Who busted up the Cerutti Mastodon? Archaeologist Steven Holen and colleagues thought the answer was clear. The Pleistocene pachyderm was a buffet for humans who lived in the vicinity of ancient San Diego 130,000 years ago. The fossil's broken bones were taken as evidence that people smashed them to get at the marrow inside. And it was that connection which seemed to make the beast so special. The mastodon bones appeared to record damage caused by some of America’s first inhabitants, tens of thousands of years earlier than previously thought.

Or, more likely, the mastodon is an unfortunate case of modern roadkill.

At the top of their 2017 paper, Holen and coauthors write that ancient sites documenting the arrival and dispersal of people into North America must first of all be found in undisturbed context. The context of the site is everything. Only, that wasn’t the case with the Cerutti Mastodon. The beast was discovered, and, ultimately, damaged by a highway construction project.

Archaeologists were immediately skeptical of the original paper’s claims. Not only is the site far older than any other, but no definitive signs of human presence – like bones or tools – were found with the mastodon. There was only the damage to the bones. Where Holen and colleagues saw breaks created by hungry humans, other researchers saw marks left through injury during the animal’s life and construction damage.

Now, according to construction plans and maps obtained by land surveyor Patrick Ferrell, the mystery seems to have been solved. The Cerutti Mastodon wasn’t broken up by humans. It was pulverized by the excavations that eventually brought shards of tooth and bone to the surface.

The key was looking at what California’s Department of Transportation set out to do while working on State Route 54. The construction crew was working on flattening a 10-foot-wide swath and create a sound barrier, using a powerful backhoe to do the job. But space was tight. There was only so much room for trucks assigned to carry away debris could get in and out. The path they had to navigate was directly over the Cerutti Mastodon site, crossing it coming and going.

Working from construction plans, excavation details, and photographs, Ferrell reconstructed what must have happened. Even though the excavator didn’t directly sit on top of the mastodon bones, and was buffered by too much sediment for the weight to cause the damage seen, the motion of the steel teeth on the digging bucket inadvertently dragged cobbles which smashed onto the buried bones. The same action could have moved a tusk found stuck vertically into the ground – said to be intentionally placed by Holen and colleagues but with no explanation as to how or why.

The dump trucks didn’t help any. The back and forth would have further mashed cobbles, some of which were drawn up by the excavator, into the sediment and against the mastodon bones. Ferrell estimates that the trucks must have made at least 150 trips over the site, probably doing more damage with each run. The path of the truck route lines up with the pulverized thigh bones critical to the archaeological case. This beast wasn’t a meal for people. There’s no evidence that humans had yet arrived in North America at the time. Instead, it’s a mastodon with a bad case of road rash.