Is the conventional chronology of human migration little more than a house of cards? Maybe. And there’s a strong wind (or at least a tantalizing breeze) blowing in from southern California, where researchers say they have evidence that the First Americans may have arrived on the continent almost ten times earlier than we thought. And here’s another kicker: the first humans in the Americas may not have been Homo sapiens.

The results, published today in Nature, came out of several different lines of inquiry, all leading to the same stunning conclusion: A partial mastodon skeleton unearthed near San Diego appears to have been processed by some kind of hominin about 130,000 years ago.

To put this in context, right now the generally accepted arrival date for humans in the Americas — from Siberia, via the land bridge Beringia — is a mere 15,000 years ago. There have been a handful of sites from Brazil and Chile to the Great Plains of the U.S. suggesting human activity up to 40,000 years ago, but academic opinion on the legitimacy of those sites is deeply divided.

This is the kind of brainshock that, once you shake off the initial surprise, questions go stampeding through your head like a bunch of spooked mastodons. So let’s take it one step at a time.

The Backstory and The Bones

The site in question, along State Route 54 in San Diego County, was discovered during highway construction in 1992. Known as the Cerutti Mastodon Site, the area was first identified as paleontologically significant when an excavator unearthed partial animal remains in an upper layer.

Construction halted, paleontologists came in and started digging down. And, more than three meters below the surface, they found a partial mastodon skeleton in a much older layer. And that’s when it gets weird.

The mastodon remains were laid out in a way that would not make sense if the animal just dropped dead, was brought down by a predator and/or later scavenged. The bones appeared to be arranged in clusters. For example, both heads of the femurs, long bones in the leg, were sitting side by side. One of the tusks was vertical, penetrating the older sedimentary layers as if someone had stuck it upright in the ground. That stuff just doesn’t happen in nature.

What’s more, several of the bones were broken. Specifically, they were spirally fractured. This is a kind of break that would only happen with fresh bone when it’s struck a certain way. Also, some of the breaks occurred in big, robust, sturdy bones not usually busted up during the natural process of being buried and fossilized.

The kinds of breakage found were not consistent with a carnivore munching on the bones, or with a herd of heavy beasties trampling the unfortunate mastodon, alive or dead.

The team, which has grown over the years as the initial researchers added colleagues from other disciplines, claims that the damage to the bones is the same as breakage found in 1.5 million-year-old elephant bones in Africa that were processed by hominins.

Some of the researchers also traveled to Tanzania to conduct experiments on the bones of a recently deceased elephant and found the breaks in both the Cerutti Mastodon and the modern elephant were identical.

The researchers took years to analyze the bones, date the samples and surrounding rocks and replicate the damage, one big reason that the results are coming out a quarter-century after the Cerutti Mastodon was unearthed.

A Rock In A Hard Place

Then there are the rocks…specifically, the cobbles. Five large stones that the authors believe were used as hammerstones and anvils to process the carcass. The presence of these stones in Bone Bed E, where the mastodon remains were deposited, is unexpected because the layer is fine-grained silt. It’s what geologists and paleontologists consider a “low energy deposition” environment. There was no raging river here when the mastodon met his end, just a gentle stream.

One of the rocks from the Cerutti Mastodon site that researchers believe hominins used to process the carcass.
(Credit Tom Deméré, San Diego Natural History Museum)

That means we don’t have a natural explanation for the big rocks being here — no rapids would have washed them here, and there is no evidence of other geological forces, such as landslides, that might have dropped these large stones into an otherwise silty spot. The only explanation, claim the authors, is that the stones were collected from elsewhere by hominins and brought to the site intentionally to process the animal carcass.

The experiments team members conducted in Tanzania used similar types of cobbles on fresh elephant bone, confirming these kinds of primitive tools could indeed cause the damage found on the mastodon bones. But were they used for that purpose?

During a press conference on Tuesday, I asked co-author Richard Fullagar of Australia’s University of Wollongong whether the team had tested the cobbles for protein residues that might confirm they were used to process the mastodon. He told me they had been looking at that possibility but were not ready to discuss results.

While we wait for those findings, consider that another team recently tested 250,000-year-old stone tools and was able to confirm not just that they were used to process animal carcasses, but even to identify the kinds of animals processed.

Research published today in Nature included not only descriptions of the bones and artifacts associated with the potential carcass processing site, but also information on how the team replicated the damage on modern elephant bones in Tanzania, using tools they believe the alleged First American hominins might have had. (Credit Kate Johnson, San Diego Natural History Museum)

The Dating Game

Of course, as with any find quite so sensational, one of the first questions that must be asked is how good the dating is. According to the researchers, they used multiple methods to date a number of the bone specimens.

Some of those methods didn’t work out. They were unable to get a result using radiocarbon dating (RCD), for example — which actually strengthens their case for the bones being 130,000 years old, because RCD is generally unusable beyond 40,000 years or so.

The researchers ran into a similar dead end using optically stimulated luminescence dating, which can calculate when a sample was last exposed to sunlight — but again, OSL is known to be generally ineffective in samples older than 100,000 years.

The team did, however, have success with Uranium series dating, which is based on the known and stable decay rates of different isotopes of uranium. Using U-series dating, the researchers found the three separate specimens tested all fell into the age of 131,000 years, plus or minus 9,000 years.

