There are few scientific questions as controversial as when humans first entered the Americas. For decades widely spread groups using the distinctive Clovis toolkit held that title with a date of first entry around 13,000 years ago. More recently a few hotly contested findings have pushed that date back by a few thousand years--for example there's now generally accepted evidence of humans living at Monte Verde, Chile, near the tip of South America, 14,800 years ago.
Now, new evidence from the depths of a cave near Puebla, Mexico, may double the length of human occupation of the Americas--to 30,000 years or more. If these new dates and the signs of human activity associated with them hold up to the intense scrutiny they are sure to provoke, it will place humans in North America before rather than after the most recent glacial maximum, dramatically re-writing the prehistory of the Americas.
Paleoecologist Andrew Somerville, at Iowa State University in Ames, Iowa, did not set out to make a potentially paradigm-breaking discovery. He and his colleagues, Isabel Casar, at UNAM* and Joaquin Arroyo-Cabrales, at INAH**, just wanted to clarify the dating of the previously excavated and studied Coxcatlan Cave in the Tehuacan Valley of Mexico, with a focus on the development of agriculture.
Earlier researchers had excavated and described 28 distinct levels of deposits in the cave extending over thousands of years, most of which included signs of human habitation. The strata revealed a progression of material culture starting with slightly worked stone blades, scrapers and choppers through the development of carefully crafted stone tools, ceramics, woven materials, grinding stones, and early signs of agriculture. Somerville and his colleagues wanted to confirm or refine the dates of the various levels.
They used accelerator mass spectrometry (ACM) to perform precision carbon-14 dating on deer, hare and rabbit bones from various levels of the cave. While their findings corroborated the dating of the site's more recent levels, they were surprised when bones from the deepest deposits consistently dated to between 33,448 and 28,279 years before the present. Previous researchers had not tried to obtain dates for the three deepest layers they excavated.
Since the previous researchers had found that those deepest levels showed signs of human habitation--for example slightly worked stone tools including flint that had to be imported to the cave, and bones showing possible signs of human modification, Somerville and his colleagues realized that they had stumbled upon a potentially revolutionary find.
"We weren't trying to weigh in on this debate, or even find really old samples," Somerville says. "We were surprised to find these really old dates at the bottom of the cave, and it means we need to take a closer look at the artifacts recovered from those levels."
Somerville is now teaming up with Matthew Hill, also at Iowa State, to re-examine the stones and bones from the deepest levels of the cave to see if they can prove or disprove that they were worked or modified by humans. "Determining whether the stone artifacts were products of human manufacture or it they were just naturally chipped stones would be one way to get to the bottom of this," Somerville says. Similarly, microscopic examination of the bone fragments may reveal proof of heating, or cut marks that could only come from stone tools wielded by human hands.
Unfortunately, the Covid-19 pandemic has so far prevented them from re-examining the bones. They think they will be able to access them during the summer of 2022. While the bones were carefully identified and categorized in the 1960s by Kent Flannery, a highly respected archaeologist, he did not study them at the level of detail Somerville has planned.
"Documenting and quantifying modifications to the bones, such as the patterns of breakage, the presence of cut marks, and thermal alterations to the bones were not part of the original study," Somerville explains. "Our plan is to return to the collection with a digital microscope and to systematically analyze each specimen to document this information. The goal is to ascertain whether or not humans had actually hunted and consumed these animals."
He is also working with the Peabody Museum, in Andover, Massachusetts, to locate and re-study the possible stone tools from the deepest levels of the Coxcatlan Cave.
According to Somerville, Flannery and his colleague, the famed archaeologist Richard S. MacNeish, were fairly sure that the bones and stones from the deepest levels of the cave were left there by humans. As a result, Somerville thinks it's likely that further study with current techniques will confirm that. "It wouldn't surprise me if they were right all along," he writes. "However the possibility remains that these levels were natural accumulations of bones and stones, so I'm remaining skeptical but with an open mind until we have more data."
Somerville and his colleagues are thoroughly aware of how contentious a finding of a 30,000-year-old human presence in the Americas would be, and accordingly have presented their current research very cautiously. "Because we very intentionally tried not to oversell our results, I think we have been able to avoid any serious criticism or backlash so far," he writes.
"So far" are the key words. American archaeology is notorious for the large number of sites at which researchers have found what they thought was evidence of human habitation dating back 20,000 years or more, only to find their work dissected, discounted and eventually dismissed by other researchers.
While we eagerly await the next round of findings from professors Somerville and Hill, it's clear from the history of the field that they will have to produce extremely convincing proof before the needle marking when people first occupied the New World budges from its current position. But if it does swing from after to before the last glacial maximum, it would mean a revolution in our understanding of the peopling of the Americas.
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