Image copyright
Ciprian Ardelean
One of the stone artifacts found in the cave.
Humans settled in the Americas much earlier than previously thought, according to new findings from Mexico.
They suggest that people lived there 33,000 years ago, twice the widely accepted age for the first settlement in the Americas.
The results are based on work in the Chiquihuite cave, a high-altitude rock refuge in central Mexico.
Archeologists found almost 2,000 stone tools, suggesting that the cave was used by people for at least 20,000 years.
Ice Age
During the second half of the 20th century, a consensus emerged among American archaeologists that the Clovis people had been the first to arrive in America, some 11,500 years ago.
The Clovis ancestors were believed to have crossed a land bridge linking Siberia to Alaska during the last ice age.
This land bridge, known as Beringia, subsequently disappeared underwater when the ice melted.
And these big game hunters were believed to have contributed to the extinction of the megafauna – large mammals such as the mammoth, the mastodon, and various species of bears that roamed the region until the end of the last ice age.
Decomposition
As the “Clovis First” idea took hold, reports of previous human settlements were dismissed as unreliable and archaeologists stopped looking for signs of previous occupation.
But in the 1970s, this orthodoxy began to be questioned.
In the 1980s, strong evidence emerged of a 14,500-year-old human presence in Monte Verde, Chile.
And since the 2000s, other pre-Clovis sites have been widely accepted, including the 15,500-year-old Buttermilk Creek Complex in central Texas.
Image copyright
Devlin A. Gandy
The entrance to the rock shelter in Zacatecas, Mexico
Now, Ciprian Ardelean, from the Autonomous University of Zacatecas, Mexico, Tom Higham, from the University of Oxford, and his colleagues have found evidence of human occupation that dates back well beyond that date, at the Chiquihuite site in the lands high in central-northern Mexico.
The results have been published in the journal Nature.
“This is a unique site, we have never seen anything like this before,” Professor Higham, director of the Oxford Radiocarbon Accelerator Unit, told BBC News.
“The evidence from the stone tool is very, very convincing.
“Anyone can see that these are deliberately manufactured stone tools and there are many of them.
“The dating, which is my job, is solid.
“And so it is a very exciting site that I have been involved in.”
Dating techniques
The team excavated a 3 m deep (10 ft) stratigraphic section, a sequence of soil layers arranged in the order in which they were deposited, and found some 1,900 stone artifacts made over thousands of years.
The researchers were able to date bones, coal, and sediment associated with the stone tools, using two scientific dating techniques.
The first, radiocarbon dating, is based on how a radioactive form of the element carbon (carbon-14) is known to break down over time.
The second, optically stimulated luminescence (OSL), works by measuring the last time sediments were exposed to light.
Image copyright
Devlin A. Gandy
Scientists sampled sediment from the cave for DNA
Using two different techniques “added a lot of credibility and strength, particularly to the older part of the timeline,” said Professor Higham.
“Optical dates and [radiocarbon] the dates agree, “he said.
And the findings could lead scientists to take a new look at controversial early-occupation sites in other parts of the Americas.
“In Brazil, there are several places where you have stone tools that seem robust to me and have a date of 26-30,000, dates similar to the Chiquihuite site,” said Professor Higham.
“This could be an important discovery that could stimulate new work to find other sites in the Americas dating from this period.”
A different perspective
Professor David Meltzer, of Southern Methodist University in Dallas, Texas, who was not involved in the research, said the findings were “interesting.” But he explained: “It is not enough to argue that the stone specimens could be cultural (artifacts), one has to demonstrate that they are not natural.” Natural processes could mimic some types of stone tools, Professor Meltzer said.
Second, he explained: “With such a long-lasting stone tool tradition, one hopes it would have been much more widespread in the region, raising the question of why that technology has not been seen elsewhere,” adding: ” Perhaps more importantly, with modern humans one expects to see evidence of technological and cultural changes over such a long period of time. “
Finally, he said, “the cave is 1,000 meters above the valley floor, but leaving aside the issue of why not camp closer to the valley floor, why keep returning to that same place ‘relatively steadily’ during So long a period of time? I find it funny. Not many sites have that kind of long-term repeated occupation unless there’s something pretty useful / available on site. “
Travel options
Between 26,000-19,000 years ago, sea level was low enough for people to easily cross from Siberia to America via the Beringia land bridge. But, what happens during the first times?
“Before 26,000 years ago, the latest data suggests that Beringia may have been an unattractive place for humans. It may well have been swampy and very difficult to traverse,” said Professor Higham.
“We still believe that the most likely scenario is that people have traveled a coastal route, hugging a coast, perhaps with some form of maritime technology.”
While people appear to have been in the Americas before 26,000 years ago, they were probably thin on the ground. It is only much later, between 14,000 and 15,000 years ago, that populations increase substantially.
It coincides with the temperature peak at the end of the last Ice Age, when jumps of 7 ° C are observed in the space of two to three years.
American natives
Scientists also used “environmental DNA” techniques to search for human genetic material in the cave’s sediments.
But they couldn’t find a strong enough signal.
Previous DNA evidence has shown that the Clovis people shared many similarities with modern Native Americans.
And scientists will now want to understand how these larger populations relate to later human groups that inhabited the continent.
In the same issue of Nature, Professor Higham and Lorena Becerra-Valdivia, also from Oxford, describe how they used the ages of 42 archaeological sites in North America and Beringia to explore how humans expanded.
The results reveal the sign of a human presence that dates back thousands of years before the Clovis people.
Follow Paul on Twitter.
