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About 24,000 years ago, when much of North America was buried under the ice of the Last Glacial Maximum, a few hunters took shelter in a small cave above the Bluefish River in what is now northwestern Yukon. The hunters had killed a Yukon horse and were butchering it using super-sharp stone shards called microblades. As they sliced out the horse’s meaty tongue, the microblades left distinctive cuts in its jaw bone. Millennia later, archaeologist and doctoral candidate Lauriane Bourgeon spotted those marks through her microscope at the University of Montreal and added the fragment of ancient jaw bone to her small selection of samples for radiocarbon dating.

The bones came from excavations led by archaeologist Jacques Cinq-Mars between 1977 and 1987 and have been in storage at the Canadian Museum of History in Gatineau, Quebec. At the time, Cinq-Mars and his team concluded that the Bluefish Caves showed evidence of occasional human use as much as 30,000 years ago. That is so much older than anything else found in the Americas that Cinq-Mars’s conclusions were widely disputed, and the three small caves were largely left out of discussions about the peopling of the Americas.

The idea of researching such a controversial site appealed to Bourgeon: “Alaska, Yukon, bone accumulations, caves, the first peopling. … That was it. That was the Spell of the Yukon!” she said by email.

Bourgeon sent six pieces of bone that showed evidence of stone-tool cuts to a lab in Oxford, England, for radiocarbon dating. The youngest, it turned out, was a 12,000-year-old caribou bone. The most ancient: the 24,000-year-old horse jaw bone.

The finding—published in the journal PLOS One—makes the Bluefish Caves the oldest known archaeological site in North America by a margin of almost 10,000 years—and confirms much of Cinq-Mars’s work.

Previously, the oldest accepted human occupations were at three sites in Alaska and one just over the border in Yukon, all dating to about 14,000 years ago.

“We had suspicions that the human presence might be old when we found cut marks on horse bones,” Bourgeon says. Horses are thought to have disappeared from the region about 14,000 years ago. “So when Tom Higham from the Oxford radiocarbon laboratory sent us the results … we were all very excited!”

Bourgeon says her results add weight to another controversial idea: the Beringian standstill hypothesis.

It was once assumed that the people of Siberia fled south when the glaciers advanced and returned to cross into North America as the ice retreated. But recent genetic studies suggest that some ancient people rode out the hostile conditions of the Last Glacial Maximum in isolation in the relatively hospitable Beringia—a continent, now mostly underwater, that once spanned from Siberia to Canada’s Mackenzie River—before moving deeper into North America when conditions improved. Archaeological evidence of their presence has been elusive, but the butchered bones of the Bluefish Caves might provide that missing link.

“Finding no cultural material doesn’t mean that people weren’t there,” Bourgeon says. “Of course, small, highly mobile groups of hunters wouldn’t have left much evidence behind and part of Beringia is now underwater.”

John Hoffecker, an archaeologist and human paleoecologist at the University of Colorado and proponent of the Beringia standstill hypothesis, agrees that the cut-marked bones are strong evidence of early human occupation. But what stunned him, he says, was a comment—taken from Cinq-Mars’s original, unpublished notes—that stone tools were found in the lowest and oldest cave deposits. “As soon as I saw the information, I realized that there is a pretty solid case here for the Last Glacial Maximum occupation 24,000 years ago.”

Not everyone is convinced. University of Alaska Fairbanks archaeologist Ben Potter has scoured Alaska for sites older than about 14,500 years without success. The research is interesting, he said by email, but the small number of cut-marked bones and lack of clear evidence that the tools found in the cave were left there at the same time as the dated bones leave him with doubts*. He still considers it uncertain that the Bluefish Caves were an early site of human occupation in North America.

Hoffecker says he expects the argument over the Bluefish Caves to continue for a while, but the bone studies and the surprising information about deeply buried stone tools have convinced him that people were there during the Last Glacial Maximum. Bourgeon is convinced that, no matter how little trace the ancient hunters left, more sites dating to that time will eventually turn up.

*Correction: This sentence was modified to more accurately reflect Potter’s view. The sentence originally read: “…the small number of cut-marked bones and lack of clear evidence that the tools found in the cave made the cuts leave him with doubts.”