A new study adds to our understanding of polar bears and brown bears' linked evolutionary histories.
Scientists have known this for a while, but the current study used a larger dataset to eke out additional detail, including DNA from an old polar bear tooth.
The tangled history of polar bears and brown bears
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The fact that these creatures were separated into different species did not prevent them from mating, as per ScienceDaily.
This has been known for some time, but the current study uses a larger dataset, including DNA from an old polar bear tooth, to elucidate the details.
The result that emerged is riddled with difficulties akin to those that have plagued human progress.
According to Charlotte Lindqvist, Ph.D., a specialist in bear genetics and associate professor of biological sciences at the University at Buffalo College of Arts and Sciences, the establishment and maintenance of species may be a chaotic process.
What's going on with polar bears and brown bears is a good analogy for what we're learning about human evolution: species separation may be imperfect.
They're witnessing multidimensional genomic mixing as diverse groups of archaic humans married with descendants of contemporary humans as more old genomes from historical population groups, particularly Neanderthals and Denisovans, have been discovered.
Another system in which this occurs is between polar bears and brown bears.
They discovered evidence of polar bear-brown bear interbreeding that preceded the study of an ancient polar bear, Lindqvist said.
Furthermore, the findings revealed a complex, entangled evolutionary history between brown and polar bears, with gene flow mostly from brown bears to polar bears.
According to the lead author, scientists previously believed that modern humans and Neanderthals evolved from a shared ancestor, and then divided into different species.
Then, she explained, researchers discovered Neanderthal DNA in current Eurasian individuals, showing that modern human groups got an influx of Neanderthal genes at some point during their common evolutionary history.
Lindqvist added that it was only afterward that scientists realized that this genetic intermingling had also enriched Neanderthal populations with current human DNA.
Interbreeding, she noted, may be complicated and not always a one-way track.
Ancient mixing and demographic fingerprints of bear genomes
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The polar bear has long been assumed to have arisen lately from its lower-latitude sister species, the brown bear, based on fossil and mitochondrial DNA data, as per the study "Polar and brown bear genomes reveal ancient admixture and demographic footprints of past climate change," that was published in the journal Proceedings of the National Academy of Sciences of the United States (PNAS).
Full mitochondrial generation sequencing, for example, has suggested a 150-kya split in these species' maternal lineages and affirmed a notably special connection between the polar bear and a hereditarily secluded population of brown bears from Alaska's Alexander Archipelago's Admiralty, Baranof, and Chichagof Islands.
A new, higher comprehensive genome for a polar bear that lived 115,000 to 130,000 years ago in Norway's Svalbard island was also created by the scientists.
The ancient polar bear's DNA was recovered from a tooth belonging to a subfossil jawbone presently kept at the University of Oslo's Natural History Museum.
Researchers predicted that polar bears and brown bears split into separate species around 1.3 to 1.6 million years ago, revising previous estimates by some of the same experts.
The age of the divide has been and continues to be a source of discussion among scientists, with historical interbreeding and a lack of fossil evidence for old polar bears among the variables that make the dating difficult to pin down, according to Lindqvist.
