Here's what we know: About 130,000 years ago, near modern-day San Diego, something or someone killed a mastodon. Whatever it was bludgeoned the creature's spine and jaw in a calculated fashion and harvested the bones for marrow and tool use. It sure looks like the kind of thing early humans would do.

There's a problem, though. At this time, humans had not left Africa—at least according to today's dominant narrative of human migration. And the earliest migration into North America that we know about occurred around 12,000 to 13,000 years ago. Yet in a paper published today in Nature, scientists put forth the idea that a mastodon was killed and its bone marrow harvested in a matter only possible by humans, in the broad sense of Homo erectus on up to Homo sapiens.

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Flakes of the tool used to harvest the creature were found near the mastodon skeleton site, the researchers say. The bones show a combination of hard bludgeons and milder scrapes, performed in such a way that they were likely the work of something possessing tools. But if this study is right, it could upend we know about early humans in North America.

"It's quite a claim," says Briana Pobriner, a research scientist in the human origins program at the Smithsonian Institute who wasn't involved in this study. "The idea that humans in some form were in North America that early is pretty jaw-dropping, but I'm intrigued by the claims the team projected."

Uranium Dating

The surface of mastodon bone showing half impact notch on a segment of femur. Tom Deméré, San Diego Natural History Museum

Because the collagen in the bones was too decayed for radiocarbon dating, the researchers estimated the age of the mastodon bones and tools using uranium dating. Luminescence dating had showen the site had to be at least 60,000 to 70,000 years old. The uranium dating put that number at a firmer estimate of 130,000 years, with a 9,000-year margin of error.

The markings on the bone don't correspond to any known carnivore indigenous to North America. Those carnivores certainly didn't bludgeon their prey in the fashion seen on the skeleton, which was repeatedly replicated by the team for the study. "I think they did their homework pretty well in replicative experiments," Pobriner says.

"If it does turn out to be true, it changes absolutely everything."

Because the finding is so unusual, people have tossed out alternative explanations for the bone markings such as that they were cause by construction equipment in the area. Steven Holen, a study co-author working out of the Archaeological Institute of America, rejects that explanation. "I've worked around construction sites my whole career and I know bones that have been impacted by equipment and those that did not," he said in a press conference.

Other animals were found at the site, though they didn't have the same wear and tear as the mastodon bones and may have been present there at an entirely separate time. "You can actually identify the particular hammer was hit on a particular anvil and trace it to the fragmented bones," Richard Fullagar, a research fellow at the Centre for Archaeological Science at University of Wollongong, said in the same press conference prior to the release of the paper.

Anthropological Whodunit

There have been several migrations of human species coming out of Africa. Homo erectus, one of the earliest relatives to cross the line from upright great apes like Australopithecus afarensis, came about 1.9 million years, establishing a presence in Southeast Asia 1.6 million years ago.

Neanderthals, most roughly defined as a cousin species to humans, became widespread in Europe around 40,000 years ago, though a 400,000-year-old bone found in Spain points to a much earlier migration of a species related to Neanderthals.That bone was actually closer in genetic relation to Denisovans, a species of humans known only from a molar and a pinky bone. They were in Siberia around 50,000 years ago. What's more, the ancestor of humans, Neanderthals, and Denisovans arose only 700,000 years ago. That species, Homo heidelbergensis, left Africa around 500,000 years ago, but never moved much beyond the Middle East and Asia.

Homo sapiens, the only surviving human species (not counting some mixture with Neanderthals and Denisovans accounting for a small fraction of DNA of their descendants), arose 200,000 years ago and arrived in Asia around 50,000 years ago. By all accounts, humans didn't arrive in North America from Asia until 12,000 years ago. So if there were indeed someone in California 130,000 years ago, who was it?

A bone breakage experiment on a modern elephant's leg bones. Kate Johnson, San Diego Natural History Museum

An accompanying article in Nature suggests erectus, Neanderthals, or Denisovans as the culprit, and indeed, little about Denisovan migration is known. The writers also don't rule out an archaic modern human from a previously unknown migration. There is some seriously controversial evidence of human activity up to 50,000 years ago in North America, and this site would be 80,000 years earlier than that.

One problem in determining who exactly killed that mastodon rests on finding the bones of the group that did it. That may be an impossibility. Even if an early human species got there somehow, whether by a land bridge or by a crude watercraft moving down the coastline, it doesn't necessarily mean that they stayed around — or even that they came in large numbers. "We think the population density would have been very low at that time and the chance of finding human remains would be very small," Holen said.

The authors said that nothing in the tools leaned toward one species hypothesis over another. The "hammers" used were crude stones, while the anvils were ... heavier crude stones. This kind of tool use isn't specific to any one human species.

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"That's almost the million-dollar question if this turns out to be credible evidence," Pobriner says. "Really any of the hominids around at the time could have broken bones to make tools."

The findings are quite audacious, and have the potential to up-end anything we've previously known not only about human migration into America but also human migration entirely. That is, so long as more evidence turns up. The paper doesn't present alternative scenarios.

The search for follow-up evidence may, in fact, have already been dug up. Pobriner suggests some museum specimen may have some of the same percussive injuries that can also be dated to this earlier era, helping map what might have been the migration.

The authors also seem relatively confident in the dating given the strata layer of the sedimentary deposits. But extraordinary claims require extraordinary proof — and this is a pretty extraordinary claim.

"I think putting them out there is really the most important first step in the scientific process," Pobriner says.

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