ALAMOSA — An Ice Age paleontological-turned-archaeological site in San Diego, California, 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 to be published in the April 27 issue of the prestigious science journal Nature.

Adams State University’s Dr. Jared Beeton, professor of physical geography, contributed to the research on the Cerutti Mastodon site and is one of 11 authors on the article. Beeton analyzed the various layers of sediment at the site in order to determine geologic events that contributed to the arrangement of fossils and stones.

In 1992, San Diego Natural History Museum paleontologists discovered the fossil remains of a mastodon. 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. (Paleontology is the study of past geological periods through fossils; archaeology is the study of past human life though material remains.) Since its initial discovery, this 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. Radiometric dating of the site completed in 2014 indicates that the site is 130,000 years old.

 Rewriting history

“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 CEO 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 for 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.

“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, curator of paleontology and director of PaleoServices at

the San Diego Natural History Museum and corresponding author on the paper. “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.”

The lead author on the paper is Dr. Steve Holen, Director of the Center for American Paleolithic Research, who was previously with the Denver Museum of Nature & Science. He said, “The bones and several teeth show clear signs of having been deliberately broken by humans with manual dexterity and experiential knowledge. 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.”

Beeton added, “These humans came over during the last major interglacial period and were using stone tools, including hammer stones and anvils to process a mastodon.” His work included describing the general geomorphology of the area, the soils and sediments at the site, and the depositional environment of the bones.

Context matters

“Understanding the depositional environment at a mastodon site is important, because if the bones were deposited in a flood, or a landslide, or some other high-energy system, then the bones will get moved around and broken by these geological processes. This is important to paleontologists and archaeologists because they are analyzing the breakage patterns on the bones to find out if humans were associated with the mastodons,” Beeton explained. “At the Cerutti Site, the bones were deposited in a low-energy fluvial environment, meaning they came to rest on the floodplain of a stream, far from fast-moving flood deposits. This establishes the geologic context and helps my colleagues tell the paleontological and, in this case, the archaeological story of the site.”

Holen invited Beeton to work on the Cerutti Mastodon site project. Over the last seven years, they’d worked together on Columbian mammoth (Mammuthus columbi) and American Mastodon (Mammut americanum) sites, with Beeton serving as geomorphologist.

Beeton conducted field work at the Cerutti Mastodon site, then analyzed and described the soils and sediments at the San Diego Natural History Museum, and conducted further analysis and sample preparation in Adams State University’s Earth Sciences Soils Laboratory. Dr. Rob Benson, Adams State professor of geology and earth sciences, assisted Beeton by running X-Ray Diffraction on soil samples in the university’s Interdisciplinary STEM Laboratory, and by collaborating on thin section analysis.

Beeton was one of 11 authors who contributed to the manuscript that is scheduled to be published in Nature. The others include: Dr. Steve Holen, director of research at the Center for American Paleolithic Research; Dr. Tom Deméré, curator of paleontology and director of PaleoServices at the San Diego Natural History Museum; Dr. Daniel Fisher, professor of paleontology and director and curator of the Museum of Paleontology at the University of Michigan; Dr. Richard Fullagar, professorial research fellow at the Centre for Archaeological Science at the University of Wollongong, Australia; Dr. James Paces, research geologist at the U.S. Geological Survey; Kathleen Maule Holen, administrative director at the Center for American Paleolithic Research; Dr. Adam Rountrey, collection manager in the Museum of Paleontology at the University of Michigan; George T. Jefferson, district staff paleontologist at Anza-Borrego Desert State Park; Dr. Lawrence Vescera, volunteer paleontologist at the California State Parks Colorado Desert District Stout Research Center in Borrego Springs; and Richard Cerutti, former paleontological monitor at the San Diego Natural History Museum.

In his 10 years on the Adams State faculty, Beeton and his students have been involved in four mammoth digs in Colorado, two of which are in the San Luis Valley. Beeton’s book, The San Luis Valley: Its Geology, Ecology, and Human History, will be published by the University Press of Colorado. He has also published articles in major international peer-reviewed journals such as Geoarchaeology and Quaternary Research. He holds a B.S. degree from Kansas State University, an M.A. degree from University of Northern Colorado, and a Ph.D. from the University of Kansas.

Julie Waechter is Assistant to the President for Communications at Adams State University

Caption: Adams State’s Dr. Jared Beeton at the San Diego Natural History Museum’s exhibit on the Cerutti Mastodon site. Courtesy photo