In 1993, construction workers building a new freeway in San Diego made a fantastic discovery. A backhoe operator scraped up a fossil, and scientists soon unearthed a full collection of bones, teeth, and tusks from a mastodon. It was a valuable find: hordes of fossils, impeccably preserved. The last of the mastodons—a slightly smaller cousin of the woolly mammoth—died out some 11,000 years ago.

But the dig site turned out to be even more revelatory—and now, with a paper in the journal Nature—controversial. See, this site wasn’t just catnip for the paleontologists, the diggers who study all fossils. It soon had archaeologists swooping in to study a number of stone tools scattered around the bones, evidence of human activity. After years of debate over the dating technology used on the mastodon, a group of researchers now believes that they can date it and the human tools to 130,000 years ago—more than 100,000 years earlier than the earliest humans are supposed to have made it to North America.

The researchers expect a bit of controversy from a discovery that pushes back the arrival of humans in North America by a factor of ten. Nature itself put together this video featuring a leading British critic of the paper. Still, lead author Steven Holen, co-director of the Center for American Paleolithic Research is confident that his colleagues have done their homework. “I was skeptical myself,” he says. “But it’s definitely an archaeological site.”

This discovery—and the inevitable pushback it will face—center on the power and peril of dating technology. After more than two decades, researchers were finally able to nail down the mastodon’s age with a more advanced kind of chemical dating. But the paper also reveals the limits of that technology in solving ancient archaeological puzzles. Tech can tell you how old things are, but now how they got there or who used them.

There are many ways to date fossils—and a lot of them didn’t work for the mastodon skeleton. Radiocarbon dating was the first no-go. The technique uses carbon 14 as a clock of sorts; after an organism dies, carbon 14 uptake stops, and it begins to decay at a constant and predictable rate. But the mastodon fossils were so old that they didn’t contain any collagen, the dominant organic component of bones carrying that carbon 14.

There’s also a technique called optically stimulated luminescence, which detects the last time quartz sediments around the bone were exposed to sunlight. It can only be used if the material isn’t exposed to light, leading some paleontologists to cover up samples as they work. That’s the method that James Paces of the US Geological Survey used to date mammoth bones from Snowmass, Colorado, a site where he met Holen and became interested in the San Diego mastodon. Holen’s group used luminescence on the San Diego bones—but it only told them that they were older than 60,000 years. The quartz sediments were so saturated that they weren’t useful.

Finally, the group tried a newer method called uranium-thorium dating, which measures how quickly naturally occurring uranium in bones decays to its daughter isotope of thorium. “Both techniques—uranium and luminescence—are widely used,” said Paces, another author on the Nature paper. “Sometimes one will work better than the other depending on the environment.” Uranium-thorium won out. The mastodon bones had held on to enough thorium that the researchers could finally placed the bones to 131,000 years ago, plus or minus 9,000 years.

So far, so good. “The job of dating is convincing,” says Warren Sharp, a geochronologist at the Berkeley Geochronology Center who is an expert in uranium-thorium dating. But now the trouble begins, as the researchers try to figure out the relationship between the bones and the human tools scattered around them at the site.

The evidence for early humans messing with the bones is strong. The San Diego specimens have bones that fractured in a spiral pattern and other markings that indicate someone broke them soon after the animals were killed. The researchers even took giant rock hammers to elephant bones in Africa to replicate the prehistoric smash party, showing that that fresh bones do actually fracture this way. More than that, the rock tools had markings indicating they were used to excavate the bones’ nutritious marrow, and the scientists say the marked-up rocks could not have been pushed there by wind or water.

Now the big unknown is whether the bones were broken by a person more than 100,000 years earlier than we thought possible, or somehow dug up and broken more recently. “The crux of the arguments is whether those bones had to be broken when they were fresh,” says Sharp, who admits that he’s not an archaeologist. “That’s the only way to associate the bone date with presence of humans.”

There’s certainly reason to believe the timing lines up. Fossilized bones fall apart after losing the collagen that keeps fresh bones strong, says Paces. “As soon as it goes away as part of the fossilization process,” he says, “you are left with an old dry bone, and it busts up in a different way.” But there’s no way of dating the tools from the mastodon site with the same precision. Researchers will never have the same degree of quantitative certainty about when early humans encountered their mastodon meal.

Alistair Pike, an archaeologist at the University of Southamton (UK) and dating expert, thinks additional finds of early human activity in North America will back up the paper’s conclusions. “It is rare that archaeology adopts a single instance of dating as a benchmark, and the smoking gun usually comes from a second, third, or fourth instance of something similar,” Pike writes in an e-mail to WIRED. “I doubt that these individuals butchered just one mastodon ever, so this is our cue to go look for more.”

More mastodon bones with chisel marks would be good, but 130,000-year-old human fossils would be better. Thomas Demere, curator of paleontology at the San Diego Museum of Natural History, and an author on the new paper, excavated the original site 25 years ago. He notes that there are additional bone beds protected by a freeway sound berm. The answer to this mystery might just lie under a San Diego neighborhood.

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