A team of scientists believe they have found evidence of human activity in North America that dates back 130,000 years — more than 100,000 years earlier than believed.

The evidence comes from an archeological site in San Diego County, Calif. In 1992, a site was uncovered containing mastodon bones, along with stone anvils and hammerstones. Dating the tools proved to be challenging. However, using recent technology, including uranium dating, the team believes they have firm evidence that humans were using tools to break apart the bones and make other tools.

"When you look at the evidence — the material, the nature of the wear pattern on the bones, the actual mapping of the site — the evidence is absolutely incontrovertible," Richard Fullagar, co-author of the paper that appeared in Nature said during a teleconference.

I was skeptical when I first looked at the material myself, but it's definitely an archeological site. - Steve Holen , researcher

The find is controversial, as there has been a consensus among paleontologists and anthropologists that humans made the journey to North America roughly 15,000 years ago.

Supporting evidence

The bones of the mastodon were arranged at the site — known as the Cerutti Mastodon site — in such a way that suggests it wasn't done naturally. It's not believed that the humans killed the animal, however. Instead, it's likely they were breaking up the limb bones, removing parts of the bones and probably making tools with them. They also may have extracted marrow from the bones for nutrition.

The bones also displayed spiral fractures, which suggests that they were broken while fresh and not some years later. Five large hammerstones and anvils also show wear and tear that didn't occur through geological processes, the researchers said.

A view of a spirally fractured mastodon femur bone. (Tom Deméré/San Diego Natural History Museum)

"What's truly remarkable about this site is that you can actually identify a particular hammer that was smacked on a particular anvil in a bunch of broken bones, and fragments of those hammers and anvils that can be refitted to the stones. It's very rare that you get that whole package together in one site," Fullagar said. "So the evidence at this site is truly remarkable and really does demonstrate human interference."

In order to replicate the types of fractures, the scientists used similar tools on elephant bones, which resulted in the same spirals.

Steve Holen, from the Center for American Paleolithic Research and a co-author, said that the breaking of the bones wasn't done by carnivore chewing or animals trampling on the bones.

"When we eliminate all the other natural processes and we can reproduce this experiment, we have very strong evidence," he said.

Skeptics challenged

The findings challenge current evidence that suggests humans arrived in North America about 15,000 years ago and will most certainly be carefully scrutinized, something the authors are well aware of. During their press conference on Tuesday, they invited other researchers to examine the evidence, some of which will be on display at the San Diego Natural History Museum next week.

I'd say the jury's still out. There's still some information missing. - Ariane Burke, Universite de Montreal

"I know people will be skeptical of this because this is so surprising," Holen said. "I was skeptical when I first looked at the material myself, but it's definitely an archeological site."

Ariane Burke, an anthropology professor at the University of Montreal, said that she'd like to see more analysis of the bones and tools before concluding that humans were indeed here so long ago.

"Normally at an archeological site you find chipped stone tools, irrespective of what they're doing at the site. People are notorious litterbugs, even back then," she told CBC News. "So you kind of expect to see some of that trace of a human presence. I'm not saying they're absolutely wrong and it cannot be a human site, but I'd say the jury's still out. There's still some information missing."

A boulder discovered at the Cerutti Mastodon site thought to have been used by early humans as a hammerstone. (Tom Deméré/San Diego Natural History Museum)

Burke said that she'd specifically like to see a closer examination of the surface damage of the bones. She's particularly puzzled by the lack of cut marks, but says theycould be obscured by the surface damage.

As for whether or not the bones could have been broken by heavy machines or vehicles, the authors are adamant that that's not the case.

The researchers are confident that under careful examination, other scientists will come to the same conclusions.

"If they look carefully at this paper, that's been through rigorous peer review that Nature insists for their publication, if people honestly look at the data … and have questions they want to pose, we are open for this sort of scholarly assessment of our work," said Thomas Demere, curator of paleontology at the San Diego Natural History Museum.

"And people are welcome to come to the museum to examine these specimens for themselves."