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 videofeaturing 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.”