Surprisingly old stone points found in a Mexican cave are the latest intriguing discovery among many to raise questions about when humans really arrived in the Americas.

For most of the 1900s, archaeologists generally agreed that humans who had crossed the Beringia land bridge from Siberia to North America only ventured further into the continent only when retreating ice sheets opened a migration corridor, about 13,000 years ago. But a few decades ago, researchers began discovering sites across the Americas that were older, pushing back the first Americans’ arrival by a few thousand years. Now, the authors of a new study at Mexico’s Chiquihuite cave suggest that human history in the Americas may be twice that long. Put forth by Ciprian Ardelean, an archaeologist at the Autonomous University of Zacatecas (Mexico), and his colleagues, the new paper suggests people were living in central Mexico at least 26,500 years ago.

Ardelean’s work was published in Nature and paired with another study that presented a broader look at 42 known early human sites across North America from the Bering Strait to Virginia. Data from those sites were used to model a much earlier peopling of the Americas, and help scientists re-imagine not only when but how the first people reached and populated the New World. The model features a number of archaeological sites, including Chiquihuite cave, which are intriguing but controversial enough, as experts disagree whether the sites actually evidence human occupation.

Chiquihuite cave is perched high in the Astillero Mountains, 9000 feet above sea level and 3,280 feet higher than the valley below. Excavations there were launched when a 2012 test pit unearthed a few stone artifacts that suggested a human presence dating back to the Last Glacial Maximum between 18,000 and 26,000 years ago. More extensive excavations detailed in the new study were carried out in 2016 and 2017, unearthing some 1,900 stone points or possible tools used for cutting, chopping, scraping, or as weapons.

The artifacts were dated by 46 different radiocarbon samples of adjacent animal bones, charcoal, and sediment samples. To the team, they represent a previously unknown technological tradition of advanced flaking skills. More than 90 percent of the artifacts were of greenish or blackish stone, though those colors are less common locally, suggesting to the authors that they were singled out as desirable. The bulk of the material is from deposits dating to between 13,000 and 16,600 years ago, leading the scientists to hypothesize that the humans may have used the cave for more than 10,000 years.

Ardelean knows that Chiquihuite’s very old dates will raise most archaeologists’ eyebrows. “As soon as you cross the limit into the Last Glacial Maximum (LGM), that’s when it gets tricky,” he says. “We have a sort of mental blockage just thinking about getting into a new continent in the middle of a glaciation.”

Yet he suggests that if people were visiting this cave during the Last Glacial Maximum they likely entered America even earlier, more than 30,000 years ago, before glaciers blocked the way from Beringia. “It takes centuries, or millennia, for people to cross Beringia and arrive in the middle of Mexico,” Ardelean says. “Even coastal arrivals wouldn’t have been landing on the Mexican coast—it’s just way too far. You need many years of previous presence to make them arrive there if they came by sea or by land.”

Sunlight shines into Chiquihuite cave (Devlin A. Gandy)

Ardelean sees the site as one point on an emerging new timeline for humans in the Americas. “This site alone can’t be considered a definitive conclusion,” he admits. “But with other sites in North America like Gault (Texas), Bluefish Caves (Yukon), maybe Cactus Hill (Virginia)—it’s strong enough to favor a valid hypothesis that there were humans here probably before and almost surely during the Last Glacial Maximum.”

As would be expected, the site has drawn scrutiny from the archaeological community. In a Nature “News & Views” article accompanying the studies, Ruth Gruhn, professor emerita at the University of Alberta, said that since the idea of an American entry date more than 30,000 years ago is double the currently popular date of about 16,000 years ago it “will be very hard for most archaeologists specializing in early America to accept.”

Southern Methodist University archaeologist David Meltzer questions why the stone tool traditions described at the site haven’t been seen anywhere else in the region, and why their technology remained unchanged for so many thousands of years. Loren Davis, an Oregon State University archaeologist, says most of the artifacts appear to have been produced by a single blow or fracture. Might they actually be broken rocks, created by natural actions like rockfall from the ceiling? “Humans don’t have a monopoly on the narrow band of physics required to break rocks,” Davis says. “I’m open to being convinced. If I’m wrong about what I seeing in their reporting, I might change my mind.”

