"Very few polar bear fossils have been found, leading to widely varying estimates of exactly when and how polar bears evolved," explains Øystein Wiig, polar bear expert and co-author at the University of Oslo's Natural History Museum. "Because polar bears live on the ice, their dead remains fall to the bottom of the ocean or get scavenged. They don't get deposited in the sediments like other mammals."
But in 2004, an Icelandic geologist found a rare, well-preserved, 110,000- to 130,000-year-old jawbone and canine tooth fossil in the Svalbard archipelago of Norway. This specimen was subsequently sent to Wiig for analysis.
Researchers sequenced of the complete mitochondrial genome of the fossil and used the information to develop mitochondrial sequencing of the other bears and to construct phylogenies showing that the ancient polar bear evolved within the lineage of brown bears.
"Since the brown bears from Alaska's Admiralty, Baranof and Chichagof Islands are the polar bears' closest relatives, it was crucial to include them in our study in order to more precisely date when polar bears appeared as a distinct species," explains Charlotte Lindqvist, assistant professor in the UB Department of Biological Sciences.
"The fact that our ancient polar bear lies almost directly at the splitting point between this unique group of brown bears and polar bears, that is, close to their most recent common ancestor of the two species, was very intriguing. It provided an ideal opportunity to ultimately settle the time of polar bear origin."
"This is, by far, the oldest mammal mitochondrial genome to be sequenced," says co-author Stephan C. Schuster from Penn State's Center for Comparative Genomics and BioinformaticsSchuster. "It's about twice the age of the oldest mammoth genome that has, to date, been sequenced."
The mitochondrial genome refers to all the DNA in the mitochondrion, the energy-producing component of most eukaryotic (complex) cells. Ancient DNA studies have tended to focus on the mitochondrial genome because it generally reveals characteristics useful for evolutionary analyses and allows for DNA to be retrieved from ancient samples most easily.