Humans are thought to have reached the Americas less than 15,000 years ago. But evidence of stone tool use on an animal carcass excavated in California points to a much earlier arrival of human relatives from the genus Homo. These findings may have opened up a whole new chapter in the history of humans in the Americas.

An Incredible 20-Year Odyssey

25 years ago, a team of researchers from the San Diego Natural History Museum, lead by Thomas Demere excavated the site located in San Diego County. The site was named Cerutti Mastodon site, in honor of Richard Cerutti, who made the discovery and led the excavation.

During 1992 and 1993, the team found cobbles and broken mastodon bones lying together. Alongside the animal’s remains, five stones were also discovered. According to the study, the stones were used as makeshift hammers and anvils. The site also contained fossils of other extinct animals, including dire wolf, horse, camel, mammoth and ground sloth.

These are some of the Mastodon bones found at the excavation site. Image credit: San Diego Natural History Museum

Uranium dating techniques (determination the age of calcium carbonate materials) revealed that these bones were 131,000 years old (Late Pleistocene period), as were the marks on them. To support their theory, researchers analyzed mastodon bones found in later North American sites, which date from 14,000 to 33,000 years ago. These bones displayed the same fracture patterns that were observed among the remains of the Cerutti Mastodon in California.

One of the study authors, Steven Holen said in the press release: “We have produced exactly the same kinds of fracture patterns that we see on the Cerutti mastodon limb bones. These bones were not broken by carnivore-chewing, they were not broken by other animals trampling on the bone. We can eliminate all of the natural processes that break bones like this.”

The surface of a mastodon bone with a notch the researchers believe was created by humans. Image credit: Tom Deméré, San Diego Natural History Museum

Science Community Reactions

The study’s findings have been met with some skepticism. One of the main critiques is that the study doesn’t definitively rule out natural causes for the presence of the purported stone tools or the breakage patterns in the mastodon bones.

An expert on early Americans, an archaeologist at the Southern Methodist University David Meltzer, claims: “It’s not impossible that human history in the Americas is older than currently thought. But to prove it, you can’t take broken bones and nondescript stones to make the case, not without demonstrating that nature couldn’t have broken those bones and modified those stones”.

Mikkel Pedersen, a researcher who has worked on studies about how and when the first humans arrived in North America, says that “from a genetics standpoint, there’s absolutely no evidence humans were in the area as early as the new paper suggests. It’s not impossible; it’s very exciting. Still, I’d like to see the more direct evidence”.

In a meanwhile, the team plans to re-excavate the site and to exam the stone tools from the site for protein residues. They claim that they’re prepared to defend their analysis from the years of fierce criticism ahead of it.