Researchers dug up what would later be identified as bones belonging to a mastodon.
Retired San Diego Natural History Museum Paleontologist Richard Cerutti (L) and Curator of Paleontology and Director of PaleoServices, Dr.Tom Deméré compare mastodon bones salvaged at the Cerutti Mastodon site in San Diego County, California, US, in this handout photo received April 26, 2017.
At the time of the find, dating techniques weren't sophisticated enough to reliably assign an age to the bones, and by association, the tool-users who acted upon them.
Stone tools from the Cerutti Mastodon site: (a-d) anvil; (a) upper surface; boxes indicate images magnified in b-d; dashed rectangle, magnified in b, small dashed square, magnified in c and solid square, magnified in d; (b) cortex removal and impact marks (arrows); (c) striations (arrows) on the highest upper cortical surface ridge; (d) striations (diagonal arrows) and impact marks with step terminations characteristic of hammer blows (vertical arrows). "For example, one tusk was positioned vertically".
But if the dates are accurate, the find has implications not just for humans in America, but also for the whole history of human migration. Over time they found more splintered bones and a smattering of large round rocks embedded in otherwise fine-grained sediment.
San Diego Natural History Museum palaeontologist Tom Deméré said until now the oldest widely accepted date for human presence in the New World was 14,000 to 15,000 years ago, making the San Diego site almost 10 times older.
The scientific community seems pleased with the dating methodology, but the presence of humans at the site is a tougher sell.
Radiocarbon dating could not be used to date the fragments.
Skeptical about the results, Paces and his colleagues continued analyzing the bones. And those objects showed wear-and-tear that the researchers say could not have been caused by geological processes.
Due to warmer temperatures and sea level rise, the Bering land bridge should have been underwater 130,000 year ago.
As for whether or not the bones could have been broken by heavy machines or vehicles, the authors are adamant that that's not the case. And the bones and stones were found in two areas, each roughly centered on what's thought to be an anvil.
Deméré added that while the large limb bones were distinctively damaged, more fragile pieces of the mastodon skeleton, such as ribs and vertebrae found at the site, were completely intact.
Parts of the bones were also used to make tools.
"When we eliminate all the other natural processes and we can reproduce this experiment, we have very strong evidence", he said.
They pointed out gaps in the research.
If verified and corroborated by other scientists, the discovery described in the journal Nature could radically rewrite the timeline of when humans first arrived in the Americas.
"The team's conclusions are paradigm-shifting, and I'm sure they will come under a lot of scrutiny in the coming days and months".
The new finding has been "rigorously researched and presented" but the paleontologists' proposed narrative about the bone data "has some gaping holes that need filling", according to Erella Hovers, an archaeology professor at the Hebrew University of Jerusalem. This is all far from certain, but interesting nonetheless. "So the evidence at this site is truly remarkable and really does demonstrate human interference".