The first humans to set foot in the Americas arrived some 25,000 years ago - or so we recently thought.
Researchers working at an archaeological dig site that runs along the 54 freeway in San Diego, California, have now uncovered what they believe is evidence of a human presence in North America that predates previous estimates by 100,000 years.
They published their findings Wednesday in a paper in the well-regarded scientific journal Nature.
"If this is true," Mikkel Winther Pedersen, a geogeneticist at the University of Copenhagen in Denmark who was not directly involved in the study, tells Business Insider, "it would rock the ground that we are standing on at the moment, not just for all archaeologists but for all the other researchers interested in this."
The proposed timeline revision is based on a set of 130,000-year old mastodon bones (dated using uranium) that show signs of having been processed by humans, according to the paper.
At the archaeological site, which was first unearthed in the 1990s, researchers discovered pieces of limb bones and teeth from the mastodon, an enormous extinct creature distantly related to the elephant.
The archaeologists say the way those bones were broken tells an important story.
Over hundreds of thousands of years, the ground beneath us gives way to tiny seismic shifts. These push-and-pull forces crush bone, hack apart human and animal remains, and turn solids to dust.
But instead of just showing the typical patterns of decay that bones exhibit over time, many of the fragments appeared to have been fractured shortly after the animal died. This is important because it signals that something other than natural processes were at work.
Furthermore, the bones don't appear to have been buried alone.
Amongst the mastodon remains, the researchers found what they believe are bones that had been fashioned into hammer-stones and anvils — two types of tools that early humans used in Africa as early as 1.7 million years ago.
And those objects showed wear-and-tear that the researchers say could not have been caused by geological processes.
"At many sites you have evidence that bones were used for hammers or anvils," says Richard Fullagar, an archaeologist at Australia's University of Wollongong.
"What's truly remarkable at this site is you can identify a particular hammer that was hit on a particular anvil."
Together, all of this data paints a picture that Fullagar calls "incontrovertible" evidence that humans were around at the time this mastodon died.
The image above shows the surface of a mastodon bone with a notch the researchers believe was created by humans.
"It's really the age of the site that's the extraordinary part of this research," Thomas Demere, a palaeontologist at the San Diego Natural History Museum, said on a call with reporters on Tuesday.
"It makes ours the oldest site in the Americas by a factor of 10."
Not everyone agrees that the evidence points to a human presence, however.
Michael Waters, director of the Centre for the Study of the First Americans at Texas A&M University, tells Business Insider via email that the bones found at the site, while intriguing, don't prove that humans were ever there.
"I am skeptical," he says, adding that "extraordinary claims require unequivocal evidence."
In addition, Waters points to "mounting genetic evidence" that suggests the first Americans arrived in the region no earlier than 25,000 years ago.
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.
But he adds, "as a scientist you need to keep your mind open. It's not impossible; it's very exciting. Still, I'd like to see more direct evidence."
This article was originally published by Business Insider.