Two stones fuel debate over when America’s first settlers arrived

Scientific debate about the most controversial archaeological site in the Americas has entered rocky new territory.

In 2017, scientists reported that around 130,000 years ago, an unidentified Homo species used stone tools to break apart a mastodon’s bones near what is now San Diego. If true, that would mean that humans or one of our close evolutionary relatives reached the Americas at least 100,000 years earlier than previously thought, dramatically reshaping scientists’ understanding of when the region was settled (SN: 4/26/17).

Critics have questioned whether the unearthed stones were actually used as tools. And other researchers suggested that supposed tool marks on the bones could have been created as the bones were carried by fast-moving streams or caused by construction activity that partially exposed the California site before its excavation in 1992 and 1993.

But new analyses bolster the controversial claim, says a team that includes some of the researchers involved in the initial finding. Chemical residue of bones appears on two stones previously found among mastodon remains at the Cerutti Mastodon site, the scientists report in the December Journal of Archaeological Science: Reports. The two Cerutti rocks also show signs of having delivered or received hard blows where bone residue accumulated, the team says. The larger stone may have served as a platform on which the bones were smashed open with the smaller stone, possibly to remove marrow for eating or to obtain bone chunks suitable for shaping into tools.

“Many repeated blows are likely to have created the concentrations of broken [mastodon] bones” found at the site, says Richard Fullagar, a geoarcheaologist at the University of Wollongong in Australia who was also part of the original research. Hominids — perhaps Neandertals, Denisovans, Homo erectus or Homo sapiens — battered the large creature’s remains on one or possibly several visits to the site, Fullagar contends.

In the new study, Fullagar, Wollongong geoarchaeologist Luc Bordes and colleagues used microscopes to determine that the chemical and molecular structure of residue on the two stones matched that of bones in general. That residue must have been acquired by pounding apart mammoth bones that were found scattered around the stones, the team argues. Since microscopic remnants of bone appeared only where stones showed signs of wear and hard impacts, it’s unlikely that the stones accumulated the residue by accidentally coming in contact with mastodon bones after being covered by sediment, the scientists say.

Parts of broken Cerutti mammoth bones are also covered with hardened crusts that formed thousands of years ago or more. The survival of those crusts, the researchers contend, contradicts the argument that Cerutti stones and bones may have been damaged by construction activity.

But the new findings haven’t settled the dispute. Repeated truck traffic over the area during construction could have jostled recently buried stones against older, fossilized mastodon bones, creating damage that has been confused for ancient, intentional tool use, says archaeologist Gary Haynes of the University of Nevada, Reno. For instance, one previously unearthed mastodon limb bone was shattered into several hundred pieces, consistent with the effects of heavy trucks frequently rumbling overhead, Haynes says.

The newly analyzed bone residue also does not include collagen. This component of bone typically degrades during fossilization, but traces from fresh bone can stick around. Stones presumably used a long time ago to break fresh mastodon bones should have picked up residue containing at least some collagen. So that lack raises the possibility that, rather than ancient stones being used to break fresh mastodon bones, truck traffic thrust buried stones against fossilized mastodon bones containing little or no surviving collagen, Haynes says.

An unpublished 2015 study, also coauthored by Fullagar, found collagen residues on three Cerutti stones, including the newly proposed hammering stone. That investigation used a special dye to identify collagen traces. Further research is needed to determine whether the techniques used in the new study can’t detect ancient collagen residues or if collagen-retaining areas of the two Cerutti stones just weren’t sampled.