Tuesday, 2 February 2021

BROWN, BLACK, POLAR & PANDA

Bears are one of my favourite mammals. Had they evolved in a slightly different way, we might well have chosen them as pets instead of the dogs so many of us have in our lives today. 

For them and for us, I think things worked out for the best that they enjoy the rugged wild country they call home. 

Bears are carnivoran mammals of the family Ursidae. They are classified as caniforms or doglike carnivorans. 

Although only eight species of bears are extant, they are widespread, appearing in a wide variety of habitats throughout the Northern Hemisphere and partially in the Southern Hemisphere —  making a home in North America, South America, Europe, and Asia. 

The relatives of our black and brown bears, a dog-bear, entered the fossil record about 20 million years ago. We've found polar bear bones that tell us more about when they split off in the lineage.

DNA from a 110,000–130,000-year-old polar-bear fossil has been successfully sequenced. The genome, from a jawbone found in Svalbard, Norway, in 2004, indicates when polar bears, Ursus maritimus, diverged from their nearest common relative, the brown bear — Ursus arctos.

Because polar bears live on ice and their remains are unlikely to be buried in sediment and preserved, polar-bear fossils are very rare. So the discovery of a jawbone and canine tooth — the entirety of the Svalbard find — is impressive. 

But far more important, is that when molecular biologist Charlotte Lindqvist, then at the University of Oslo's Natural History Museum and now at the University at Buffalo in New York, drilled into the jaw, she was able to collect intact mitochondrial DNA. Yes, a bit Jurassic Park-esque.

Mitochondria — organelles found in animal cells — have their own DNA and can replicate. And because there are many mitochondria per cell, mitochondrial DNA is easier to find in fossils than nuclear DNA. 

Lindqvist wondered whether this mitochondrial DNA could illuminate the evolutionary history of how and when polar bears diverged from brown bears. To find out, she worked with Stephan Schuster, a molecular biologist at Pennsylvania State University in University Park, and a team of colleagues to sequence the genetic material she had collected and was successful.

It is the oldest mammalian mitochondrial genome yet sequenced — about twice the age of the oldest mammoth genome, which dates to around 65,000 years old. From Lindqvist's work, we learned that polar bears split off the lineage from brown bears about 150,000 years ago. They evolved rapidly in the Late Pleistocene, taking advantage of their hunting prowess to become the apex predators of the northern arctic region.