David Meltzer, an archaeologist at Southern Methodist University in Dallas, Texas, said the study is fascinating because it provides a first look at the history of ancient bison populations.
"If we have evidence bison populations were going through bottlenecks, there isn't any reason mastodons, camels, horses, and all the other critters weren't also experiencing similar [disturbances] in their populations," he said.
Shapiro, the study author, said that relying on the mere abundance of fossils to estimate ancient animal populations is inaccurate, because bones are better preserved under certain environmental conditions than others.
A lack of fossils could mean a population was extinct or rare, she said. Or it could mean that geological conditions were not good for preserving bones.
Comparing bison genes from ancient herds throughout Beringia enabled Shapiro and her colleagues to draw a diagram, or tree, of how the populations were related. A statistical model, based on what are known as coalescence events, allowed the researchers to make inferences about population size.
A coalescence event is an instance when two genetic lineages, or branches, come together and share a common ancestor, Shapiro said. These are more likely to occur when populations are small. The frequency of coalescence events determines the shape of the tree.
"One way to think about it is to imagine sampling two people from a small village and two people from a large city like London," Shapiro said, noting that a diagram relating the pairs will likely show that the two village residents share a common ancestor more recently than the two Londoners.
In the case of the ancient bison, the researchers divided the animal's tree up into periods of time. They then estimated the shape of the tree in each time period and used the tree shape to estimate what the bison population looked like over time.
"We repeated this process for 27,000 of the trees that best describe the relationships among bison and came up with a picture that the population was increasing to a point and then begins to crash," Shapiro said.
Shapiro and her colleagues are unclear why the bison populations suddenly plummeted. But they believe the decline coincides with a time of environmental and climate change that immediately preceded the last North American ice age.
At the time some areas of vast grasslands converted to forests, which were more difficult for bison to move through and forage in. Further analysis of the climate and environmental record may reveal the forces that ultimately drove the mammals' decline, Shapiro said.
What is clear, according to Shapiro, is that the decline began long before the arrival of large human populations. "We are not arguing that humans had no effect. We are arguing that humans didn't start it," she said.
In addition to extinction insights, the bison DNA analysis also allowed Shapiro and her colleagues to reconstruct how herds from Siberia, Alaska, Canada, and the lower U.S. moved around and interacted through time.
Previous research indicates that the first bison came from Asia, moved into North America, and diversified. The new DNA analysis adds to that picture, suggesting that these North American bison later dispersed back to Siberia and replaced the bison that were there about 140,000 years ago.
Until the peak of the last ice age about 22,000 years ago, bison roamed freely across Siberia, much of North America, and south to Mexico. Then the climate began to cool. Ice sheets moved across North America. By 22,000 years ago the ice formed a geographic barrier that prevented bison herds of Beringia and central North American from mixing.
After the glaciers receded, creating what is known as the ice-free corridor, humans may have moved through the corridor toward central North America. Previously, researchers believed the bison living in North America today descended from herds that moved south through the ice-free corridor at the same time early humans migrated.
Shapiro and her colleagues found, however, that forest cover and peat swamps prevented the Beringian bison from moving south. "The modern bison, both the wood and plains bison, are descended from the maternal lineage of bison that were already present south of the ice sheets when the glaciers formed," Shapiro said.
Today the wood and plains bison in North America are the only surviving populations. Over the last 8,000 years bison stampeded across the plains of the North American West and their numbers swelled into the hundreds of thousands. But in the late 1800s the animals were hunted to the brink of extinction. Today only a few herds remain.
Meltzer, the Southern Methodist University archaeologist, said: "I'm not at all surprised that the record shows the population did have boom and bust periods and did have bottlenecks through which they passed. It happens to all other critters, and there's no reason to think bison were any different."
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