They camped only briefly, leaving little evidence of their stay: a flaked-stone spear or arrow point, a few shards suggesting tool-making or sharpening, a grinding stone and – most important for researchers – several piles of excrement preserved in the dry cave floor.
From these unintended time capsules, scientists say they've extracted DNA that is unquestionably human. And carbon dating suggests that people first occupied the caves 14,300 years ago – more than a thousand years before the rise of the Clovis hunters, long presumed to be the first Americans.
If confirmed, the Oregon find reported Thursday in the journal Science would stand as the oldest direct evidence of human presence in the Americas. Other researchers have uncovered sites in North and South America with what appear to be pre-Clovis artifacts, but those claims remain controversial.
"What you are seeing here is very strong, unequivocal evidence for people being in America before Clovis," said Michael Waters, director of the Center for the Study of First Americans at Texas A&M University, who was not involved in the study.
Dennis Jenkins, the University of Oregon archaeologist who led the excavation, said many questions remain unanswered, but he is optimistic that other artifacts and ancient human feces, or coprolites, that have yet to be analyzed will verify his team's conclusions.
Others are not convinced. Gary Haynes, a professor of anthropology at the University of Nevada, Reno, said he's troubled by a lack of tools and other artifacts. And he questions whether the coprolites might be animal droppings contaminated by DNA from much later human cave dwellers.
"I remain skeptical," Haynes said.
Stone Age Oregon
Ice sheets covered nearly all of Canada 14,000 years ago. Camels, mammoths and bear-sized ground sloths roamed a wetter, swampier Oregon. A massive lake spanned the dry basin that now contains Summer Lake and Lake Abert, about 220 miles southeast of Eugene. Its waves carved a row of caves into a west-facing ridge, now called Paisley Caves.
Oregon archaeologist Luther Cressman first excavated three caves in 1938, finding bones of extinct horses, camels and other animals mixed with stone tools. Jenkins led excavations using updated and more exacting methods starting in 2002.
The work began to draw attention when Jenkins reported finding very old animal bones with signs of human butchering, and what appeared to be many human coprolites. One scientist, Eske Willerslev of the University of Copenhagen in Denmark, urged Jenkins to let his team try to recover human DNA.
When the first DNA tests came back positive, and carbon dating showed the samples to be more then 14,000 years old, Jenkins realized that the find had the potential to be a milestone in American archaeology.
"That's when we started getting excited," Jenkins said.
Mindful of the fierce controversy over earlier claims of pre-Clovis sites, the researchers went to great lengths to validate their conclusions. Separate laboratories in Florida and England performed carbon dating. The oldest coprolites ranged from 14,000 to 14,300 years old.
Experts in Denmark, Sweden and Germany independently isolated human DNA from coprolites. To rule out contamination with modern human DNA, they compared the ancient DNA sequences with those from every researcher and technician involved in the excavation and handling of the samples – 67 people in all.
But the coprolites hold clues about what the cave dwellers ate: horse, bison, squirrels, lizards, small fish and grains from wild grasses. Pollen grains, Jenkins said, may help establish the seasons when people used the caves. "These coprolites are just wonderful little packets of information," Jenkins said.
Debate not over
Three of the six oldest coprolites also yielded canine DNA, possibly fox, coyote or dog, the researchers noted. They tried to confirm they came from people by testing for protein markers, which they managed to find in four of the oldest samples. Willerslev said the team also tested for wood rat DNA to see if DNA from the copious rat droppings in the cave could migrate through soil and contaminate the coprolites. They found none.
But critics, such as Haynes and Marcus Hamilton of the University of New Mexico, said it remains possible that animals left the feces 14,000 years ago, and that later human cave dwellers dug up the cave floor and contaminated the coprolites with urine.
"Some people would consider that a more parsimonious sequence of events," Hamilton said.
Haynes said his overall doubts stem from the lack of evidence for a human presence in northeastern Asia and Alaska 14,000 to 15,000 years ago. "I wonder, where are the Asian precursor sites and how did people migrate through ice-covered landscapes in Canada that did not become passable until just a short time before the date on the coprolites?" he said.
But Waters, the Texas A&M scientist, said the Paisley Cave find fits the emerging pattern of evidence suggesting that people from the east started making their way to the Americas about 15,000 years ago – when a lower sea level opened a land bridge across the Bering Strait and receding glaciers cleared a coastal corridor.
"It's getting harder and harder for people who want to hold on to the Clovis-first model to keep ignoring these sites," Waters said. "The evidence has become too strong to dismiss."