Mexican cave contains signs of human visitors from 30,000 years ago

At first glance, Chiquihuite Cave in Mexico’s Zacatecas state is an unlikely place to find signs of early humans, let alone evidence that might change the story of the peopling of the Americas. It sits a daunting 1000 meters above a valley, overlooking a desert landscape in the mountains north of Zacatecas. Getting there requires a 4- or 5-hour uphill scramble over a moonscape of jagged boulders.

But in the soil below the cave’s floor, a team led by archaeologist Ciprian Ardelean of the Autonomous University of Zacatecas, University City Siglo XXI, dug up almost 2000 stone objects that researchers think are tools. By combining state-of-the-art dating methods, the team argues that humans were at the site at least 26,000 years ago—more than 10,000 years before any other known human occupation in the region. “Chiquihuite is a solitary dot” of human occupation, Ardelean says.

The dates place humans there during the height of the last ice age when ice covered much of what is now Canada and sea levels were much lower. To have settled in Mexico by then, Ardelean says, people must have entered the Americas 32,000 years ago or more before the ice reached its maximum extent.

“If it is true people were in Zacatecas by 32,000 years ago, that changes everything—it more than doubles the time people have been in the Americas,” says Oregon State University, Corvallis, archaeologist Loren Davis, who was not part of the research team. But he remains sceptical, in part because he isn’t convinced the artefacts are tools. “I’m not going to say it’s impossible,” he says. “But if all they found are fractured rocks without any corroborating evidence, it’s natural to be skeptical.”

Still, he and others say they’re willing to be convinced. For decades, most researchers thought humans arrived in the Americas approximately 13,000 years ago; occasional claims of an earlier arrival met strong criticism. But over the past decade, evidence for earlier migrations has emerged at sites from Canada to southern Chile.

Most researchers now think people travelled by boat along North America’s west coast, exploiting marine resources, as early as 16,000 years ago, when the interior of the continent was mostly frozen over.

Just one other site—Bluefish Caves, in Canada’s Yukon territory—has yielded dates as old as Chiquihuite. Researchers attribute thousands of broken animal bones there—dated to about 24,000 years ago—to human hunting. But the site remains controversial, in part because few stone tools or cut marks have been found among the bones.

Ardelean heard about the cave from local villagers. Beginning in 2012, he and his team spent 1 month or more at a time at Chiquihuite, resupplying every few weeks using donkeys. Although forbidding today, the site would have looked far more hospitable 26,000 years ago.

A spring-fed creek flows near the cave’s original entrance, which was blocked long ago by rockslides. DNA and other evidence the researchers extracted from inside the cave show it opened onto a lush landscape harbouring cranes, condors, marmot, goat, sheep, horses, and bears. “It looked a lot more like British Columbia or Oregon than desert,” Ardelean says.

Digging into the cave floor over the past 8 years, Ardelean and his team found stones shaped into what look like scrapers, hand axes, spear points, and other tools at depths of up to 3 meters.

Dating experts at the University of Oxford, the University of New South Wales (UNSW), and elsewhere determined when the rocks had last been exposed to light and radiocarbon-dated more than 50 samples of animal bone and charcoal found near the tools.

As the group reports today in Nature, the artifacts were deposited starting 26,000 years ago and accumulated on the cave floor for the next 16,000 years. The authors argue that it adds up to a continuous human presence, with people regularly visiting the cave over millennia.

But the team found no human DNA or bones cut marked by human hands. Nor did they discover a central hearth, so they can’t be certain whether the bits of burned wood analyzed for radiocarbon dates are from wind-blown wildfires or human-made campfires.

“The evidence is what the evidence is,” says team member Lorena Becerra-Valdivia, a radiocarbon dating expert at UNSW. “We’re quite confident that the stone tools are, indeed, stone tools.”

Critics point out that the tools are simple and don’t resemble other toolkits from the Americas, raising the possibility they’re the product of natural breakage. “They look like they could be artifacts, but why aren’t they found anywhere else in the landscape?” wonders David Meltzer, an archaeologist at Southern Methodist University.

The tools’ consistency is also remarkable, he says. “If these tools are real, why are they only found—so far at least—in this one spot over a 10,000-year period? Humans adapt and adopt new technology.”

Becerra-Valdivia says work in other sites, especially those south of the United States, may turn up corroborating evidence. “We need to take a really good look at South America.”

 And Ardelean says Chiquihuite has more secrets to reveal: “This is not a hit-and-run discovery. There’s more evidence coming.”