An Ice Age paleontological-turned-archaeological site in San Diego preserves 130,000-year-old bones and teeth of a mastodon that show evidence of modification by early humans.

Analysis of these finds dramatically revises the timeline for when humans first reached North America, according to a paper published in the April 27 issue of the journal Nature.

The 11 authors of the Nature paper include two University of Michigan paleontologists.

The fossil remains were discovered by San Diego Natural History Museum paleontologists during routine paleontological mitigation work at a freeway expansion project site managed by Caltrans, the California Department of Transportation. 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.

“This discovery is rewriting our understanding of when humans reached the New World. The evidence we found at this site indicates that some hominin species was living in North America 115,000 years earlier than previously thought,” said Judy Gradwohl, president and chief executive officer of the San Diego Natural History Museum, whose paleontology team discovered the fossils, managed the excavation, and incorporated the specimens into the museum’s research collection. “This raises intriguing questions about how these early humans arrived here and who they were.”

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 (named in recognition of field paleontologist Richard Cerutti, who discovered the site and led the excavation) 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.

U-M paleontologist Daniel Fisher, a co-author of the Nature paper, said several lines of evidence from the Cerutti Mastodon site point to “one interpretation that is almost unavoidable because of the way these different lines of argument interlace.”

“We don’t know how this animal died. We don’t know whether humans were part of that death. All that we know is that humans came along some time after the death, and they very strategically set up a process involving the harvesting of marrow from the long bones and the recovery of dense fragments of bone that they could use as raw material for producing tools,” said Fisher, who has been excavating mammoths and mastodons in the Great Lakes region for 38 years and who has also worked on woolly mammoth remains in Siberia for about 20 years.

Since its initial discovery in late 1992, the Cerutti Mastodon site has been the subject of research by top scientists to date the fossils accurately and evaluate microscopic damage on bones and rocks that authors now consider indicative of human activity. In 2014, James Paces, a research geologist with the U.S. Geological Survey, used state-of-the-art radiometric dating methods to determine that the mastodon bones—which were still fresh when they were broken by strategically placed blows from hammerstones — were 130,000 years old, with a conservative error of plus or minus 9,400 years.

“The distributions of natural uranium and its decay products both within and among these bone specimens show remarkably reliable behavior, allowing us to derive an age that is well within the wheelhouse of the dating system,” said Paces, a co-author of the paper.

The finding poses a lot more questions than answers: Who were these people? Are they part of an early — but failed — colonization attempt? Or is there a long, but as of yet, scarcely recognized presence of humans in this hemisphere?

“There’s no doubt in my mind this is an archaeological site,” said Steve Holen, director of research at the Center for American Paleolithic Research, former curator of archaeology at the Denver Museum of Nature & Science, and the lead author of the paper.

Both Holen and U-M’s Fisher have conducted experiments with the bones of large modern mammals, including elephants, to determine what it takes to break the bones with large hammerstones and to analyze the distinctive breakage patterns that result.

“It’s this sort of work that has established how fractures like this can be made,” said Fisher, director of the U-M Museum of Paleontology and a professor in the Department of Earth and Environmental Sciences and in the Department of Ecology and Evolutionary Biology.

The specimens recovered from the Cerutti Mastodon site will be displayed at the San Diego museum beginning April 26, and a public lecture featuring several of the Nature article authors will take place April 29.

Digital 3-D models of a selection of specimens pointing toward human association at this site can be viewed interactively at the U-M Online Repository of Fossils ( Animations featuring these models are also presented as supplementary information associated with the Nature paper.