Polar Bear Evolution
The polar bear is a bear native largely within the Arctic Circle encompassing the Arctic Ocean, its surrounding seas and surrounding land masses. It is often considered a symbol of the pristine Arctic. An analysis of newly sequenced polar bear genomes is providing important clues about the species' evolution, suggesting that climate change and genetic exchange with brown bears helped create the polar bear as we know it today. The international study, led by researchers at Penn State and the University at Buffalo, found evidence that the size of the polar bear population fluctuated with key climatic events over the past 1 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 four to five 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, bringing the two species into greater contact as their ranges overlapped, said Charlotte Lindqvist, the study's senior author.
"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 bear family, Ursidae, is believed to have split off from other carnivorans about 38 million years ago. The Ursinae subfamily originated approximately 4.2 million years ago. The oldest known polar bear fossil is a 130,000 to 110,000-year-old jaw bone, found on Prince Charles Foreland in 2004. Fossils show that between ten to twenty thousand years ago, the polar bear's molar teeth changed significantly from those of the brown bear. Polar bears are thought to have diverged from a population of brown bears that became isolated during a period of glaciation in the Pleistocene.
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 sequenced and analyzed the nuclear genomes of 28 different bears.
"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-thousand-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.
"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 to 5 million years ago."
"This result means polar bears definitely persisted through warming periods during Earth's history," Lindqvist said. "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."
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."
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.
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Polar Bear image via Wikipedia.