A freeway find rewrites history
In November 1992, paleontologists were performing routine survey work during the expansion of California State Route 54. Field paleontologist Richard Cerutti spotted something unusual: the bones and tusks of a single American mastodon (Mammut americanum). The find was not typical. The remains were buried nine feet down in fine-grained sand and silt from an ancient stream. Found alongside the mastodon skeleton were five large, rounded stones, or cobbles, that did not seem to belong in that sedimentary layer.
The bones themselves were the biggest puzzle. Thick limb bones and molars were shattered, while more delicate bones like vertebrae and ribs were mostly intact. The fracture patterns on the dense bones—sharp breaks and spiral fractures—suggested they were broken while still fresh. Researchers led by Thomas Deméré of the San Diego Natural History Museum and Steven Holen of the Center for American Paleolithic Research proposed a hypothesis: hominins had used the large cobbles as hammerstones and anvils to process the mastodon bones, likely to extract nutritious marrow or to get raw material for making tools. The arrangement of the broken pieces and the wear marks on the stones supported this interpretation. The team published their analysis in the journal Nature in 2017.
A date that defies convention
The most challenging aspect of the discovery was its age. For decades, the consensus has been that the first humans arrived in the Americas around 15,000 to 20,000 years ago. The geological context of the Cerutti site suggested it was much older. Because the bones lacked enough collagen for standard radiocarbon dating, scientists turned to a different method. In 2014, U.S. Geological Survey geologist James Paces used Uranium-thorium (U-Th) dating on over 100 samples from the mastodon bones. The results provided a date of 130,700 years ago, with a margin of error of about 9,400 years.
This date places the site more than 100,000 years before the accepted timeline for human presence in the Americas. It would mean that a species of pre-modern human, perhaps Neanderthals, Denisovans, or even Homo erectus—made it to the continent. This is a migration there is currently no other evidence. The scientific community remains largely unconvinced. Critics point to the absence of definite, flaked stone tools and the lack of any hominin fossils at the site. They propose alternative explanations for the breakage, such as trampling by other large animals or damage from the heavy construction equipment that originally uncovered the site. The original research team has countered these points, arguing that the specific fracture patterns and the geological stability of the site make natural causes or modern damage unlikely. The debate continues, leaving the Cerutti Mastodon site as a deep and unresolved scientific question.