The School: Polar Bear Update
A weekly dose of education in the ocean.
This is the fifth in a five-part series on the polar bear’s adaptations to the Arctic environment.
Last year, in honor of International Polar Bear Day and the release of our film To the Arctic, we wrote a series of blogs all about polar bear adaptations. Polar bears are amazingly adapted to their life in the Arctic, where they depend on their ability to successfully hunt on sea ice, swim, and survive cold. We also wrote about how polar bears were thought to have arisen from grizzly bears about 150,000 years ago. But just since last year, a lot has developed in our understanding of the species. New science has changed what we know about the origin of polar bears.
The belief that the polar bear species arose 150,000 years ago, relatively recently, was based on research that examined mitochondrial DNA collected from an ancient polar bear jawbone. While most DNA is found in the nucleus of a cell, mitochondrial DNA (also called mtDNA or mDNA) is DNA that is located in a different part of a cell, in the mitochondria. It is only passed down from the mother and not the father, so it can be used to track a maternal lineage, and because it’s just a portion of the whole gigantic genetic code, it’s easier to study. Scientists studying it found that the mtDNA from the two bear species was similar. This suggested that the species were closely related, and that the polar bear had only separated into a new species recently.
However, scientists are always making new discoveries: it turns out that studying the mtDNA solved only part of the mystery. Just within the last year since we published that blog, new research has come to light suggesting this understanding of polar bears’ origin is not accurate.
A study that was published in April 2012 analyzed DNA from polar bears, brown bears, and black bears. Instead of just testing the mtDNA, these researchers were able to look at samples from the whole genome; this is more difficult and expensive but provided more accurate and detailed information. It showed that the polar bear was not as closely related to the brown bear as previously thought. They concluded that the species diverged about 600,000 years ago. This would give them more time to develop different traits more suitable for their icy ocean habitat.
But if polar bears diverged 600,000 years ago, how do we explain the mtDNA that traces polar bears back to brown bears just 150,000 years ago? Interbreeding. Although brown bears and polar bears separated much earlier, there was a more recent period in history when the two had overlapping territories and successfully mated.