Nature

San Diego Natural History Museum

San Diego Natural History Museum

San Diego Natural History Museum

Tom Deméré, San Diego Natural History Museum

Tom Deméré, San Diego Natural History Museum

Tom Deméré, San Diego Natural History Museum

San Diego Natural History Museum

San Diego Natural History Museum

Kate Johnson, San Diego Natural History Museum

In 1992, a group of archaeologists found something extraordinary buried below a sound berm next to the San Diego freeway in Southern California. They had been called in during a freeway renovation to do some excavation because the fossil-laced earth of the California coast often yields scientific treasures. After digging about three meters below the construction area, Center for American Paleolithic Research archaeologist Steve Holen was deep into a pristine layer of soil that hadn't been disturbed for millennia. There, he found what appeared to be an abandoned campsite, where humans had left stone tools and hammered mastodon bones behind. This wasn't too unusual; it's fairly well-established that humans were hunting mastodons in the Americas as early as 15,000 years ago.

Evidence and skepticism

But when Holen's colleagues used several techniques to discover the age of the bones, the numbers sounded crazy. Test after test showed that the bones had been buried more than 100,000 years ago. The result flew in the face of everything we think we know about the spread of humanity across the globe. It took 24 years before Holen and his fellow researchers were certain enough to publish their findings in Nature . Now, based on a reliable dating method using Uranium decay rates and years of repeated tests, the researchers say that an unknown type of early human lived in California roughly 130,000 years ago. If true, it completely changes the story of how humans reached the Americas.

What the researchers found at the site, dubbed Cerutti Mastodon, looks very much like a perfectly preserved tableau of an ancient tool-making workshop. Stone "anvils," or flat rocks, are surrounded by a scattering of smaller "cobbles" used as hammers. Mixed in with these items are shattered mastodon bones and teeth, many of them crushed in a way that could only be done by a human with a stone tool. It's clear that the stones were deliberately struck against each other because there are stone flakes that perfectly fit into the cobble hammers, possibly broken off while the bones were being worked. The breaks and scoring marks on the bones suggest that people were breaking them open to get at the marrow, as well as crafting them into tools. Some of the mastodon bones, like the tusks, were left untouched. Holen and his colleagues argue that if these bones had been disturbed by animals or some kind of natural disaster, they would all have been damaged in the same way. But that's not what we see.

"People were breaking up the limb bones of a mastodon, removing some of the big, thick pieces," Holen said at a press conference. "They were extracting marrow for food. And they were using old technology. We have evidence of people in Africa 1.5 million years ago breaking up elephant limb bones with this same [stone] technology." Another author on the paper, University of Wollongong archaeologist Richard Fullagar, is an expert on the kinds of microdamage that humans leave behind on stone tools. "The evidence at this site is remarkable," he said. When he first identified stone tools from the site, he didn't know their age. "They were pounding stones. All the materials indicated that they had been used for smashing up bones. You can see fragments of hammers and anvils that can be fitted back into the stones... It's rare that you get all that evidence at one site. It really does show humans have been there."

Just to make sure these kinds of stone tools could damage mastodon bones in the way they found them at Cerutti Mastodon, Holen and his colleagues conducted several experiments. They built stone-age tools and smashed elephant bones. After several such experiments, it was clear that one of the only ways to produce the damage they saw on those 130,000-year-old mastodon limbs was to crush them with rock cobbles on rock anvils.

Current data suggests Homo sapiens spread out of Africa around 100,000 years ago, across Asia about 60,000 years ago, into Europe and Australia 50,000 years ago, and finally the Pacific coasts of both North and South America, and probably interior continental locations by 13,500 to 15,000 years ago. To confirm the presence of an undefined species of Homo in the Americas at 130,000 years ago requires indisputable evidence. While the Cerutti Mastodon site challenges our knowledge of the peopling of America and forces us to think beyond what is currently known, fresh fractured bone together with five large cobbles is not extraordinary evidence.

Reactions to the paper in the scientific community are veering between skepticism and shock. Some say the Cerutti Mastodon site may have been too disturbed by freeway construction to be trustworthy. Others say it's clear that the site is undisturbed, but we still can't be sure that the evidence found by Holen and his colleagues is enough. Trent University archaeologist Daniel Rafuse , who has worked on some of the most ancient human sites found in Argentina, quoted Carl Sagan's famous line, "extraordinary claims require extraordinary evidence." Speaking via e-mail to Ars, he continued:

University of Wisconsin, Madison paleoanthropologist John Hawks was flummoxed. He told Ars, "I’m not usually at a loss for words, but this one has left me and many of my friends speechless."

