In an announcement sure to spark a firestorm of controversy, researchers say they’ve found signs of ancient humans in California between 120,000 and 140,000 years ago—more than a hundred thousand years before humans were thought to exist anywhere in the Americas.
If the researchers are right, the so-called Cerutti mastodon site could force a rewrite of the story of humankind.
“I realise that 130,000 years is a really old date and makes our site the oldest archaeological site in the Americas,” says study leader Tom Deméré, the palaeontologist at the San Diego Natural History Museum, whose team describes their analysis today in Nature. “Of course, extraordinary claims like this require extraordinary evidence, and we feel like the Cerutti mastodon site presents this evidence.”
To be clear, the team has not found human bones at the site. But as Deméré and their colleagues tell it, their evidence—a mastodon skeleton, bone flakes, and several large stones—shows that the area was a “bone quarry,” where an unknown hominin allegedly smashed fresh mastodon bones with stone hammers, perhaps to extract marrow or to mine the skeleton for raw materials.
However, many of the world’s leading experts in American archaeology already have expressed some form of scepticism to the paper’s claims. Some have rejected it outright.
“The earliest occupation of the Americas is a highly contentious subject,” says University of Southampton archaeologist John McNabb. “The date of the find at 130,000 years ago is a really big ask for archaeologists who are used to talking about 12, 13, 14,000 years ago. It’s a big, big time difference.”
Here's everything you need to know about the discovery and the reactions of other archaeology experts.
Where is the mastodon site, and how was it found?
The site was discovered in 1992 when an excavator dug up large bone fragments as California’s State Route 54 was being built through San Diego County.
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The California Department of Transportation requires that palaeontologists be on call at major excavations, just in case any fossils pop up amid the carnage. Richard Cerutti, the San Diego Natural History Museum field palaeontologist who was on call at the time, flagged the site for follow-up.
He and Deméré then blocked off the area and excavated it, digging down to material undisturbed by the heavy machinery overhead. (Deméré received a grant from the National Geographic Society in January 1993 to support emergency excavations of the site.)
The site struck Deméré as unusual from the beginning, given the large rocks associated with the site, apparent concentrations of bone flakes around the largest stones, and the mastodon remains’ odd arrangement. One of its tusks, for example, was embedded vertically in the dirt.
Wait, the site was found in 1992 and excavated in 1993 ... why is it only being announced now?
The main delay came from the sheer difficulty in accurately dating the site. It wasn’t until 2011 and 2012 that Jim Paces of the U.S. Geological Survey could provide state-of-the-art ages for the mastodon bones, based on the relative amounts of uranium and thorium within them.
Deméré, a palaeontologist by training, also had a lot of other projects on his plate. After writing a brief report on the site in 1995 for the California Department of Transportation, he turned his attention to his other research projects, including the evolution of baleen whales.
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In the meantime, a small band of online commenters had seized upon his 1995 report as a sign that evidence on the peopling of the Americas was being “suppressed.” Deméré appears surprised and confused by the charge. “Nobody ever said, Forget this ever happened,” he says. “That’s just baloney.”
OK. What’s the new evidence being presented?
To start, the wear features on the rocks match what one would expect from stone tools, specifically those used for smashing up bones, says research team member Richard Fullagar, an archaeologist at Australia’s University of Wollongong.
There’s also the matter of the stones’ placement. The site was entombed in siltstone, a type of sedimentary rock that forms from fine-grained sediments—the sort that would settle out only in very slow-moving, low-energy water. But the large stones that appear to be rudimentary tools are far heavier than the surrounding particles. One is roughly 30 pounds. If water didn’t move the rocks there, then perhaps people carried them to the site.
In addition, fractures on the mastodon bones suggest they were broken while fresh—and the researchers say they couldn’t have been smashed by natural processes. The skeleton likely were not trampled by other large creatures, the team argues since some of the mastodon’s more fragile bones—such as its ribs and vertebrae—are less shattered than the sturdy limb bones. Nor could smaller animals have done it, the team argues because scavenging carnivores can’t chew their way through the middle of a fresh mastodon femur.
The team also provided experimental evidence that hammerstones and anvils make similar fracture patterns in fresh elephant bones, suggesting by analogy that such a process could have been at work at the Cerutti site.
Sounds intriguing! Why are other experts so sceptical?
The Cerutti site also revealed unbroken mastodon ribs and vertebrae.
