Researchers discovered the 14,000-year-old DNA in pieces of dried feces -- called coprolites -- unearthed in a cave in Oregon, according to a study published this week in the journal Science.
"This is the earliest direct evidence of a human presence in the Americas," Eske Willerslev, director of the University of Copenhagen's Center for Ancient Genetics and one of the study's authors, told the Boston Globe.
Until about a decade ago, scientists thought that the earliest humans in the Americas arrived via a land bridge from Asia around 13,000 years ago. That bridge, which was uncovered during the last ice age, disappeared when the climate warmed and sea level rose again.
The humans, called the "Clovis people" after a site in New Mexico where they were first found, spread throughout North America -- their remains, with their trademark stone spearheads, have been found through the continent.
Some scientists, however, have been puzzled by how quickly the Clovis people appear to have spread throughout North America. The new findings may help explain it, by providing evidence that humans reached the continent much earlier than previously thought.
The finding also lends weight to the theory that early North Americans might have accessed different parts of the continent via water routes -- taking boats down the Pacific coast -- because if humans arrived in North America during the ice age, overland routes through what is now Canada and the Northern United States would have been blocked by glaciers.
Other recent archeological excavations, including one in Monte Verde, Chile, have also provided evidence that pre-Clovis humans may have inhabited the Americas. But the new study is the first to find actual human DNA.
Human excrement itself doesn't contain any DNA, but it does contain DNA-holding flakes of tissue from the intestine.
"People shed gut tissue just like they shed skin flakes," lead author M. Thomas Gilbert told the Boston Globe.
The researchers used two methods to analyze the DNA. Radiocarbon dating put its age at more than 14,000 years old, and an analysis of the mitochondrial DNA -- a part of the DNA that is passed through the maternal line -- suggested that the humans descended from people who came from Northeast Asia.
Some scientists have cautioned that the feces could have been from dogs rather than humans, or could have been contaminated by DNA from later humans.
But most researchers in the field are convinced.
University of California, Davis anthropologist David Glenn Smith says the work was "a carefully designed and comprehensive study."
"I am convinced of [the researchers'] evidence for the pre-Clovis origins," he told the San Francisco Chronicle.
And, according to study author and archeologist Dennis Jenkins, of the University of Oregon, what matters is that the feces contains human DNA, not that it was human feces.
"Whether the coprolites are human or canine is irrelevant, since for a canine to swallow human hair, people had to be present," he told the Boston Globe. "Any way you cut the poop, people would have been present at the site."
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