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 (California, USA). 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.
     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 exposed the California site before its excavation.
     But new analyses by a team of researchers bolster the controversial claim: chemical residue of bones appears on two stones previously found among mastodon remains at the Cerutti Mastodon site, the scientists report. 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. 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 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 accidentally, the scientists say. Parts of broken Cerutti mammoth bones are also covered with hardened crusts that formed thousands of years ago or more, contradicting the argument that the stones and bones may have been damaged by construction activity.
     But the new findings haven't settled the dispute. 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. Stones presumably used a long time ago to break fresh mastodon bones should have picked up residue containing at least some collagen, which is missing from the newly analyzed bone residue.
     
Edited from Science News (4 December 2020)