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It's true that over the years, researchers have excavated multiple sites from Pennsylvania to Brazil that have pointed to an earlier human presence in the New World, perhaps 20,000 to 50,000 years ago. But all of those finds have drawn intense criticism by mainstream archaeologists, and have been effectively sidelined.
Today however, a team of paleontologists, geologists and archaeologists writing in the prestigious journal Nature, present evidence pushing the first human presence in the Americas back by close to a factor of ten, to between 120,000 and 140,000 years ago.
"This discovery is rewriting our understanding of when humans reached the New World," says Judy Gradwohl, President and CEO of the San Diego Natural History Museum, whose team of paleontologists led by Richard Cerutti first studied the site, located along the route of a freeway in southwest San Diego, California.
"It's a very urban area now," says Thomas Deméré, also at the Natural History Museum, "but 130,000 years ago it was a meandering stream close to sea level, with camels, horses, ground sloths, capybara and deer--probably a very nice place to live."
130,000 year old mastodon bones
and possible stone tools, in situ
Credit: Holen et al./Nature
The core of the evidence consists of two clusters of fossilized teeth and bones of a single mastodon that, according to the researchers, show clear signs of having been smashed open by humans using stones as hammers and anvils. In addition to the shattered bones and teeth, the "smoking guns" are five hefty stones found among the bones--hammerstones that still bear impact marks from their use at the site, with flakes knocked off by the blows nearby. Tellingly, the mastodon's ribs, which would have been broken by geological forces, are still intact, while the extremely tough, marrow-containing long bones were cracked open.
Possible hammerstone wielded 130,000 years ago
Credit: Holen, et al./Nature
To verify that the marks they found on the stones and bones were caused by humans rather than by animals or geological processes, the team used similar stone hammers and anvils to smash open fresh elephant bones. "We found the same fracture patterns," said Steven Holen, a paleontologist at the San Diego Natural History Museum and the paper's lead author. "It reproduced what we saw at the Cerutti site." [Note: the researchers experimented on the bones of already-deceased elephants; no animals were harmed in their experiments.]
The research team used state-of-the-art uranium-thorium dating to determine that the bones were buried 131,000 plus-or-minus 9,400 years ago. This high-tech technique was what finally allowed them to definitively date the bones.
Since anatomically modern humans are not thought to have left Africa until around 100,000 years ago, just who these extremely early Americans might have been remains an open question. The authors speculate that they may have been a far-flung branch of Homo erectus, the mysterious Denisovians, or even Neanderthals. "The simple answer is that we don't know," says Richard Fullagar, at the University of Wollongong, in New South Wales, Australia.
Still, he adds, assemblies of stone hammers and anvils along with similarly smashed bones occur in archaeological sites in Africa dating back up to 1.5 million years, and in Europe to 350,000 years ago. Cracking open the long bones of large animals to extract marrow or make tools has been part of the repertoire of many human ancestors.
Although the research team carried out an extremely careful and detailed study of this site and its artifacts--a 24-year research odyssey that has been validated by the publication of their work in Nature--only time will tell if their conclusions will hold up in the face of the withering scrutiny they will no doubt encounter. In science, extraordinary new understandings require extraordinary proof, and setting back the date when the first humans reached the Americas by a staggering 115,000 years is certainly extraordinary.
The researchers know that their groundbreaking findings will be highly controversial, but believe that they have dotted every i and crossed every t. "It's taken a very long time to get to this stage," says Fullagar. But now, he says, "we have enormous confidence in the evidence we've put together. It's truly incontrovertible."
Incontrovertible or not, it's already eliciting great skepticism on the part of many archaeologists, for whom one isolated site with no actual human remains simply isn't sufficient to overturn the current 15,000-year-old first entry date. However, a study published just one day later in the equally prestigious journal Science, may offer the possibility of a definitive proof.
A team of geneticists, archaeologists and paleontologists at the Max Planck Institute of Evolutionary Anthropology, in Leipzig, Germany report being able to recover Pleistocene-era Neanderthal and Denisovian DNA, along with DNA from various animals, from the sediments of caves occupied or visited by ancient humans, even though no bones or other human remains were found. I won't go into more detail here, since this story has been heavily covered elsewhere. You can see what the New York Times had to say about it here.
If the same technique can be used on materials from the Cerutti site, it could pin down a revolutionary claim that otherwise is likely to remain at best controversial.
The Nature article by Steven Holen and colleagues can be found here.
You can view a YouTube video about this discovery at this URL.
And click here for video of the team's elephant-bone-breaking experiment.
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