Who Dis?

So…the researchers behind today’s study laid out a multidisciplinary case for hominins being in San Diego 130,000 years ago or so, but if you accept that, the big question is, well, which hominin?

Let’s consider the usual suspects, by species:

Anatomically Modern Humans: The conventional (and increasingly iffy) timeline for AMHs leaving Africa is around 100,000 years ago, though each year researchers uncover more evidence that challenges that chronology, such as potential AMH remains in the Middle East that are 115,000 years old and other partial remains from China that may be up to 125,000 years old.

As the first footsteps of our species appear to get pushed further and further back, it’s no longer cray-cray to consider that AMHs were racking up the mileage far from our ancestral African homeland by 130,000 years ago. And that’s assuming we’re all still signed on to the “ancestral African homeland” idea.

Some researchers are pushing for a reassessment of where AMHs evolved, arguing that the species may have arisen out of the intermingling of regionally evolved archaic hominins that were living in East Asia and elsewhere well before the conventional 100,000 year Out of Africa moment.

Neanderthals: Our stocky elder siblings got around, too, as the fossil record from Spain to Siberia shows. And in the past decade, new discoveries have shown that they were remarkably handy and ingenious with tool-making and other skills requiring both brains and brawn.

Denisovans: Another possibility. Though I feel that ever since this mysterious branch of our genus was genetically confirmed at a single site in Siberia in 2010, they’ve become kind of like Mikey. Remember Mikey from the old Life cereal commercials? When in doubt give it to Mikey. When you’re not sure which hominin was involved, consider the Denisovans.

Homo erectus, Homo habilis or some other distant relative: A number of much older H. erectus fossils, such as Java Man, have been found in Indonesia and East Asia. The much-debated Dmanisi hominins, which some researchers suspect are neither H. erectus nor the earlier H. habilis, were already as far north of Africa as western Asia by 1.87 million years ago.

The truth is, there are probably at least a few more (possibly many more) members of our genus out there waiting to be discovered. And it’s possible that one of these early hominins, physically and cognitively capable of making primitive tools, made it as far as San Diego. The land bridge known as Beringia has, after all, connected Siberia with North America on and off for 70 million years during periods of intense glaciation.

In fact, the authors of today’s study pointed out that Beringia was high and dry for some time immediately before 130,000 years ago, after which rising sea levels sunk it for a good long while. So it is possible that one or more of these hominin species took a road trip out of Siberia, crossing into the Americas via Beringia before it was swallowed by the seas (temporarily) 130,000 years ago.

What happened to whoever bashed the Cerutti mastodon? We don’t know. But humans are like any other living organism, lead author Steven Holen pointed out during the press conference: “It’s quite possible early populations came in and didn’t make it.”

UFOs: Seriously. Another reporter broached the idea during Tuesday’s press conference, as a joke, I think (I hope). And while a find that causes this much of a paradigm shift is bound to bring out the wild speculation, no. Just no. Unearth me a well-preserved Stargate from a pristine geological context and we’ll talk but until then, there is no, repeat no evidence of E.T showing up to bash some mastodon bones.

But mere mention of some wild speculation brings me to…

And Now A Word From My Inner Skeptic

Whenever scientists make a discovery, it should be met with skepticism. That’s how the scientific method works and other researchers should feel obligated to try to replicate their colleagues’ results or bust them apart like fresh mastodon bones. The researchers behind today’s paper said they continue to test and analyze the mastodon remains and the cobbles, and that they welcome scrutiny from others in the field.

My two cents on the matter is that I am willing to believe it’s possible, and I think that out of the gate, the team has done a good job of anticipating the doubts raised. Already, questions such as “how do we know the road construction heavy equipment didn’t cause the damage to the bones?” have been raised and answered, usually with multiple lines of evidence.

(Bone Bed E, where the goodies were found, was more than three meters under the surface, insulated from damage by heavy equipment, and you can’t get spiral fractures from heavy equipment rolling over old bones anyway. And oh, by the way, don’t forget the mastodon remains were found in a freaky arrangement that appears intentional, such as one tusk stuck upright and femur heads placed beside each other.)

If protein residue studies can definitively tie the cobbles to the processing of the carcass, that would be a big boost in my confidence about the study. But I want even more. We humans, like all living things, are constantly shedding our DNA everywhere we go. Yucky for fastidious types, but having some ancient DNA, on the cobbles or mastodon bits, that could be tested (and verified as uncontaminated by modern human DNA) would be the cliché holy grail.

Until the team’s results are replicated and additional information added in support of their argument, though, I can’t stop thinking about a 2016 study that showed at least one smashy-bashy group of capuchin monkeys in Brazil busts up rocks to create shapes remarkably similar to human-made stone tools. Those monkeys, by the way, live rather close to two of the highly controversial sites where some researchers assert humans were present 20,000 years ago, based entirely on what they claim are stone tools made by our species. Hmm.

So stay tuned to see whether additional evidence proves this is the archaeological find of the century or just another tantalizing possibility that will fuel debate for decades to come. For more on this potential bombshell in the story of the First Americans, check out Nature‘s video on the research.

More into ancient rock-breaking than all that pesky academic talk? Feast your eyes on a video of the techniques researchers used to re-create the damage seen on the mastodon bones.