When researchers first arrived at a cave high in the desert mountains of north-central Mexico, they hoped to learn what the environment was like there thousands of years ago. But the unexpected discovery of what they believe is an ancient projectile point led to a decade-long excavation that could rewrite the history of the Americas.
According to a paper published today in the journal Nature, the site, known as Chiquihuite Cave, may contain evidence of human occupation that places people in North America around 30,000 years ago—roughly twice as early as most current estimates for when the first humans arrived on the continent.
The question of when people first arrived in the Americas has been debated for more than a century. For much of that time the reigning theory put the arrival around 13,500 years ago. But archaeologists are now exploring sites that keep pushing the date farther back, including some who have reported finding signs of human presence beyond 30,000 years ago. The evidence supporting those claims is hotly contested, and this latest discovery is already stirring more controversy.
“Everybody knows that when you step in the ring at this level, you are looking for an international debate, you're going to get it, and you should have your defense prepared,” says Loren Davis, an archaeologist at Oregon State University. “To me, it's inevitable. We're going to continue to push this back until there's no farther back to go.”
Excavating the sloping floor of the cave to a depth of 10 feet, excavation director and the paper’s lead author Ciprian Ardelean, an archaeologist with the Autonomous University of Zacatecas, and his fellow researchers have unearthed thousands of stones they identified as blades, projectile points, and flakes produced by the tool-making process. What they say are implements and flakes are of a type of limestone that has not been found in the cave and is believed to have been brought there.
The archaeologists also discovered bits of charcoal throughout the layers of sediment. While it's impossible to say whether the material was burned by human hands or in natural events, radiocarbon dating showed they ranged in age from 12,000 to 32,000 years old.
The researchers found no human remains and very few animal bones. They did, however, detect the presence of human DNA in the sediment layers. But it’s unclear whether the genetic material was left by ancient people, or whether the excavation was contaminated by DNA from modern humans.
"Chiquihuite's main contribution is that it brings you another tiny light, another tiny signal, that there is something there," says Ardelean, who has directed excavations at the cave since 2011.
Any visitor to Chiquihuite Cave today would assume that humans would never have picked such a place to live because it would have been so inhospitable. But they would be wrong.
The team extracted ancient pollen and DNA from the sediments and found signs that the cave’s desolate environs once were much cooler, greener, and wetter. At a deep level of the excavation reliably dated to 28,000 years ago, they discovered evidence of Douglas fir, a tree no longer native to Mexico, as well as a piece of stone the researchers believe to be a human-crafted blade.
When glaciers worldwide reached their maximum extent some 24,000 years ago—known as the last glacial maximum, or LGM—the landscape around the cave would have been forested with juniper, pine, spruce, and fir and studded with lakes and hot springs. “It would have looked like Oregon or British Columbia,” says Ardelean. “You would completely not recognize it.”
Questioning the evidence
The extreme age claimed for the Chiquihuite Cave doesn’t accord with the widely accepted view that people from Asia walked over a land bridge via the Bering Strait and into the Americas as the ice sheets that covered Canada during the Last Glacial Maximum (26,500 to 19,000 years ago) began retreating. For this and other reasons, the discovery is being greeted with caution by outside experts who’ve reviewed the data presented in Nature.
The stones that the researchers believe are tools fashioned by human hands have come under particular scrutiny. While the researchers demonstrated that the stone came from outside the cave, some experts question whether they are actual human artifacts or were created by natural geological processes.
Loren Davis, the Oregon State archaeologist who directs excavations at the early site of Cooper’s Ferry in Idaho, points out that cave environments also create plenty of naturally fractured stones that can be misinterpreted as artifacts.
“The thing to remember,” says Davis, “is that humans don’t have a monopoly on the physics required to break rocks.”
He’s also troubled by the lack of other signs of human occupation in the cave deposits, such as hearths and animal bones bearing cut marks.
“You can have a big list of all the things you might expect to see in a site, and [the Chiquihuite researchers] don't have anything except for some broken rock,” Davis says. “And if you take the rocks away, there’s really nothing.” While he calls the research “intriguing,” he’s reserving judgment.
Then there’s the startling fact that the style of toolmaking—the distinctive way the stones appear to have been shaped—is utterly unique.
“It's very curious that the assemblage is so different from anything anyone has known before,” says archaeologist Tom Dillehay of Vanderbilt University. “How is it possible that it’s not related to anything previously found? Well, it’s possible.”
In 1997 Dillehay presented evidence that people arrived at the tip of South America, at a site known as Monte Verde, some 14,500 years ago, a thousand years earlier than then-accepted estimates. His claim sparked a furor, but ultimately his findings were confirmed.
Dillehay notes that he received similar criticism about stone tools he found at Monte Verde, and he thinks some of the Chiquihuite projectiles may be forerunners of later points found in north-central Mexico.
But he also expressed misgivings about the seeming lack of technological evolution in the stone artifacts. Based on radiocarbon dates of the charcoal dug from its floor, the cave’s use by humans spanned some 20,000 years. Dillehay notes that there are marked changes in the stone tools used at Monte Verde over the course of thousands of years, yet over a much longer period the stones from Chiquihuite show no signs of evolving toolmaking techniques.
University of Oregon professor Dennis Jenkins, who directs excavations at the early site of Paisley Caves, is concerned that many of the purported stone blades don’t appear particularly sharp, based on the photos presented in the Nature paper. “However, there were some that definitely looked like potential artifacts,” he says. And the fact that the stone originated outside the cave adds weight to the claim that it was brought in and fashioned for use by humans, Jenkins says.
Several other outside researchers contacted by National Geographic declined comment and directed inquiries to Michael Waters, director of The Center for the Study of the First Americans at Texas A&M University and a preeminent expert on the peopling of the Americas. Waters reviewed the paper but also declined to comment, instead providing National Geographic with a 2019 Science review he authored that concludes current genetic and archaeological data do not support an occupation of the Americas before 17,500 years ago.
The Chiquihuite team is currently preparing another paper on their research, and Ardelean is confident that the new data will provide additional support for their conclusions. Nonetheless, Ardelean repeatedly reminds his team that Chiquihuite Cave is just one site of many that are drawing a fuller and more complex picture of when and how people arrived in the Americas.
“I think this study shows that we need to re-examine what we think we know about the peopling of the Americas, and we need to be open to a much longer time span,” says Chiquihuite team member Devlin Gandy, an archaeologist at the University of Cambridge.
The deep dates provided by the Chiquihuite data, however, are “undoubtedly going to be contested voraciously,” says Jenkins. “I don't have any doubts about that.”