The caves were hidden high above the Yukon's Bluefish River, at the base of a limestone ridge in the middle of a sprawling wilderness. When a helicopter reconnaissance of the river spotted the caves in 1975, it may well have been thousands of years since the last humans entered them—or so hoped archaeologist Jacques Cinq-Mars.

Between 1977 and 1987, Cinq-Mars led a team into the remote wilderness, battling clouds of mosquitoes and cold weather to excavate the layers of sediment and bones. What he found was a game-changer.

At the time, the prevailing theory was that the Clovis were the earliest human inhabitants of the Americas, with sites across North and Central America containing their iconic spearheads. As early as the 16th century, Europeans proposed that a land bridge between Asia and North America might have provided the route for early human migration; by the 1940s scientists were actively looking for and finding evidence for the bridge’s existence. And in the 1930s, spear points discovered near Clovis, New Mexico were discovered to match the artifacts found in Beringia, convincing people that the Clovis came first, approximately 13,000 years ago.

But when Cinq-Mars brought the fragments found at the Bluefish Caves back to the laboratory, he came to an incredible conclusion: humans had actually occupied North America as early as 24,000 years ago.

Naturally, the scientific community was skeptical. Other archaeologists raised a number of doubts about the bone samples. Anything in the environment can leave marks on artifacts: freeze-thaw cycle can snap bones, wolves and other carnivores chew on them, rocks fall on them from the ceiling of the cave. And it just didn’t fit into the Clovis hypothesis.

“For at least 70 years, everybody was stuck on ‘Clovis first,’” said anthropologist Dennis Stanford with the Smithsonian's National Museum of Natural History. “Anybody that came up with another kind of site was shouted down or disproved.”

The discord surrounding Cinq-Mars’ discovery resulted in a portion of the collection never being thoroughly analyzed, and researchers eventually lost interest. But now, 40 years after Cinq-Mars’ initial discovery, it seems the archaeologist has been vindicated.

Canadian scientists Lauriane Bourgeon and Ariane Burke, assisted by University of Oxford professor Thomas Higham, conducted a two-year re-analysis of the bones found in the Bluefish Caves, poring over 36,000 bone fragments held in a collection at the Canadian Museum of History and studying fragments that hadn’t previously been taphonomically classified. After doing a thorough classification of the markings on the bones as made by natural forces or humans, they conducted radiocarbon dating of those they deemed to have been marked by humans. The earliest bone to show distinct human-made marks—a horse jaw, sawed by a stone tool that indicates the hunter was attempting to remove the tongue—dates to 24,000 years ago.

The horse mandible was the most exciting find for Bourgeon. It bears multiple straight cuts, very similar to those made by stone tools and distinct in shape from marks made by carnivore teeth and natural abrasion. Additionally, the cuts match the patterns that would be created from butchering the horse. Altogether, Bourgeon says, the marks on the bone fulfill multiple criteria that would classify them as having a cultural origin, whereas it would be hard to explain their existence by natural processes.

“It was fairly exhausting,” said Bourgeon of their investigation in an interview conducted in French. “But I was really passionate about the project. When you see those traces of cuts on the bones, and know that horse is believed to have disappeared 14,000 years ago, that means we can guess humans were here before. It was a huge discovery.”

Bourgeon and Burke’s research provides new evidence for a more recent hypothesis that aims at overturning the old ‘Clovis first’ assumption. Known as the Beringia standstill hypothesis, it states that there was a pause in human migration from Asia to North America between 30,000 and 15,000 years ago due to the frigid climate. The standstill coincides with the last glacial maximum (about 26,000 years ago to 19,000 years ago), the most recent period in Earth’s history when the ice sheets were at their furthest southward extension (think glaciers down to New York City).

If the evidence bears out, it would also mean that humans came to North America a whole lot earlier than previously believed: 10,000 years earlier. Humans were living in the Siberian Arctic prior to the last glacial maximum, when the climate was milder and hunting options were abundant. Archaeologists have found evidence of human habitation in western Beringia (the landmass now beneath the Bering Strait) from 32,000 years ago, near the Yana River. But as the climate grew colder, humans would have been forced to migrate in search of food and shelter.

“Think of Arctic deserts as sets of lungs,” writes archaeologist Brian Fagan. “In warmer and moister times they breathe in people and animals, then they exhale them when aridity and cold intensify. This is what happened in Siberia during the last glacial maximum”—and what presumably chased humans out and likely decreased the size of their population.

Luckily for the early humans, the Beringia land bridge had a relatively mild climate despite its high latitude, thanks to the North Pacific Ocean circulation patterns bringing humidity to the region. That humidity allowed for more plant growth, in turn giving humans the fuel to build fires. And with the new evidence from the Bluefish Caves bones, researchers can see that humans did migrate sometime during the last glacial maximum, and were likely trapped on the Beringia land bridge due to the presence of glaciers all around them. In other words, they were stuck at a standstill.

Of course, not all archaeologists are completely convinced by the Bluefish Caves research. “I’ve seen pictures of the new bone they found, and it does look like it could possibly be human [markings],” said Stanford, who was not involved in the study. “But they didn’t leave much of an echo of a record if they were there. If there was a human in [the caves], why haven’t they been able to find any real artifacts? What technology did they have and why didn’t they leave anything?”

Bourgeon agrees that she’d like to do far more research on the region. Based on their discovery, she’s convinced they’ll find more equally ancient sites with evidence of human habitation. She’s never been to the Bluefish Caves and would like to visit that site, and look for others in the Yukon. But between the financing and logistics of such an expedition, it’s no easy undertaking.

“You can only work in the summertime, between June and August,” Bourgeon said. “It’s a very vast, sparsely populated region, an environment that’s hostile.” But, she added, the standstill hypothesis is starting to be more widely accepted, meaning more scientists will want to develop projects in the region. And as they do, she hopes they’ll find more pieces in the puzzle of human colonization of North America. 

Editor's note, February 1, 2017: This article originally missplaced the Bluefish River in Alaska. It also stated that the horse jawbone in question was dated to 24,800 years, rather than 24,000.