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A concentration of fossil bone and rock. The unusual positions of the femur heads, one up and one down, broken in the same manner next to each other is unusual. Mastodon molars are located in the lower right hand corner next to a large rock comprised of andesite which is in contact with a broken vertebra. Upper left is a rib angled upwards resting on a granitic pegmatite rock fragment. Photo: San Diego Natural History Museum

Smashed mastodon bones unearthed in California show that hominids were in the Americas 100,000 earlier than previously thought, contends a new paper in Nature.

The controversial claims, based on an assortment of animal bones, tusks, molars and rocks, would upend a significant part of human prehistory, according to experts. Prior research has indicated that Homo sapiens only began colonizing the Western Hemisphere 20,000 years ago.

“This discovery is rewriting our understanding of when humans reached the New World,” said Judy Gradwohl, president and CEOP of the San Diego Natural History Museum, which made the find and managed the excavation. “The evidence we found at this site indicates that some hominin species was living in North America 115,000 years earlier than previously thought.”

The Cerutti Mastodon site was discovered just outside San Diego during road work projects in 1992. The San Diego Natural History Museum took over the work, and led a five-year excavation of the work.

The conclusion: the bones were fresh when they were purposefully broken by blows from two stones. A larger rock functioned as a kind of anvil, while the smaller one was used as a kind of hammer, they found.

Radiocarbon dating could not be used to date the fragments. Instead, the scientists used a radiometric dating method based on minute levels of uranium and thorium. That method indicated they are 130,000-years-old.

“We don’t know how this animal died. We don’t know whether humans were part of that death,” said Daniel Fisher, of the University of Michigan, one of the authors. “All 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.”

But the authors acknowledge that there will be skepticism by extending the hominid timeline in the Americas so far into the past.

“There’s no doubt in my mind this is an archaeological site,” said Steve Holen, lead author, director of research at the Center for American Paleolithic Research, based in South Dakota. “The bones and several teeth show clear signs of having been deliberately broken by humans with manual dexterity and experiential knowledge.”

Holen’s prior work has indicated a history of hominid activity extending far into the past, before the Clovis peoples, he added.

“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,” Holen added.

But one thing conspicuously lacking remains: there are no human bones or other specific tools or man-made materials found at the various sites.

The history of the peopling of the Americas has grown complicated over the last five years, with better DNA technology and dating methods. What once was a tidy narrative about a single group of people crossing a frozen “land bridge” from Siberia to Alaska several thousand years ago has become a series of competing narratives involving multiple waves of migration – even perhaps from water crossings across the Pacific. The many languages of natives in the Americas has most recently pointed toward complex interconnections between tribes and lineages that are only beginning to be understood.

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