Last year, archaeologists in California made a surprising announcement: They had discovered a 130,000-year-old prehistoric human campsite in the Golden State. According to currently accepted evidence, the first modern humans were just then venturing out of Africa and no one would set foot in the Americas for about 115,000 years. Not surprisingly, the claim was met with skepticism by other archaeologists, who said the evidence at the site wasn’t enough to rewrite the whole story of human migration.
That evidence is a collection of broken mastodon bones, round cobblestones and flat stones. These bits and pieces were excavated during a 1992 highway construction project in San Diego and are now known as the Cerutti Mastodon Site. Archaeologist Steven R. Holen of the San Diego Natural History Museum and his colleagues say early humans used the cobblestones as hammers and the flat stones as anvils to break open mastodon bones to get the marrow inside. They say the Cerutti site represents a very early wave of human migration to the Americas, perhaps one that didn’t last long enough to gain a foothold. Which explains why no other human campgrounds this old have surfaced in America.
There is no doubt about the age of the site. Radiometric tests based on uranium decay and several other methods put the bones at more than 100,000 years old, and even critics agree the dating is reliable. The debate revolves around whether the items at Cerutti really are evidence of human activity, and competing arguments are published in today’s issue Nature.
“There are still significant issues with the evidence from the Cerutti Mastodon archaeological site, in particular with the taphonomic processes of the site,” University of Trent archaeologist Daniel Rafuse said, commenting on the debate.
Leave no stone unturned
Part of the problem is that if the stones found in Cerutti are actually tools, they are among the simplest of types — the kind that people simply pick up from the ground and use without chipping or flaking them into clearly recognizable shapes. such as hand axes or spear points. That can make it difficult to distinguish a cobblestone that has turned a bit from a cobble that someone once used as a hammer.
“The only tools needed to break these bones, based on experimental archeology of breaking elephant limb bones, are boulder anvils and hammers. We argue that there is no evidence that humans hunted/killed or slaughtered the mastodon, so you wouldn’t need chipped stone tools,” Holen said. “At the Cerutti site, the mastodon bone was apparently captured from a mastodon that was already dead.”
Holen and his colleagues say the fractures and chips on the cobblestones look exactly like what you’d expect if the cobbles had been slammed against a stone anvil. The flakes broken off by the impact even fit perfectly in their old spots on the stone hammers. But Ferraro and his colleagues argue that while the marks on the rocks could certainly be the work of people hitting the rocks together, they could also be the product of natural forces, such as rocks bumping into each other as they washed downstream during a flood.
If Ferraro and his colleagues are right, that doesn’t rule out human activity, but it means there could be other explanations. And that’s important because it would mean we can’t be sure that this site is the big problem that Holen and his colleagues claim.
“We urge caution in interpreting the current record,” Ferraro and his colleagues wrote. Holen et al.’s extraordinary claim of prehistoric hominin involvement in the [Cerutti Mastodon] site should not rely on evidence open to multiple, contrasting interpretations.”
Thomas A. Deméré, Museum of Natural History of San Diego
Bones of twist
Another important debate revolves around how the mastodon bones at Cerutti were broken: were they crushed under accumulated layers of dirt, or did humans do it? Holen and his colleagues say the damage to the bones is clearly human handiwork. They even made their own set of similar stone hammers and anvils, then smashed elephant bones with them. The resulting damage matched the fractures on the mastodon bones at Cerutti, and the archaeologists say none of their other experiments succeeded in producing that series of fractures.
But Baylor University anthropologist Joseph Ferraro and his colleagues argue that natural processes may also have been to blame. They say they’ve seen the same types of spiral fractures, nicks and flake scars on bones in other locations — such as a group of 24,000-year-old mammoth bones at Inglewood Mammoth Site in Maryland, a group of 66,800 to 51,300-year-old mammoth bones at Waco Mammoth National Monument in Texas. , and others dating to the Triassic.
No one has ever suggested human habitation in these locations. Instead, the fractures were found to be the result of postmortem trampling by other mammoths, the weight of several feet of sediment, or heavy earth-moving equipment. And in many of those sites, paleontologists found stone cobblestones similar to those at Cerutti.
Holen and his colleagues argue that the bones at the Waco Mammoth National Monument actually show a different type of fracture, which occurs long after death when the bones are dry. New fractures, they say, produce smoother surfaces like those on the Cerutti Mastodon bones. And the indentations in the bones at other locations are shallower than those at Cerutti, whose depth indicates deliberate human activity rather than natural wear and tear.
Thomas A. Deméré, Museum of Natural History of San Diego
Meanwhile, Ferraro and his colleagues say the Cerutti bones lack other types of fractures, which should be on at least some of the bones if people actually broke them — a type of damage called “hamerstone stretch marks” or “hammerstone pits.” (Archaeologists know this, again, from the time they spent making stone tools and then crushing elephant bones with them.)
San Diego Natural History Museum archaeologist Thomas A. Deméré indicates that at least one bone fragment from Cerutti showed hammerstone stretch marks and much of the mammoth’s skeleton was missing from the site. And while experimental archeology tells researchers to expect these features in the damage caused by stone hammers with hooks, the Cerutti Mastodon tools would have been hand-held and could show different patterns.
And Holen and his colleagues offer yet another piece of evidence. A crust of a substance called pedogenic carbonate – which precipitates from certain types of soil as they form – has been found around most of the bones and rocks in Cerutti. Because the crust appears to have formed around the broken bones and the crust hasn’t broken, the bones must have broken before the crust formed, the archaeologists say.
“Therefore, these features, as well as the clustered distributions of [Cerutti Mastodon] bones and stones, cannot be the result of more recent events such as heavy equipment or even sediment compaction,” explains Deméré.
Need more proof
The debate over Holen and his colleagues’ extraordinary claim is far from settled, and in fact it seems to be getting quite heated.
“We never felt that this one site would represent a paradigm shift. We are currently in the field looking for additional locations of this age,” Holen told Ars Technica. “The answer to the question of when people first arrived in America requires that people do a lot of fieldwork and have an open mind about human capabilities.”
At this point, there’s no clear evidence that the stuff at Cerutti wasn’t the work of people who got here much earlier than everyone else. But there’s also not nearly enough evidence to say they were, as there are other explanations that don’t require such a big jump.
“The taphonomic issues need careful thought. More convergent evidence is needed to overturn the current scientific consensus on the timeline of human dispersion in the Americas,” Rafuse said.
Nature2017. DOI: 10.1038/nature22065 (About DOIs).
Frame image by Thomas A. Deméré, San Diego Natural History Museum