Some scientists were absolutely convinced, however. Berkeley Geochronology Center geologist Warren Sharp, who is an expert in the kind of Uranium dating used to verify the age of the mastodon bones, praised the careful work the researchers had done. "The samples they used are some of the actual broken bones, so the samples and the context are clear," he told Ars by phone. He explained that Uranium dating measures the decay of the element after it has been taken up by bones from minute amounts of water in the soil. Because Uranium is very mobile, seeping into and out of porous objects like bone, the test can sometimes result in artificially old ages. But after examining the researchers' work in this paper, Sharp was satisfied that they had accounted for this potential problem by dating three different samples, taking multiple samples from each bone. This technique corrects for the problems caused by Uranium's mobility. "This is a reliable approach and it’s been done with care by people who have analyzed standards and ensured quality of the data," he concluded.

Holen, who first discovered the 130,000 year-old Cerutti Mastodon site, said he was prepared for the skepticism. At a press conference, he said that he and his colleagues had found other sites dating back to 30,000 years ago in the American Great Plains. Though these findings were controversial, he sees this new discovery as validating a new version of history where human ancestors were in the Americas long before most modern Homo sapiens had made it out of Africa.

Who were these mystery humans?

Currently, scientists believe that several different waves of humans came to the Americas from Asia between 12,000-20,000 years ago. Some nosed their way along the rich coastal ecosystems in boats, possibly walking part of the way through Beringia or the Bering Land Bridge. Some may have walked most of the way after the planet warmed and ice sheets covering most of Canada and the northern US withdrew. The Cerutti Mastodon discovery, say the researchers, doesn't change that story. It just adds a new wave of human explorers who came over 100,000 years earlier.

The timing makes sense, at least in terms of climate. About 130,000 years ago, the planet went through a warm, interglacial period when the coastal route would have been relatively ice-free, allowing people to find food as they gradually made their way from Asia to the Americas. It's even possible that Beringia would have been as walkable as it was in the late Pleistocene.

But who were these humans who made it all the way to the Americas and turned a mastodon into bone tools? That's what paleoanthropologist Hawks is wondering, too:

The key question is, which hominin population had the ability to establish a sustained population in Beringia? We don’t even know how many populations were in mainland Asia at the time—certainly multiple, deeply divergent populations of Denisovans and Neanderthals, maybe others with even older origins. Maybe modern humans. From the biological side, we have no reason to think that any of them were incapable of entering the New World. We’ve just been stuck with the assumption that they didn’t cross into North America—because if they did, we should see a hundred millennia of discarded tools and butchered animal bones.

Holen and his colleagues essentially agree with Hawks' point. In their paper, they suggest that the humans whose tools they found in California might have been Neanderthals or Denisovans. Probably, they conceded, these humans were a hybrid of several early humans, and possibly even a form of early Homo sapiens who roamed Asia more than 200,000 years ago. There is also evidence that humans in Indonesia had boats over 180,000 years ago, because there are human remains from that time on Indonesian islands that could not have been reached any other way. So we have humans and boat technology in Asia. And we have a climate that would allow for their passage to the Americas.

If we accept all this evidence, the question remains: what happened to those early Americans, and why haven't we seen more of their discarded stones and bones? Holen believes that these early mastodon hunters probably lived in very dispersed, small bands. The population was low. Plus, he noted, "It’s possible that the early population came in and did not make it. Humans could become locally extinct if the environment wasn't favorable for human adaptation in that area." So we may not ever find many of these people's tools because there were so few of them, or they simply died out before leaving much behind.

Still, while they were alive, they enjoyed a good life on the California coast, says San Diego Natural History Museum paleontologist Thomas Deméré, who worked on the paper with Holen. 130,000 years ago, he said, the Cerutti Mastodon site "was a meandering stream close to sea level. It was occupied by ice age mammals including camels, horses, ground sloths, dire wolves, deer, and capybara. It was very nice I would think, not far from the coastline." As scientists gather more evidence and search for more signs of these ancient Americans, we're left with a startling new vista on an alternate history that might just turn out to be our own.

Nature, 2017. DOI: 10.1038/nature22065

Listing image by San Diego Natural History Museum