WERE PEOPLE OCCUPYING A CAVE IN MEXICO 30,000 YEARS AGO?

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. In recent years 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.

Coxcatlan Cave and its location--Credit Andrew Somerville/ISU/Latin American Antiquity

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 (UNAM*) and Joaquin Arroyo-Cabrales (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 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. 

Since respected previous researchers had found that those deep 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 signs of heating and butchering, 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 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 Somerville and his colleagues 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 plans. 

"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."

Somerville 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 equally highly respected 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 that its 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.

REA

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You can access the original, open-source Latin American Antiquity article reporting these findings here

*Institute of Physics, National Autonomous University of Mexico

**Subdirectory of Laboratories and Academic Support, National Institute of of Anthropology and History

FIRST AMERICANS MAY HAVE ARRIVED 130,000 YEARS AGO--TEN TIMES EARLIER THAN WE THOUGHT

Most archaeologists believe that the first Americans arrived no more than 14,500 years ago, as the waning ice age opened up overland and coastal routes from Eurasia into North and South America.

It's true that over the years,  researchers have excavated multiple sites from Pennsylvania to Brazil that have pointed to an earlier human presence in the New World, perhaps 20,000 to 50,000 years ago.  But all of those finds have drawn intense criticism by mainstream archaeologists, and have been effectively sidelined.

Today however, a team of paleontologists, geologists and archaeologists writing in the prestigious journal Nature, present evidence pushing the first human presence in the Americas back by close to a factor of ten, to between 120,000 and 140,000 years ago.

"This discovery is rewriting our understanding of when humans reached the New World," says  Judy Gradwohl, President and CEO of the San Diego Natural History Museum, whose team of paleontologists led by Richard Cerutti first studied the site, located along the route of a freeway in southwest San Diego, California.

"It's a very urban area now," says Thomas Deméré, also at the Natural History Museum, "but 130,000 years ago it was a meandering stream close to sea level, with camels, horses, ground sloths, capybara and deer--probably a very nice place to live."

130,000 year old mastodon bones

and possible stone tools, in situ

Credit: Holen et al./Nature

The core of the evidence consists of two clusters of fossilized teeth and bones of a single mastodon that, according to the researchers, show clear signs of having been smashed open by humans using stones as hammers and anvils. In addition to the shattered bones and teeth, the "smoking guns" are five hefty stones found among the bones--hammerstones that still bear impact marks from their use at the site, with flakes knocked off by the blows nearby. Tellingly, the mastodon's ribs, which would have been broken by geological forces, are still intact, while the extremely tough, marrow-containing long bones were cracked open.

Possible hammerstone wielded 130,000 years ago

Credit: Holen, et al./Nature

To verify that the marks they found on the stones and bones were caused by humans rather than by animals or geological processes, the team used similar stone hammers and anvils to smash open fresh elephant bones. "We found the same fracture patterns," said Steven Holen, a paleontologist at the San Diego Natural History Museum and the paper's lead author. "It reproduced what we saw at the Cerutti site." [Note: the researchers experimented on the bones of already-deceased elephants; no animals were harmed in their experiments.]

The research team used state-of-the-art uranium-thorium dating to determine that the bones were buried 131,000 plus-or-minus 9,400 years ago. This high-tech technique was what finally allowed them to definitively date the bones.

Since anatomically modern humans are not thought to have left Africa until around 100,000 years ago, just who these extremely early Americans might have been remains an open question. The authors speculate that they may have been a far-flung branch of Homo erectus, the mysterious Denisovians, or even Neanderthals. "The simple answer is that we don't know," says Richard Fullagar, at the University of Wollongong, in New South Wales, Australia.

Still, he adds, assemblies of stone hammers and anvils along with similarly smashed bones occur in archaeological sites in Africa dating back up to 1.5 million years, and in Europe to 350,000 years ago. Cracking open the long bones of large animals to extract marrow or make tools has been part of the repertoire of many human ancestors.

Although the research team carried out an extremely careful and detailed study of this site and its artifacts--a 24-year research odyssey that has been validated by the publication of their work in Nature--only time will tell if their conclusions will hold up in the face of the withering scrutiny they will no doubt encounter. In science, extraordinary new understandings require extraordinary proof, and setting back the date when the first humans reached the Americas by a staggering 115,000 years is certainly extraordinary.

The researchers know that their groundbreaking findings will be highly controversial, but believe that they have dotted every i and crossed every t. "It's taken a very long time to get to this stage," says Fullagar. But now, he says, "we have enormous confidence in the evidence we've put together. It's truly incontrovertible."

Incontrovertible or not, it's already eliciting great skepticism on the part of many archaeologists, for whom one isolated site with no actual human remains simply isn't sufficient to overturn the current 15,000-year-old first entry date. However, a study published just one day later in the equally prestigious journal Science, may offer the possibility of a definitive proof.

A team of geneticists, archaeologists and paleontologists at the Max Planck Institute of Evolutionary Anthropology, in Leipzig, Germany report being able to recover Pleistocene-era Neanderthal and Denisovian DNA, along with DNA from various animals, from the sediments of caves occupied or visited by ancient humans, even though no bones or other human remains were found. I won't go into more detail here, since this story has been heavily covered elsewhere. You can see what the New York Times had to say about it here.

If the same technique can be used on materials from the Cerutti site, it could pin down a revolutionary claim that otherwise is likely to remain at best controversial.

The Nature article by Steven Holen and colleagues can be found here.

You can view a YouTube video about this discovery at this URL.

And click here for video of the team's elephant-bone-breaking experiment.

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