, Engineering School
Mandibular movement monitoring may help improve oral sleep apnea devices
06.11.2018 | Elsevier
Biochips have been developed at TU Wien (Vienna), on which tissue can be produced and examined. This allows supplying the tissue with different substances in a very controlled way.
Cultivating human cells in the Petri dish is not a big challenge today. Producing artificial tissue, however, permeated by fine blood vessels, is a much more...
Faster and secure data communication: This is the goal of a new joint project involving physicists from the University of Würzburg. The German Federal Ministry of Education and Research funds the project with 14.8 million euro.
In our digital world data security and secure communication are becoming more and more important. Quantum communication is a promising approach to achieve...
On Saturday, 10 November 2018, the research icebreaker Polarstern will leave its homeport of Bremerhaven, bound for Cape Town, South Africa.
When choosing materials to make something, trade-offs need to be made between a host of properties, such as thickness, stiffness and weight. Depending on the application in question, finding just the right balance is the difference between success and failure
Now, a team of Penn Engineers has demonstrated a new material they call "nanocardboard," an ultrathin equivalent of corrugated paper cardboard. A square...
Physicists at ETH Zurich demonstrate how errors that occur during the manipulation of quantum system can be monitored and corrected on the fly
The field of quantum computation has seen tremendous progress in recent years. Bit by bit, quantum devices start to challenge conventional computers, at least...
09.11.2018 | Event News
06.11.2018 | Event News
23.10.2018 | Event News
15.11.2018 | Earth Sciences
15.11.2018 | Physics and Astronomy
15.11.2018 | Physics and Astronomy