for National Geographic News
"We've always thought that anthrax was an Old World disease that was brought to the New World by Europeans" around 1500, said study coauthor Paul Keim of Northern Arizona University.
But the new report suggests that ancient humans entering the continent thousands of years earlier imported the disease after crossing the Bering land bridge, which once connected present-day Alaska and eastern Siberia.
Scientists think anthrax originated in Africa or the Middle East thousands of years ago. One popular theory to explain its presence in North America is that it was brought over by Christopher Columbus and others beginning in the late 15th century.
But new genetic tests of hundreds of samples of one of North America's most common forms of anthrax suggest the disease actually spread from north to south. Anthrax in the Arctic Circle, for example, is more closely related to strains in Europe and Asia than to anthrax in the southern United States.
The tests also suggest anthrax first appeared in North America about 13,000 years ago, about when scientists think humans crossed the Bering land bridge.
Humans traversing the land bridge could have scavenged carcasses of bison and other mammals killed by anthrax.
"The movement of animal products such as bones and meat and hide and hair is very effective at spreading anthrax," Keim said.
While some parts of the new theory are speculative, it "makes a lot of sense" overall, said Gary Andersen, a microbial ecologist at California's Lawrence Berkeley National Laboratory who was not involved in the study.
"I think a lot of scientists will be convinced on the strength of the DNA evidence."
Findings detailed online in the journal PLoS One.
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