Stone tools unearthed from a rock shelter in Southern Oregon were last used more than 18,000 years ago, radiocarbon dating suggests. That makes the site one of the oldest-known human living spaces in the Americas. But the people who lived in Oregon more than 18,000 years ago almost certainly weren’t the first to call the continent home.

A home where the buffalo roam

Buried deep beneath a layer of volcanic ash, archaeologists excavating Rimrock Draw Rockshelter found two stone scraping tools, which ancient knappers had skillfully shaped from pieces of orange agate. A residue of dried bison blood still clung to the edges of one scraper, a remnant of the last bit of work some ancient person had done with the tool before discarding it. The layer of volcanic ash above the tools had blasted out of Mount St. Helens, a few hundred kilometers north of the rock shelter, 15,000 years ago, long after the fine agate scrapers, and the people who made and used them, had been forgotten.

But how long?

In a layer of dirt below the volcanic ash but above the stone tools, archaeologists found broken teeth from now-extinct relatives of modern camels and bison. Radiocarbon dating on a piece of bison tooth enamel (first in 2012, and confirmed recently by more testing) suggests the teeth belonged to animals that lived about 18,250 years ago. And because those teeth were buried in a layer of dirt above the stone tools, they must have ended up in Rimrock Draw sometime after the tools. That makes the agate scraper, complete with bloody evidence of its use, more than 18,000 years old—and one of the oldest traces of human presence in North America.

New ideas about old events

“The identification of 15,000-year-old volcanic ash was a shock,” said University of Oregon archaeologist Patrick O’Grady, who runs an archaeological field school at the site. “Then [data from Tom Stafford of Stafford Research] 18,000-year-old dates on the enamel, with stone tools and flakes below, were even more startling.”

A decade ago, archaeologists would have been completely flabbergasted to find evidence that people were living in North America 18,000 years ago. At that point, the oldest evidence of people anywhere on the continent dated to around 13,000 years ago, in the form of long projectile points with flutes: narrow notches at their bases, creating a shallow groove to fit a wooden shaft for hafting.

These points belonged to people we now call the Clovis Culture. And for most of the 20th century, it looked like the Clovis people were the first ones to arrive in North America, probably following a huge gap in the middle of the kilometers-thick ice sheet that covered most of what’s now Canada and the northern US.

But today, the “Clovis First” theory is now just part of the history of science. The stone tools at Rimrock Draw are just the latest in a growing pile of evidence that people arrived in North America thousands of years before the ice sheets opened enough to create an ice-free corridor down the middle of the continent. Most archaeologists who study how people first reached the Americas now agree that they probably followed the edge of the ice sheet along the western coast of Canada sometime between 20,000 and 16,000 years ago.