Fossilized excrement found in an Oregon cave has given scientists the clearest evidence to date that humans roamed the New World at least 1,000 years earlier than previously believed.
The prehistoric feces, deposited in a cave some 14,300 years ago, contain DNA from the forebears of modern-day Native Americans, according to the research.
The discovery reported yesterday by the journal Science added fresh weight to emerging theories that Stone Age people from Asia somehow bypassed ice sheets sealing off North America before 11,000 BC.
Nearly all scholars agree that humans were present by then, but until recently few archeologists accepted that an earlier arrival was even possible because of the formidable ice barriers. So the Oregon discovery and work at other sites may help solve one of archeology's most enduring mysteries - how and when did humans reach the Americas?
The new timeline comes from 14 pieces of fossilized excrement, called coprolites, found within the Paisley Caves complex by University of Oregon archeologist Dennis L. Jenkins and painstakingly analyzed by genetic anthropologists in Denmark.
"This is the earliest direct evidence of a human presence in the Americas," said Eske Willerslev, director of the University of Copenhagen's Center for Ancient Genetics, which extracted human mitochondrial DNA from six of the coprolites.
"There were humans doing their business in a cave in Oregon long before many scientists believed there were any humans at all in North America," Willerslev said in an interview.
According to the research, the coprolites contained DNA signatures that could belong only to native Americans. Excrement contains no genetic material, but typically carries particles of tissue from the intestines. These fragments yield DNA.
"People shed gut tissue just like they shed skin flakes," said M. Thomas P. Gilbert, lead author of the study.
The new research doesn't set an exact arrival date for humans, but it shakes up long-held assumptions - especially the notion, still dear to many archeologists, that humans couldn't have punched past the glaciers covering nearly all of present-day Canada and the northern United States much before 13,000 years ago. That's when warming would have allowed easier transit across a land bridge from Siberia and into the heart of the new continent by interior passageways.
Because this research puts humans in the New World more or less concurrent with the ice wall, the find supports emerging theories that the first Americans followed a rugged, coast-hugging route down the Pacific Northwest - perhaps coursing from peninsula to peninsula in primitive watercraft.
"These people got around the ice somehow, and the idea that they followed a marine route is plausible," Willerslev said.
The research triggered immediate controversy, with some archeologists arguing that the Danish team had failed to entirely rule out the possibility that the coprolites were tainted by DNA from later humans.
But there were more cheers than boos, because an earlier arrival date helps clear confusion over how humans managed to migrate so widely in the New World in such a short time span. If the new DNA dating is right, they didn't. They simply had forged past the ice much earlier than many scientists had suspected and, thus, had more time to reach the far corners of the continent.
"This is an important site and an important discovery," said geoarcheologist Michael Waters, director of the Center for the Study of the First Americans at Texas A&M University, who believes that the Oregon findings, together with archeological digs underway in Florida, Washington, and other states, will push back the time of human arrival to an even more distant past.
"It's part of a real paradigm shift in thinking in archeology," said Waters, who was not involved in the Paisley findings.
Until a decade ago, there was broad consensus among scientists that the first Americans were the so-called Clovis people, known for their delicately fluted spearheads and other stone blades. Clovis sites are numerous across North America, but none has proved much older than 13,000 years.
The Paisley Cave coprolites were dated using radiocarbon methods. The dry, stable temperatures of the caves are ideally suited to preserving organic material.
"Too often we've had to worry over whether supposed pre-Clovis specimens are as old as they are claimed to be," said David J. Meltzer, an anthropologist at Southern Methodist University in Dallas. "Direct radiocarbon dating of human coprolites would seem to cut that Gordian knot."
Some scientists remain deeply dubious about the significance of the fossil feces.
Anthropologist Gary Haynes of the University of Nevada, Reno, noted that the Paisley Caves served as the Stone Age equivalent of a public toilet. "The rock shelter was used for thousands of years by Native Americans urinating, sweating, defecating, and otherwise depositing their DNA in the sediments . . . [so] the presence of DNA in the coprolites may be the result of contamination" from a later period, he said.
Haynes and other skeptics also suggested that the Paisley Caves excrement may be that of wolves or wild dogs, not people.
Archeologist Jenkins disagreed, noting that human hair as well as microscopic tissue was contained in the ancient waste.
"Whether the coprolites are human or canine is irrelevant, since for a canine to swallow human hair, people had to be present," he said. "Any way you cut the poop, people would have been present at the site. The dating and the DNA are what's important."