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Polar bear genomes show the complexities of species formation

A new study of brown and polar bear genomes challenges the linear ideas of species splitting and shows the complexities of interbreeding and DNA influxes, as well as updates the timeline of when brown and polar bears split from their common ancestor.

(CN) — A new study analyzing bear genetics reveals complicated evolutionary histories, challenging the commonly-held belief that species-splitting is linear and distinct.

Scientists at the University at Buffalo, the Alaska Department of Fish and Game and seven other universities studied the genomes of 64 modern brown and polar bears. Some of the genomes came from Alaska where both bears can be found. On top of the current genomes, the scientists got their hands on an ancient polar bear's jawbone.

Found in Norway's Svalbard archipelago, the jawbone and tooth is a rare find and is estimated to be between 115,000 and 130,000 years old. Tracking down polar bear history can be difficult. Polar bears tend to die on sea ice, and their remains fall to the sea floor. Other bears tend to die in caves, making their fossils more accessible.

The polar bear fossil limitations hinder discovering when brown bears and polar bears became two distinct species. With the new data, it is now thought that the species split around 1.3 million to 1.6 million years ago — different than the previous study done by the same scientists in 2010.

"Our results demonstrate a complicated, intertwined evolutionary history among brown and polar bears, with the main direction of gene flow going into polar bears from brown bears. This inverts a hypothesis suggested by other researchers that gene flow has been unidirectional and going into brown bears around the peak of the last ice age," said study author Charlotte Lindqvist.

As climate change continues to shrink polar bear habitats, the interbreeding histories can aid in understanding how they have adapted to environmental shifts in the past and what hope they may have moving forward.

"It's clear from the history of polar bears that they are clearly marked by having had a drastic population decline, and their genetic diversity is very low. So species like that are probably already quite vulnerable to loss of habitats and loss of population sizes, and loss of individuals- and increased inbreeding because of it… We still can't forget about this kind of threat hanging over us called climate change that we may not necessarily feel in our day-to-day lives. But it definitely is something that we need to act on and be concerned with," Lindqvist said.

The findings answered some questions and left more work for researchers. The genomes show that the split isn't one-directional and that interbreeding between the two bears has gone on for over 130,000 years. It also adds weight to human species splits — it was once thought that the divide between modern humans and Neanderthals was simple. Then, Neanderthal DNA was found in modern Eurasian people. This discovery proved that there was inbreeding at one point in the shared evolutionary history, and the bear genomes demonstrate how multi-directional and complex DNA influxes can be.

"I feel this is a very important message to show that [science is] sort of a very rigorous, incremental process that we go through, and that we can learn a lot about our pasts that hopefully can help us also understand the present that we live in… And only by doing these kinds of sort of careful studies… we're really better off in trying to understand both our past and certainly our present," said Lindqvist.

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