Artifacts recently unearthed at a site in western Idaho called Cooper’s Ferry indicate that humans were living there 16,000 years ago, pushing back the timeline of human habitation in North America.

The find is more evidence to overturn the “Clovis First” hypothesis, reports Megan Gannon at National Geographic. Archaeologists previously believed that the oldest culture to settle the interior of North America came through a gap in the ice sheets in central Canada that appeared roughly 14,000 years ago. These people have left behind distinctive Clovis points, found in various places in North America throughout the 20th century, the oldest dating back 13,500 years.

But in recent years, archaeologists have found numerous sites and artifacts older than that migration timeline, suggesting that early humans didn’t travel through the ice but followed the coast, likely using boats. A site called Monte Verde at the southern tip of Chile is at least 15,000 years old, a sinkhole in Florida recently yielded a knife and butchered mammoth bone more than 14,500 years old and the Gault site in Texas has yielded thousands of artifacts that could be 16,000 to 20,000 years old.

The finds at the Cooper’s Ferry site are the final nail in the coffin of the Clovis theory argues Todd Braje of San Diego State University, who reviewed the new paper in the journal Science “[T]he Clovis-first model is no longer viable,” he tells Gannon bluntly.

The Cooper’s Ferry site—located at the confluence of Rock Creek and the lower Salmon River—has long been familiar to the Nez Perce Tribe, who occupied the site for generations as the ancient village of Nipéhe. In 1997, Loren Davis, Oregon State University anthropologist and lead author of the new study, excavated the site, finding some non-Clovis points that were about 13,300 years old. That find was controversial at the time since it was close to or even older than the Clovis points.

Davis still had some lingering questions about the site, so a little over a decade ago he returned to Cooper's Ferry and set up a field school there. “I was hoping we could evaluate if the site was really 13,300 years old,” he tells Ewen Callaway at Nature. As the summer excavations progressed, the team sent samples of charcoal from hearths and animal bones to researchers at Oxford University for dating. The oldest sample turned out to be 16,500 to 16,300 years old. “It just absolutely blew our minds how early this stuff was,” Davis says.

The simplest explanation is that the earliest migrants to North America traveled up river to reach Idaho. “The Cooper's Ferry site is located along the Salmon River, which is a tributary of the larger Columbia River basin. Early peoples moving south along the Pacific coast would have encountered the Columbia River as the first place below the glaciers where they could easily walk and paddle in to North America,” Davis says in the press release. “Essentially, the Columbia River corridor was the first off-ramp of a Pacific coast migration route. The timing and position of the Cooper's Ferry site is consistent with and most easily explained as the result of an early Pacific coastal migration.”

Geologist Alia Lesnek, who is studying coastal migration, tells Katherine J. Wu at Nova that the new research “drives home the idea that while the Clovis were a really important cultural tradition in North America, they probably weren’t the first humans living [there].”

Not all experts are convinced. Archaeologist Ben Potter at the University of Alaska Fairbanks tells Callaway it’s not clear if the oldest radiocarbon dates at the site are associated with human habitation. “Cooper’s Ferry is intriguing, but not paradigm-shifting,” he says.

One of the big questions remaining is just who the earliest North Americans were. Davis has speculated that the oldest artifacts found at Cooper’s Ferry are similar in form to artifacts found in northwestern Asia, in particular Japan. He’s currently comparing his dig’s finds with Japanese artifacts and also has lots of other material queued up for carbon dating from a second dig site in the area. “We have 10 years’ worth of excavated artifacts and samples to analyze,” he says. “We anticipate we’ll make other exciting discoveries as we continue to study the artifacts and samples from our excavations.”