PHOTOGRAPH BY SAN DIEGO NATURAL HISTORY MUSEUM
One of the main critiques is that the study doesn’t definitively rule out natural causes for the presence of the purported stone tools, the breakage patterns in the mastodon bones, or the patterns of breakage and wear on their surfaces.
For one, the paper doesn’t satisfyingly rule out the possibility that natural processes carried the large rocks to the scene, says Vanderbilt University archaeologist Tom Dillehay. Nor does it fully rule out the possibility that the wear patterns on the stones were a result of rocks bumping against one another in a stream, he says.
“When you put the total package together, there’s certainly more evidence to reject [the study] than accept it,” Dillehay says.
It’s not impossible that human history in the Americas is older than currently thought, says Southern Methodist University archaeologist and National Geographic grantee David Meltzer, an expert on early Americans.
“But to prove it, you cannot take broken bones and nondescript stones to make the case, not without demonstrating that nature could not have broken those bones and modified those stones,” he says.
Andy Hemmings, the lead archaeologist at a site of ancient human habitation in Florida called Old Vero, agrees. “We just have to accept that the stones were carried in, [but] from where, and by whom? These are things I’d like to see answered before I’d be willing to put much faith in the conclusions.”
Are there any other concerns about the tools?
Archaeologists also take issue with the stone tools that aren’t there. Usually, hammer-and-anvil sites also come with lithics, flaked stone tools and the debris from their manufacture and use, notes Jim Adovasio, the Florida Atlantic University archaeologist who excavated Meadowcroft Rockshelter, one of North America’s oldest archaeological sites.
These types of tools are missing entirely from the Cerutti site, even though it supposedly dates to a time when hominins were perfectly capable of making sophisticated hand axes.
“They make a statement that the [evidence at Cerutti] is consistent with many other sites,” says Adovasio. “Well, I’m sorry, it’s not—that just isn’t simply true.”
Steve Holen of the Center for American Paleolithic Research, one of the research team’s archaeologists, rebuts the charge, saying that there’s evidence for archaeological sites in the Americas that don’t have flaked stone tools. For the last 25 years, Holen has studied two sites in Kansas and Nebraska that are about 14,000 to 33,000 years old. He claims these sites are also bone quarries where humans did not use flaked stone tools, much like the Cerutti site.
Hemmings, however, isn’t convinced that the evidence uniformly supports the idea that humans at Cerutti were trying to use the mastodon bones as tools. In particular, one of the mastodon’s teeth is shattered for no obvious reason.
San Diego Natural History Museum palaeontologist Don Swanson points at a rock fragment near a large mastodon tusk fragment.
PHOTOGRAPH BY SAN DIEGO NATURAL HISTORY MUSEUM
“Everything that’s broken was still there, so it wasn’t mined for tools, and you’re certainly not getting marrow out of the bone of a mastodon tooth,” he says. “So what exactly is supposed to have gone on?”
If the stones are tools, though, does that cinch the case for humans?
Not necessarily. The human line doesn’t have a monopoly on tool use, after all. For at least 4,000 years, chimpanzees in Côte d'Ivoire have been cracking nuts with stone hammers. And in Brazil, bearded capuchin monkeys have smashed cashews with rocks for at least a hundred generations.
However, the fossil record of the Americas lacks a marrow-munching, non-human primate at 130,000 years ago. One of the site’s rocks is also nearly 30 pounds—far heavier than the rocks Brazil’s capuchins wield. In addition, “capuchins are too small to generate the kinetic force needed to crack a mastodon bone,” says the University of Georgia’s Dorothy Fragaszy, a National Geographic Explorer who studies capuchin tool use. “I agree with the authors that, if these are hammer stones, humans used them.”
Michael Haslam, an Oxford archaeologist who studies tool use in non-human primates, agrees. “I think that the evidence presented in this paper backs up the authors’ claim that a mastodon has been broken apart using stone tools,” he says. “Overall, I think that we need to consider humans as the starting hypothesis for this site, and go from there.”
Well, that sounds more promising! So if the authors are right, is it possible I’m related to these ancient mastodon miners?
Maybe, although even if the interpretation holds up, it’s unlikely that the tool bearers were anatomically modern humans. At present, there’s no evidence that Homo sapiens sapiens left Africa earlier than 120,000 years ago. But at least four sister species were living in East Asia around the time, and three would be contenders for crossing into the Americas. (The fourth is Homo floresiensis, the much-ballyhooed “hobbits” of Indonesia’s island of Flores—but they’re probably not involved.)
