A research paper in 2020 made headlines by claiming that humans reached North America at least 30,000 years ago, but some archaeologists are raising concerns that the evidence was misread.
Conventional estimates have it that humans reached North America at some point between 15,000 and 20,000 years ago. A Nature paper published in July 2020 blew the lid off this estimate by claiming an earlier arrival date, as evidenced by 30,000-year-old stone tools and flakes found at the Chiquihuite Cave site in Zacatecas, Mexico.
The finding was taken as further proof that humans reached the Americas by traveling along a Pacific coastal route, as the gigantic continental ice sheets were still firmly in place at the time. The paper, led by archaeologist Ciprian Ardelean from Universidad Autónoma de Zacatecas in Mexico, would be evidence against the Clovis-first hypothesis, which posits that the first humans to reach the Americas did do some 13,000 years ago, after the last ice age came to an end.
So yeah, a real bombshell of a paper—except that the physical evidence was completely misinterpreted, at least according to the authors of new research published in the science journal PaleoAmerica. The paper, co-authored by archaeologist Ben Potter from the Arctic Studies Center at Liaocheng University in China, argues that the items described in the Ardelean study are not actually stone tools and flakes but are instead the products of natural cave processes.
Ardelean and his colleagues analyzed nearly 2,000 stone artifacts found inside Chiquihuite Cave. The oldest objects were found in a layer dated to between 31,000 and 33,000 years old, with evidence of more intense occupation at the cave dating to around 26,500 years ago. The apparent artifacts were made from limestone and hafted into an unknown lithic style, according to the Ardelean study. No human bones or human DNA were found inside the high-altitude Mexican cave.
The objects found at the site were classified as cores, scrapers, blades, and flakes, among other tool types. But where these researchers saw human fabrication, Potter and his team see only natural processes.
“In the high-energy cliff-face environment where Chiquihuite Cave is found, falling and tumbling rocks strike one another and drive off shards, which often have some of the features of rocks broken by people,” James Chatters, the first author of the new study and an archaeologist from Applied Paleoscience in Bothell, Washington, explained to Gizmodo in an email. “A stone striking a stone can produce similar looking products regardless of how the force is initiated.”
Chatters said systematic human behavior tends to produce overlapping chips of similar size, but none of the items showcased in the Ardelean study exhibited those characteristics. And where the Ardelean team saw wear-and-tear on tool edges, Chatter and his team saw patterns of damage produced by natural events.
The Ardelean team has already prepared a response to these and other concerns, which has likewise been published in PaleoAmerica. The team is standing by their initial interpretation of the evidence, saying they “dismiss” the claim that the stone tools described in their paper are mere “geofacts,” that is, rocks, bones, or shells that have been modified by natural processes to appear as human artifacts. I reached out to Ardelean with specific questions, but he declined the opportunity to comment, saying “All I may say is written there,” in reference to his team’s response paper.
It’s important to point out that Chatters and his colleagues didn’t inspect the items gathered in Chiquihuite Cave first-hand, and instead relied “on the evidence provided in the original article and supporting documentation,” as the scientists wrote in their study. That caveat aside, I asked Potter how it’s possible for two sets of specialists to reach such drastically different conclusions when looking at the same thing.
“In a word: equifinality,” he replied. “It is a very common problem in archaeology—multiple processes can often leave the same or similar results.”
A percussive blow from one rock hitting another rock can produce the same result as a human tool builder, he said, so it’s therefore important to “evaluate the context of the finds.”
For example, the “purported artifacts occur essentially randomly throughout” the cave, but “appear more concentrated in strata with more rocks,” a distribution that’s “expected under the natural hypothesis,” he said. Potter was also concerned about what wasn’t found—things like hearths and butchered animal remains—the absence of which he described as “red flags.” Moreover, the “lack of any cultural change in how they made the tools over 10,000 years is something that does not occur in modern human cultures.”
Another key point made in the critique is that hunter-gatherers tend to use all sorts of stones when making their tools, including both local and non-local toolstone, and stones of varying quality. The purported lithics in the cave lack this dynamic, which Potter found to be quite unusual, “especially for a site occupied for millennia,” he said. Or as Chatters put it, “when there is a variety of stone available in an area, as it is in the Zacatecas valley where Chiquihuite Cave occurs, people will leave behind examples of that variety in their living sites.”
The scientists also believe it unlikely that this population would leave no genetic evidence behind. The “likelihood of human populations persisting for many thousands of years, even overlapping with Clovis in the region for over 1,000 years, yet then leaving no genetic trace, is vanishingly small,” said Potter.
Anna Marie Prentiss, an anthropologist at the University of Montana and a co-author of the critique, said the Ardelean team used an “interpretative language” that ascribed cultural meaning to the cave objects, without considering alternatives. By doing so, the team avoided having to confront the possibility that these objects might have formed through geological processes, she said.
“Thus, Ardelean et al. describe the objects as ‘artifacts’ as opposed to the non-inferential, ‘clasts’,” Prentiss wrote in an email. “They note ‘fine percussion retouch’ on items that are better described as simply having marginal flake detachments...and [they] discuss ‘point preforms,’ a highly interpretive construct for angular clasts with snapped lateral margins,” she wrote, adding that: “Language makes a big difference and we hope our critique leads to consideration of such issues in future research.”
In their response, the Ardelean team said “Chatters et al., misunderstood our evidence,” and “they failed to recognize human-made stone items in the illustrations, as well as the concise descriptions we provided in our paper, of an assemblage whose traits would not occur naturally and under the circumstances alleged by our critics.” At the same time, the team note that the research was “preliminary” and that “further data” will help to “support our claims.”
The researchers clearly agree to disagree, but the good news is that more evidence is apparently forthcoming. The 2020 paper included results from the 2016-2017 excavation seasons, but the team performed more work at the cave in 2019. A subsequent study, delayed by the covid-19 pandemic, “will provide more in-depth assessments of the site and will allow readers to better evaluate the human involvement,” according to the Ardelean response.
We’re very much looking forward to this follow-up research, given the serious implications of the original paper. It may be that humans migrated into the Americas much earlier than the old Clovis-first hypothesis claims, but archaeologists are still searching for the slam-dunk evidence.