Archaeological sites in North America have led most researchers to believe that the continent was first reached by humans like us, Homo sapiens, about 15,000 years ago. But inspection of the broken mastodon bones, and large stones lying with them, point to a radical new date for the arrival of ancient humans. If the claim stands up, humans arrived in the New World 130,000 years ago.
Thomas Deméré, curator of palaeontology at the San Diego Natural History Museum which led the project, said: “Of course extraordinary claims like this require extraordinary evidence,” adding that the team believed “the site preserves such evidence”. Anticipating the disbelief of many experts in the field, Steven Holen, another project scientist at the Center for Paleolithic Research, said: “I know people will be sceptical about this.” That caution was summed up by one scientist who preferred not to be named: “They are going to face a shitstorm,” he said.
The partial remains of the American mastodon, a long gone relative of the modern elephant, were discovered in San Diego in the winter of 1992 during a freeway expansion project. When researchers moved in they found layers of fine sediments deposited by streams, bearing shells, rodent teeth, and wolf and horse bones. In one layer they found the mastodon, a beast that could reach a height of three metres and weighed eight tonnes when fully grown. The animals had roamed North America for millions of years.
The bones posed an immediate puzzle. The pattern of the fossilised limbs, the obvious damage, and stones found alongside them raised enough questions that the scientists brought in other experts and launched a detailed analysis of the remains and surrounding site.
Using leg bone used from an elephant that had recently died of natural causes, a breakage experiment was carried out in an attempt to determine the kinds of breakage patterns that might result from hammerstone percussion.
The results of the investigation, reported in the journal Nature, build a case for the mastodon bones being “processed”, a term that translates into more frank terms such as smashed, cracked and snapped. Unlike the wolf and horse bones found in other layers at the site, the ends of some of the mastodon bones had been broken off, as if to extract nutritious bone marrow. Others had been battered. One of the animal’s tusks poked upright in the ground, perhaps by chance, or perhaps to serve as a marker for the remains.
Intriguingly, the bones were found in two rough piles, each with two or three large rocks measuring 10 to 30cm across. The scientists believe the stones are too heavy to have been carried there in the flow of a stream, and instead suspect they were carried by humans for use as hammerstones and anvils to break the bones apart. “What is truly remarkable about this site is that you can identify particular hammers that were smacked on particular anvils,” said Richard Fullagar, a stone tools expert on the team from the University of Wollongong in New South Wales. Pieces knocked off the stones and bones were found too.
“We have no evidence that this is a kill or butchery site, but we do have evidence that people were here, breaking up bones of the mastodon, removing some of the big, thick pieces of mastodon limb bones, probably to make tools, and perhaps extracting some of the marrow for food,” said Holen.
Most remarkable of all is the apparent age of the bones. Carbon dating and another procedure did not work in this case, so the scientists turned to a method that infers age from the radioactive decay of natural uranium which infiltrates the buried remains. The tests dated the bones to 130,700 years old, give or take 9,000 years. James Paces, a researcher at the US Geological Survey who performed the dating, said it was a “robust, defensible age” for the materials.
If the scientists are right and the bones were broken by humans while fresh – rather than by other animals, natural processes or bulldozers building roads – and the dating is sound, it raises major questions about the peopling of the Americas. Who were these pioneers? How did they get there? What happened to them? There is little to suggest that Homo sapiens had dispersed from Africa 130,000 years ago, but Homo erectus, the Neanderthals and the little-known Denisovans had reached Eurasia.
The usual assumption is that humans came to America from eastern Asia across the Bering strait. The crossing itself would have been easiest in the cold period that ended 130,000 years ago when sea levels were low and a land bridge formed. But could these early humans have survived the harsh conditions at that latitude? “It’d be bloody cold up there,” said John McNabb, a palaeolithic archaeologist at Southampton University.
Emboldened by claims that human ancestors reached Indonesian and Mediterranean islands by raft more than 100,000 years ago, the authors suggest that instead of walking to America, the humans, perhaps archaic Homo sapiens, arrived from east Asia on “watercraft” and followed south what is now the coastline of California.
It will take more evidence to convince many scientists, however. “This is a really extraordinary claim. There are questions about everything,” said Jean-Jacques Hublin at the Max Planck Institute for Evolutionary Anthropology in Leipzig. “Let’s imagine it happened. We have humans in America 130,000 years ago. What happened to them? They disappeared? When humans arrived in Australia, they were immediately very successful because they had no competitors. In the Americas, there is a huge range of environments where humans could be very successful. But to this date we have nothing in America until modern humans arrive.”
Another question lies with the dating. The uranium method works well on stalagmites and stalactites found in caves, because uranium around at the time is locked into their crystal structure. But it is far harder to date bone with uranium, because bone is porous, and uranium can seep in and out with water all the time. “I personally would never use uranium series dating of bones alone to assign age, it needs to be supported and consistent with other dating results,” said Dirk Hoffmann, an expert on uranium dating at Leipzig. “I am not saying the presented age range is wrong, but I am very cautious with this dating method on bones.”
David Meltzer, professor of prehistory at the Southern Methodist University in Dallas, is even more wary of the claims. “Nature is mischievous and can break bones and modify stones in a myriad of ways,” he said. “It is not enough to demonstrate that they could have been broken by humans. One has to demonstrate that they could not have been broken by nature.”
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To test their theory that humans smashed the bones with stones, the scientists smacked large rocks onto elephant bones and found that the violent impacts produced similar fracture patterns as seen on the mastodon bones.
“If you are going to push human antiquity in the New World back more than 100,000 years in one fell swoop, you’ll have to do so with a far better archaeological case than this one,” Meltzer added. “I’m not buying what’s being sold.”
Chris Stringer at the Natural History Museum in London said: “If the results stand up to further scrutiny, this does indeed change everything we thought we knew about the earliest human occupation of the Americas.” He added: “Many of us will want to see supporting evidence of this ancient occupation from other sites, before we abandon the conventional model of a first arrival by modern humans within the last 15,000 years.”