Could these mysterious mastodon bones rewrite the history of the Americas?

American history may have begun more than 100,000 years earlier than previously thought.

At least that's what a team of scientists suggest in a paper published Wednesday in the journal Nature.

The paper's authors point to an assemblage of broken mastodon bones and chipped rocks unearthed in southern California as evidence that a stone tool-wielding people snacked on the meat and marrow, or perhaps shaped tools out of the massive animal's skeleton, when it died some 130,000 years ago.

Such a megafauna-human interaction from that period wouldn't have been shocking to find almost anywhere else in the world, as various archaic human species had already spread across much of the globe. But humans are thought to have first settled the Americas around 15,000 years ago, give or take a thousand years, not 100,000.

Rewriting history is not an easy thing to do. The researchers' findings have been met with widespread skepticism, highlighting just how hard it is to reframe historical narratives.

"It's an extraordinary claim. It would rewrite the prehistory of the Americas, and the prehistory of human migrations around the world," says Jon Erlandson, an archaeologist at the University of Oregon. Still, he says, "Extraordinary claims require extraordinary evidence, and I didn't find it here."

But Thomas Deméré, a paleontologist at the San Diego Natural History Museum and one of the paper's authors, disagrees. "Of course extraordinary claims like this require extraordinary evidence and we feel that [this site] preserves such evidence," he said in a press conference.

Skeptics largely suggest that the evidence for a hominin presence could too easily be explained away. For example, the authors point to spiral fractures in the bones as being key evidence of a hominin smashing the bones with hammerstones, which matches behavior thought to be associated with prehistoric humans in Africa at the time, and even tried smashing elephant bones themselves as a proxy. But Joseph Ferraro, an anthropologist at Baylor University who studies archaeological and paleontological materials across humanity’s history in East Africa, suggests that there may be another explanation.

The research team ruled out another carnivore chewing or bashing the bones, but Dr. Ferraro says that proboscideans, a group that includes elephants, mammoths, mastodons, and other tusked megafauna, are known to have tussled, using their tusks and whacking each other's flanks. "It's not uncommon to get broken ribs, not uncommon to get broken legs, and so forth," he says. "That could easily result in a fracture, and if it results in the death of an individual, there's not going to be any signs of any healing," much like the breaks found on the mastodon that is the focus of this study.

"You can spin so many different equally or more plausible stories about how and why this assemblage formed, without having to invoke any sort of hominin activity whatsoever," Ferraro says.

So just what would it take for this discovery to revise the prehistory of the Americas?

Although cutmarks and flaked stone tools would make this site more compelling, Ferraro says, all that is really needed would be one human fossil. If you had an unquestionably well-dated Homo erectus, or Neanderthal, or Denisovan, or even Homo sapiens bone, he says, then the prehistory books would certainly need to be rewritten. But, he says, "This is not that."

The prehistory of the Americas has actually been rewritten before. For decades, archaeologists thought they knew exactly how and when humans first spread across the Americas.

The story, called the Clovis-first model, had the Clovis people as the first population to spread south into the Americas from the region near the Bering land bridge when an ice-free corridor opened up through the middle of Canada, around 13,500 years ago at the earliest. As this model reigned, older archaeological sites, like an underwater 14,500-year-old site in Florida or a 15,000-year-old site in Chile, were dismissed as insufficient evidence. The thinking was that anything dating before the distinctive Clovis spearpoints showed up in the archaeological record couldn't possibly be evidence of a human presence.

But as fresh evidence poured in from sites across the Americas, including genetic analysis, the Clovis-first model was eventually discarded and the history books were rewritten.

Tom Dillehay, an anthropologist at Vanderbilt University, helped lead efforts countering the Clovis-first narrative through his work at the Monte Verde archaeological site in Chile. But, he says, although the San Diego site is a "classic early site" made up of bones and stones, the Monte Verde site also had other evidence pointing to a human presence, such as burned wood, knotted reeds, chunks of hide and meat, and even footprints.

Dr. Dillehay advises that it's best to try to disprove any potentially history-shattering claims, rather than trying to prove them, saying it's a stronger way to rule out all the other possible explanations for the evidence.

In the case of debunking the Clovis-first model, more archaeological sites bolstered the claim, and the same could help support Deméré and his colleagues' claim, too.

There have been previous suggestions of such shockingly early human occupation of the Americas, similar to the current claim, Dr. Erlandson says. Items suggested to be artifacts of particularly ancient human settlements have been described from other sites in southern California, for example. But when this was proposed before, scientists went out looking for more evidence, Erlandson says, "And they never came up with anything convincing."

Erlandson himself looked for evidence of human-caused fire, but was unable to find evidence that old scorched materials were the result of anything other than wildfires.

Still, Steven Holen, lead author on the new paper, said in the press conference that he has already been looking for similar fractures in megafauna bones, which may have been overlooked by paleontologists who wouldn't have even considered a human impact at the time. Dr. Holen says evidence may have fallen through the cracks between archaeology and paleontology, as archaeologists wouldn't have been looking at materials this old before and paleontologists wouldn't have been considering a human factor when they examined the bones.

But Ferraro says such an assertion isn't giving the experts enough credit. "There's a big literature out there on bone damage," he says. Paleontologists who devote their lives to studying bone damage can even identify something as specific as which species of termite once munched an old bone, he says, so he suspects paleontologists wouldn't have missed something as significant as evidence of human activity.

Skeptics are also concerned about the bigger-picture implications of shifting the story of human occupation in the Americas so dramatically.

"As scientists we're supposed to keep an open mind, but this discovery is hard to wrap my mind around because it falls so far beyond the realm of accepted knowledge," Erlandson says. "I'm not opposed to controversial theories," he says, "but if it's really 130,000 years ago, it just raises so many questions": for example, who those people were, where they came from, how they got there, and what happened in the subsequent 100,000 years.

To answer that last question, the authors did suggest in the press conference that, like any other population of animals, this group of humans may have died out and therefore not left a trace in the years before the ancestors of today's Native Americans trekked across the land bridge from Siberia and spread across the region.

Filling in the other gaps of the background story implied by Deméré, Holen, and colleagues' claim would require other extraordinary claims, Ferraro says. To explain how humans got to southern California would require a scenario such as one in which Homo erectus, Denisovans, or another archaic human species would have had to have been making boats in Siberia and following the coastline east, then down the western coast of the Americas, for example.

And each detail needed to support such a tale, from whether they possessed boating technology to which archaic human species made the journey, would be an additional extraordinary claim in its own right, he says, which would in turn require its own set of extraordinary evidence.

"It just requires so many individual extraordinary claims," Ferraro says. "It's not just one claim, but the whole argument is resting on a very shaky foundation."

Perhaps eventually the prehistory books of the Americas will need to be revised to include a human presence 130,000 years ago, but first, Dillehay says, all other possible explanations need to be ruled out. "In other words," he says, the question must be asked: "Are we being fooled in this case once again?"