Might the tool users have been Homo erectus, our direct ancestors and the earliest known fire-starters? What about Homo neanderthalensis, which had made it to present-day Kazakhstan around the time of the activity at Cerutti? Or could they have been the Denisovans, the enigmatic East Asian group known from DNA samples collected in a single Russian cave?
At present, it’s impossible to say.
How would any of these species have made the crossing?
The answers vary, since the span of possible ages for the site—120,000 to 140,000 years ago—cover the beginning of the last interglacial, the warm period before the last ice age.
If the site is 140,000 years old, then it’s possible that any tool-wielding hominins could have come to the Americas via Beringia, the land bridge that once connected Siberia and Alaska, says McNabb. But if it’s 120,000 years old—well into the last interglacial—then sea levels would have been much higher. Any would-be migrants would be facing a sea crossing at least 50 miles long.
By 130,000 years ago, hominins undoubtedly figured out how to cross open water. A site in Crete called Mochlos bears stone tools that could be about 130,000 years old, but Crete was never connected to the Greek mainland by a land bridge.
But at present, there’s no solid evidence that hominins had made it into northeastern Siberia before about 30,000 years ago—much less any evidence that they floated across the Bering Strait a hundred thousand years before that.
A close-up view of a spirally fractured mastodon femur bone from the site.
PHOTOGRAPH BY TOM DEMÉRÉ, SAN DIEGO NATURAL HISTORY MUSEUM
“I think a sea crossing to the Americas with a much higher sea level is a much more difficult proposition,” says McNabb.
Have claims for sites this early been made before?
For the most part, American archaeology has been caught in a bitter debate over whether humans arrived a couple thousand years earlier than previously thought—not a hundred thousand years before.
“The earliest occupation of the Americas is a highly contentious subject,” says McNabb.
For decades, convention had held that around 13,000 years ago, an ice-free corridor opened up from Alaska to Montana, providing passage to a group of humans who had trekked across Beringia. Within 500 years, these migrants were thought to have successfully peopled the Americas—leaving behind a type of fluted stone point that became a calling card of sort for the Clovis culture.
But beginning in the 1970s, archaeologists—including Adovasio and Dillehay—began to find evidence of sites that predated the Clovis culture, such as Pennsylvania’s Meadowcroft Rockshelter and Chile’s Monte Verde, excavations that the National Geographic Society helped fund.
A brutal—and sometimes deeply personal—debate over the legitimacy of these “pre-Clovis” sites ensued. In the case of Monte Verde, it took Dillehay nearly two decades to convince many of his peers that the site predates the Clovis culture. Some Clovis-first holdouts still don’t buy it.
Dramatically extending the timeline for humans in the Americas makes the Cerutti claim even more contentious. And few previous claims of sites this much older have survived the academic wringer.
In the 1960s, for instance, famed archaeologist Louis Leakey received several National Geographic Society grants to fund excavations at Calico Hills, a site in California that had been rumoured to contain stone tools at least a hundred thousand years old.
Unfortunately for Leakey, the site’s particular geology produced stone flakes that looked a lot like tools—making it all but impossible to tell the difference between natural and human-made objects. Modern consensus is that the stone “tools” are naturally occurring.
“Nobody’s gone back and seriously touched that site at all,” says Dillehay. “Leakey was burned, [and] the people associated with the site were burned, so to speak. These things happen.”
Can the researchers do anything else to back up their claim?
Yes. Deméré and his colleagues are currently examining the stone tools from the site for protein residues. If they really were used to smash the mastodon bone, microscopic bits of mastodon likely would have been ground into the rocks’ nooks and crannies.
The researchers also say they’re interested in re-excavating the site, part of which remains in an earthen dam in San Diego County intended to dull the noise from California’s State Route 54. In the meantime, the authors say that they’re prepared to defend their analysis from the years, if not decades, of fierce criticism ahead of it.
“I think we’ve made a very good case that this is an archaeological site,” says Holen. “I think we’re quite prepared for the firestorm that’s coming.”
“This is a hypothesis that begs for careful scrutiny and attempts to falsify it; I’m open to that,” adds Deméré. “That’s the way science should work, right? Bring it on.”
Header Image: Two mastodon femur balls, one face up and one face down, are among the remains found at the Cerutti site in San Diego. PHOTOGRAPH BY SAN DIEGO NATURAL HISTORY MUSEUM