Davis also laments a lack of evidence for domestic life in the cave. “We usually see things like butchering animals and making food,” he says. “They did find lots of animal bones but they say there is no evidence of butchering and that’s really strange. There’s also an absence of things like fire pits, or pits in the ground for storing things, or unusual distributions of objects.”

Ardelean believes some of those features might lie tantalizingly close by, yet be difficult or impossible to uncover. The current excavation is taking place far inside the large cave. “Most activities like cooking and eating happened right at the entrance,” he says. “And that entrance isn’t accessible, it’s buried under tons of debris that has fallen from the top of the mountain.”

That cave’s mountainous location, thousands of feet above the valley floor, has David Meltzer asking another question. “Why keep coming back to that same place on a relatively constant basis over such a long period of time?” he says. “I find that curious. Not many sites have that kind of long-term repeated occupation, unless there is something quite useful or available at the spot to attract people to it.”

Scientists work in Chiquihuite cave (Mads Thomsen)

The study shed some light on environmental conditions that existed over many centuries at the cave—a shifting landscape of mixed forests and grasslands revealed by plant samples from 31 DNA extractions from the surrounding soil materials. But while tests of cave sediments revealed lots of ancient plant and animal DNA, scientists recovered no unambiguous signal of ancient people. Ardelean says the lack of verifiable human DNA up to this point is a disappointment. “Until we would have DNA available, there’s nothing to tell us who these people were or where they came from,” he says.

In recent years archaeologists working on various New World sites have stacked up evidence to refute the once-ubiquitous theory that the Clovis People, with their distinctive points, were America’s oldest culture. Scientists estimated that they passed through a corridor between Canada’s great ice sheets some 13,000 years ago.

At Paisley Caves in Oregon people made an entirely different type of projectile point and left behind fossilized poop at least 14,000 years ago. Butchered mastodon bones and stone tools in a sinkhole suggest humans had reached Florida by at least 14,500 years ago. Evidence suggests humans made tools and butchered animals at Cooper’s Ferry in Idaho 16,000 years ago and made it all the way to tip of South America at Monte Verde, Chile by 14,500 years ago.

Many of those sites are represented in the second study, co-authored by Lorena Becerra-Valdivia, an archaeological scientist at the University of Oxford and the University of New South Wales, and Thomas Higham, an archeological scientist at the University of Oxford. They compiled radiocarbon and luminescence dating data from 42 archaeological sites across America, creating a model that maps scenarios of human distribution across the continent in time and space, from the Bering Strait to Virginia. When the oldest pre-Clovis sites are plugged in, the model suggests that humans populated the Americas before and during the Last Glacial Maximum some 19,000 to 26,500 years ago. That would mean that humans not only arrived in the Americas earlier than is commonly believed, but that they somehow circumvented the era’s massive ice sheets.

A theory that these peoples migrated by travelling down the Pacific coastline 14,000 to 15,000, or even as long as 20,000 years ago, has been steadily gaining support as excavations turn over more evidence, though uncovering their tracks is complicated due to past changes in sea levels. Another possibility is simply that people entered the Americas by land before the glaciers blocked the route into the continent’s interior. The model also suggests that a second, more widespread peopling of the Americas unfolded during a period of sudden and dramatic warming about 12,900 to 14,700 years ago. Becerra-Valdivia says this is evidenced by a spike in archaeological sites and the emergence of stone tool traditions like Clovis. Genetic research, she adds, also “points to marked population growth between around 15 to 16 thousand years ago.”

If the Americas begin to look more heavily populated by distinct groups of people after these dates, Ardelean believes the earlier pre-Clovis sites, each with distinct types of technologies or artifacts, tell a different tale.

“I think that the human presence during the Last Glacial Maximum was extremely diverse, and there were multiple arrivals from multiple directions,” he says. “I believe humans were culturally diverse and potentially genetically diverse. There was no such thing as a single arrival.”