It looked like we had the polar bear’s origin story nailed down. Genetic studies suggested that between 111 and 166 thousand years ago, a group of brown bears, possibly from Ireland, split off from their kin. In a blink of geological time, they adapted to the cold of the Arctic, and became the polar bears we know and worry about. Fossils supported this story: the oldest polar bear bone is between 110 and 130 thousand years old.
But according to Frank Hailer at the Biodiversity and Climate Research Centre in Frankfurt, this story is wrong in two important ways. First, the polar bear aren’t just a branch of the brown bear family tree. They’re a separate lineage in their own right. Second, they around four times older than anyone had thought, arising around 600 thousand years ago.
If this new vision is right, the bear’s journey to polar dominance wasn’t a speedy sprint, but a more leisurely stroll. As a species, polar bears have seen many ice ages. Rather than being a symbol of extraordinarily fast evolution, they’ve actually had plenty of time to adapt to life in the freezer.
We have two separate origin stories for polar bears because, like all animals, they have two genomes. There’s the nuclear genome that hoards most of their DNA in the centre of their cells, and the smaller mitochondrial one, housed in small battery-like structures. The nuclear genome is inherited both parents, but the mitochondrial one is only inherited from mothers. That’s important, for reasons we’ll come to.
Until now, scientists had studied polar bear evolution by looking at their mitochondrial DNA. These studies produced a model in which the polar lineage is nested within a wider brown dynasty, meaning that some brown bears are more closely related to polar bears than to other brown bears. This called the polar bear’s status as a separate species into question.
Hailer changed that by sequencing the nuclear DNA of 45 brown, polar and black bears, at 14 different sites. His results showed that polar and brown bears are actually sister groups, that branched off from each other around 603 thousand years ago. Polar and brown bears are both distinct species, and they’re as genetically different from each other as they are from black bears.
It’s understandable why the fossils told a different story. Polar bears live in a world of shifting ice and freezing water, so many of their bones have been lost. But why did the mitochondrial DNA results differ so radically?
Hailer thinks that the two bears would often meet and mate with one another, long after they diverged. Indeed, they can still create hybrids today. If female brown bears mated with male polar bears, the hybrid (a pizzly bear) would carry a brown bear’s mitochondrial genome. This hitchhiking DNA could have invaded the wider polar population if the hybrids mated with other polar bears.
If you looked at the mitochondrial DNA, you’d see the footprint of these recent polar-brown liaisons, and mistake them for a split between the two groups. If you looked at the nuclear DNA, you’d see when the split actually happened. That’s exactly what Ceiridwen Edwards from Oxford University suggested when she analysed the bear’s mitochondrial DNA last year.
There are interesting parallels to our understanding of human evolution. For many years, scientists had been studying the mitochondrial DNA of Neanderthals, recovered from their bones. These sequences suggested that Neanderthals were a separate species to modern humans, and we never bred with one another. But the Neanderthal nuclear genome, unveiled in 2010, put paid to that idea. It revealed that Neanderthals and modern humans must have had sex, for 1 to 4 per cent of every European or Asian genome now comes from Neanderthals. Again, the nuclear genes uncovered a story that was invisible to the mitochondrial ones.
Charlotte Lindqvist, who led one of the earlier mitochondrial studies, isn’t surprised that polar and brown bears turn out to be distinct lineages. “These two species are clearly recognized as separate based on morphological and behavioral characteristics,” she says.
However, she thinks that it’s too early to say when the two species split from one another. Lindqvist says, “It doesn’t surprise me that it is older than previously suggested [but] I believe it is premature to settle on a date estimate based on relatively little molecular evidence.”After all, Hailer only looked at around 9,000 letters in each bear’s DNA – just a tiny fraction of their full genomes.
Edwards says that the study is a “welcome addition”. She’s now keen to analyse ancient DNA from bear fossils, to better understand how the two species diverged over time, and how they may have shared genes after their split.
Reference: Hailer, Kutschera, Hallstrom, Klassert, Fain, Leonard, Arnason & Janke. 2012. Nuclear Genomic Sequences Reveal that Polar Bears Are an Old and Distinct Bear Lineage. Science http://dx.doi.org/10.1126/science.1216424