A close-up view of a fractured mastodon femur bone, thought to have been damaged by ancient humans Tom Deméré, San Diego Natural History Museum

Stone tool marks found on the 130,000-year-old remains of a mastodon reveal that ancient humans may have landed in North America some 115,000 years earlier than previously thought.

"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 and managed the excavation.


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"The evidence we found at this site indicates that a hominin species was living in North America 115,000 years earlier than previously thought. This raises intriguing questions about how these early humans arrived here and who they were."

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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

The Cerutti Mastodon site in San Diego, California, was first discovered in 1992 by palaeontologist Richard Cerutti during routine work being carried out as part of a major road expansion. The first bones were found when grading equipment hit a tusk. The site was then excavated over the following five months, during which time the researchers discovered a single mastodon skeleton alongside hammerstones and stone anvils close to the remains. The site dates back to the early late Pleistocene epoch.


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The bones were found embedded in fine-grained sediments that were deposited much earlier than ancient humans were first thought to have arrived on the continent. Attempts to accurately date the bones had been unsuccessful and as there was no collagen preserved in the bones, radiocarbon dating was not an option. Eventually, in 2014, researchers used uranium–thorium dating from multiple bone specimens to determine that the smashed mastodon carcass is 130,000 years old.

A digram of the mastodon skeleton shows which bones and teeth were found at the site Dan Fisher and Adam Rountrey, University of Michigan

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"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,” said Dr. Tom Deméré, from the San Diego Natural History Museum who dated the remains.

For example, numerous bone fragments were found displaying spiral fractures indicating they were broken while fresh. Several of the bone and tooth pieces also show evidence of being hit with hard objects and many of the damaged bones were found with the five large hammerstones. This, along with the way in which they were distributed around the anvils, suggests they were broken at this location and the researchers believe ancient humans extracted from the mastodon limb bones and may have also used them for tool production.

San Diego Natural History Museum Paleontologist Don Swanson pointing at rock fragment near a large horizontal mastodon tusk fragment San Diego Natural History Museum


"The bones and several teeth show clear signs of having been deliberately broken by humans with manual dexterity and experiential knowledge," said Dr Steve Holen, director of research at the Center for American Paleolithic Research. "This breakage pattern has also been observed at mammoth fossil sites in Kansas and Nebraska, where alternative explanations such as geological forces or gnawing by carnivores have been ruled out."

The bones recovered from the Cerutti mastodon site are now on display at the San Diego Natural History Museum.

The research is published in the journal Nature.