The scientific debate over the most controversial archaeological site in the Americas has entered new and painful territory.
In 2017, scientists reported that about 130,000 years ago, an unidentified homo species used stone tools to break the bones of a mastodon near what is now San Diego. If true, that would mean that humans or one of our close evolutionary relatives arrived in the Americas at least 100,000 years earlier than previously thought, dramatically reshaping scientists' understanding of when the region settled (SN: 26/04/17) .
Critics questioned whether the unearthed stones were actually used as tools. And other researchers have suggested that alleged tool marks may have been created on the bones as the bones were transported by fast-moving streams or caused by construction activity that partially exposed the California site before its excavation in 1992 and 1993.
But new analyzes reinforce the controversial claim, says a team that includes some of the researchers involved in the initial finding. Scientists report in the Journal of Archaeological Science: Reports in December, the chemical residue of bones appears on two stones previously found among mastodon remains at the Cerutti Mastodon site. Cerutti’s two rocks also show signs of having given or received hard blows where bone debris has accumulated, according to the team. The larger stone may have served as a platform on which the bones were shattered with the smaller stone, possibly to remove the marrow to eat or to obtain pieces of bone suitable for shaping tools.
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“Many repeated blows are likely to create concentrations of broken bones (mastodons)” found at the site, says Richard Fullagar, a geoarchaeologist at the University of Wollongong in Australia who was also part of the original research. Hominids – perhaps Neanderthals, Denisovans, Homo erectus or Homo sapiens – struck the remains of the large creature in one or possibly several visits to the site, Fullagar argues.
In the new study, Fullagar, Wollongong geoarchaeologist Luc Bordes and colleagues used microscopes to determine that the chemical and molecular structure of the debris in the two stones matched that of the bones in general. That debris must have been acquired by hitting mammoth bones that were found scattered on the rocks, according to the team. Since microscopic bone remains appeared only where the stones showed signs of wear and hard impacts, the stones are unlikely to accumulate residue when they accidentally come in contact with mastodon bones after being covered by sediment, scientists say.
Parts of broken Cerutti mammoth bones are also covered with hardened crusts that formed thousands of years ago or more. Researchers argue that the survival of those crusts contradicted the argument that Cerutti's stones and bones may have been damaged by construction activity.
But the new findings did not resolve the dispute. Repeated truck traffic through the area during construction could result in recently buried stones against older, fossilized mastodon bones, creating damage that has been confused for ancient and intentional use of tools, says University of Nevada Reno archaeologist Gary Haynes . For example, a bone from the extremities of a previously unearthed mastodon was shattered into several hundred pieces, according to the effects of heavy trucks that frequently bounced off their heads, Haynes says.
The newly analyzed bone residue also does not include collagen. This component of bone usually degrades during fossilization, but traces of fresh bone may remain. Stones used long ago to break fresh mastodon bones should pick up debris that contains at least some collagen. So that lack increases the possibility that, instead of using old stones to break fresh mastodon bones, truck traffic had pushed buried stones against fossilized mastodon bones that contained little or no surviving collagen, Haynes says.
An unpublished 2015 study, also co-authored by Fullagar, found traces of collagen in three Cerutti stones, including the recently proposed hammering stone. That research used a special dye to identify traces of collagen. Further research is needed to determine whether the techniques used in the new study cannot detect ancient collagen remnants or whether the collagen retention areas of the two Cerutti stones were simply not sampled.