Like humans, brown bears are thought to have migrated from Eurasia across a temporary land bridge and then spread out across North America. Genetic research on living bears suggested that they settled their modern ranges in successive waves of migration. But now DNA found in bones plucked from the permafrost suggests there was a diverse array of immigrants that later broke up into isolated populations. The finding suggests current populations may not be genetically distinct enough to warrant separate conservation efforts.
Brown bears, also known as grizzlies, live in four different areas in North America: Alaska, the Yukon, the northern Rockies, and several islands off of British Columbia. Each population has a unique type of mitochondrial DNA, suggesting they haven't mingled in a long time. A few years ago, Alan Cooper, a molecular evolutionist at the University of Oxford, United Kingdom, came across a golden opportunity to study the genetic history of these bears. Stored in museum collections are more than 100,000 bones of bears and other large mammals that gold miners washed from the Alaskan permafrost--an ideal freezer for preserving DNA.
In the 15 February Proceedings of the National Academy of Sciences, Cooper and his colleagues describe how they extracted mitochondrial DNA from seven bear bones that had been collected at different locations within a few hundred miles of Fairbanks. To their surprise, they found no traces of past waves of immigration. Instead, bones some 35,000 to 42,000 years old suggest that all modern types of mitochondrial DNA were already present in Alaska. This ancient population was "more diverse than any one population of brown bears in the world today," says Cooper. After that heyday, the bears must have been split up into small, isolated populations---genetic bottlenecks---perhaps as they sought refuge on islands and other areas not covered by advancing glaciers during the most recent ice age.
This kind of work "is of great value [for understanding] the history of populations," says molecular geneticist Svante Pääbo of the Max Plank Institute for Evolutionary Anthropology in Leipzig, Germany. And it bears on modern creatures too, says Cooper. Since all the grizzlies coexisted just 35,000 years ago, he says, they may not be as genetically distinct as mitochondrial DNA suggests--and conservation plans for threatened populations may need to be reexamined.