Early humans probably reached North America 130,000 years ago — 115,000 years earlier than previously thought, claims a study.

The findings, published in the journal Nature, are based on analysis of bones and teeth of a mastodon discovered in an an Ice Age site in San Diego, California.

"This discovery is rewriting our understanding of when humans reached the New World," said Judy Gradwohl, President and CEO of the San Diego Natural History Museum, whose palaeontology team discovered the fossils, managed the excavation and incorporated the specimens into the museum's research collection. 

"The evidence we found at this site indicates that some hominin species was living in North America 115,000 years earlier than previously thought," Gradwohl said.

The fossil remains were discovered by museum paleontologists during routine paleontological mitigation work at a freeway expansion project site managed by the California Department of Transportation (Caltrans). 

The bones, tusks, and molars, many of which are sharply broken, were found deeply buried alongside large stones that appeared to have been used as hammers and anvils, making this the oldest in situ, well documented archaeological site in the Americas.

Until recently, the oldest records of human sites in North America generally accepted by archaeologists were about 14,000 years old. 

But the fossils from the Cerutti Mastodon site were found embedded in fine-grained sediments that had been deposited much earlier, during a period long before humans were thought to have arrived on the continent.

"When we first discovered the site, there was strong physical evidence that placed humans alongside extinct Ice Age megafauna. This was significant in and of itself and a 'first' in San Diego County," corresponding author on the paper Tom Demere, Curator of Paleontology and Director of PaleoServices at the San Diego Natural History Museum, said.

"Since the original discovery, dating technology has advanced to enable us to confirm with further certainty that early humans were here significantly earlier than commonly accepted," Demere said.

The researchers found evidence that 130,000-year-old bones and teeth of a mastodon at the side were modified by early humans.