In 1992, Thomas Deméré, a paleontologist at the San Diego Natural History Museum, and several of his colleagues were called in to inspect an array of bones that had been unearthed by highway workers building Route 54, just south of the city. What turned up was the Pleistocene. The site was rich with fossils tens of thousands of years old, including the remains of a camel, a horse, a dire wolf, a ground sloth, and, most impressive, a mastodon. Today, the area stretches with ranch homes and water-restricted lawns; way back then, it was a broad floodplain with a single shallow ribbon of water winding through it. “It was a very nice place to live, I’d think, not far from the coastline,” Deméré said at a press conference yesterday.

For years now, Deméré and his collaborators have been studying specimens from the site with mounting astonishment. In 2014, James Paces, a research geologist with the U.S. Geological Survey, determined that the mastodon remains were a hundred and thirty thousand years old. On its own, this fact would not be so surprising, but a recent examination of the animal’s bones suggests that they were smashed open with large cobbles, most likely for the marrow inside. What seemed at first to be a paleontological site, in other words, could be an archeological one. If that’s the case, then it predates some of the earliest evidence of hominins in North America by more than a hundred thousand years. Eleven scientists, including Deméré and Paces, joined to publish their results today in the journal Nature.

Unbroken mastodon ribs and vertebrae. PHOTOGRAPH COURTESY SAN DIEGO NATURAL HISTORY MUSEUM PHOTOGRAPH COURTESY SAN DIEGO NATURAL HISTORY MUSEUM

“The scholarship over the earliest occupation of America is a battlefield,” John McNabb, an archeologist at the University of Southampton, said by phone. (Nature asked McNabb to independently comment on the paper and the team’s results.) “But those folks are arguing about differences of two hundred years here and there. This is an order of magnitude beyond anything that’s been talked about before.” McNabb, who characterized himself as “skeptical,” predicted that the announcement would be met by “an uproar—outrage, anger, utter dismissal.” Still, he said, “If it is true, it changes everything about the occupation of the Americas and the story of the movement of people out of Africa.”

Deméré, at the press conference, conceded that “extraordinary claims require extraordinary evidence” and argued that the study “provides conclusive evidence that this is an archeological site.” The sediment in which the remains rested, a fine-grained silt, was laid down gradually by a slow-moving stream, which would not have been powerful enough to shift big stones or rearrange large bones. The cobbles that served as hammerstones and anvils, then, could not have entered the layer by natural means. And the bones and rocks showed cracks, scars, flakes, and abrasions typical of repeated battering. Careful comparison with bones from other sites, the authors suggested, ruled out damage inflicted by trampling or the gnawing of carnivores.

Using rocks to break apart large bones, either for the marrow or to create hardy tools, is an old hominin trick, dating back at least a million years in Africa. To make their case, Deméré and his colleagues tried their hand at it. In one experiment, they attached a rock to a long wooden handle and used it to break apart the bones of a dead elephant that they had been allowed to dig up. (“It was extremely fresh and smelled extremely bad,” Deméré said.) The fragments and fracturing that resulted were virtually identical to what they saw on the mastodon bones in San Diego. They conducted a similar experiment in Tanzania on the femur of an elephant that had died along a road. The video of the activity, with two Maasai men looking on as two white scientists hammer away with a rock club, resembles a fun-house version of a nature documentary.

The scientists I spoke to who were not part of the Nature team were quick to praise the authors’ credibility and the quality of their research. “They’ve taken the best body of evidence and explored it in a very logical manner,” McNabb said. “They have tried to falsify all the possible natural explanations and come to a point where they can’t see any other interpretation than human habitation. It’s just a question of whether they really have excluded everything possible.” But, he added, with “something as big and important as this, you need absolutely incontrovertible evidence, and there’s too much here associated with a question mark.” For instance, he found it strange that the hammerstones and anvils weren’t accompanied by the smaller, sharper stone tools also typically found at prehistoric butchery sites. “Disarticulating the leg of an elephant isn’t easy,” he said. “If you’re going to break into the bone, you have to cut through the hide and flesh first.”

Ariane Burke, an archeozoologist at the University of Montreal, said she was also struck by the absence of knapped stone tools. (In January, Burke co-authored a study suggesting that the first North Americans arrived from Siberia about twenty-four thousand years ago, some ten thousand years earlier than the previously accepted date.) She added that she’d be excited to see the team compare the activity at the mastodon site with sites of a similar age in Eurasia where woolly mammoths were butchered by humans, to see how the physical evidence compares. “I still find the site puzzling—you can say ‘intriguing’ if you don’t want to say ‘puzzling’—for various reasons,” she said. “One, of course, is the date. A hundred and thirty thousand years before present is unprecedented; there’s nothing in North America to compare it to.”

The looming question is who, or what, might have dismembered this mastodon so long ago. “The simple answer is, of course, we don’t know,” Deméré said. The researchers found no human remains at the site. The generally accepted theory for the settlement of North America has modern humans, Homo sapiens, entering across a land bridge connecting Siberia and Alaska between about twenty-seven thousand and eleven thousand years ago. But the bridge was ephemeral; around the time the mastodon’s bones were smashed, it would have been underwater, as a warm climate melted glaciers and raised sea levels. Back then, humans were still a fairly new and remote species. Homo sapiens were believed to have left Africa in several waves, the first no earlier than a hundred and thirty thousand years ago. They made it to China between eighty thousand and a hundred and twenty thousand years ago and to Australia by fifty thousand years ago; a later wave out of Africa reached Western Europe by about forty-two thousand years ago. For humans proper, North America would have been a bridge too far, too fast.

A boulder thought to have been used by early humans as a hammerstone. PHOTOGRAPH BY THOMAS DEMÉRÉ / SAN DIEGO NATURAL HISTORY MUSEUM PHOTOGRAPH BY THOMAS DEMÉRÉ / SAN DIEGO NATURAL HISTORY MUSEUM

But earlier members of the genus Homo, including the species that eventually evolved into Neanderthals, had already left Africa and populated Eurasia. At the press conference, Deméré speculated that they or another species of Homo might have entered North America, if not by land bridge then by boat. Burke doesn’t think the existing evidence is compelling enough; there is no genetic indication that Neanderthals ever reached this continent and nothing to suggest that they were sailors. “I would not push that boat out into the water,” she said with a laugh. Deméré also mentioned the Denisovans, a cryptic hominin species that inhabited Western Siberia sixty or seventy thousand years ago. “It’s premature to start invoking the Denisovans given how little we know about them,” Burke cautioned.

And if one or another of these hominins did make it to San Diego, what happened to them? Did they come this far only to fade away, like so many ill-fated colonists? “We can become locally extinct,” Deméré said. Even so, he added, there may well be more evidence out there to find. He encouraged archeologists to look in older deposits than they typically explore, and for paleontologists to reëxamine museum collections that might contain evidence of hominin—or human—activity after all. “I like to say that a lot of this evidence has fallen into the academic cracks between archeology and paleontology,” Deméré said. Another possibility is that there is nothing to be found. How do you prove that something doesn’t exist? As Burke put it, “The believers will continue to believe, the disbelievers will continue to disbelieve, and the rest of us will sit in the middle and say, ‘We’d like to see more data.’ ”