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, double the generally accepted age for the first colony in the Americas.
The results are based on work done at Chiquihuite Cave, a high altitude rock shelter in central Mexico.
Archaeologists have found thousands of stone tools suggesting that the cave has been used by people for at least 20,000 years.
During the second half of the 20th century, a consensus emerged among North American archaeologists that the Clovis people had first reached the Americas some 11,500 years ago.
The Clovis are believed to have crossed a land bridge connecting Siberia to Alaska during the last ice age.
This land bridge subsequently disappeared underwater when the ice melted.
And these big game hunters are believed to have contributed to the extinction of megafauna – large mammals such as mammoths, mastodons and various species of bears that roamed the region until the end of the last Ice Age.
As the idea of ’Clovis First’ took hold, reports of earlier human settlements were dismissed as unreliable and archaeologists stopped looking for signs of earlier occupation.
But in the 1970s, this orthodoxy began to crumble.
In the 1980s, strong evidence of a 14,500-year-old human presence in Monte Verde, Chile, emerged.
And since the 2000s, other pre-Clovis sites have become widely accepted – including the 15,500-year-old Buttermilk Creek site in central Texas.
Now, Ciprian Ardelean, of the Universidad Autónoma de Zacatecas, Mexico, Tom Higham, of the University of Oxford, and his colleagues have found evidence of human occupation that goes back well beyond that date, on the Chiquihuite site in the highlands of north-central Mexico.
The results were published in the journal Nature.
“It’s a unique site, we’ve never seen anything like it before,” Professor Higham, director of the Radiocarbon Accelerator Unit at Oxford, told BBC News.
“The evidence for the stone tool is very, very convincing.
“Anyone can see that these are purposefully made stone tools and there are a lot of them.
“Dating – which is my job – is robust.
“And so, this is a very exciting site to participate in. “
The team dug a stratigraphic section 3 m deep (10 feet) – a sequence of layers of soil arranged in the order they were laid – and found some 1,900 stone artifacts from thousands of years.
The researchers were able to date the bones, charcoal 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 decay over time.
The second, optically stimulated luminescence (OSL), works by measuring the last time the sediment was exposed to light.
Using two different techniques “added a lot of credibility and strength, especially to the older part of the timeline,” Professor Higham said.
“Optical dates and [radiocarbon] the dates are in good agreement, ”he said.
And the findings could lead scientists to take a fresh look at the controversial early occupation sites elsewhere in the Americas.
“In Brazil there are several sites where you have stone tools that I think are sturdy and are dated 26-30,000, dates similar to the Chiquihuite site,” Professor Higham said.
“This could be an important discovery that could spur new work to find other sites in the Americas dating from this period. “
Scientists also used “environmental DNA” techniques to search for human genetic material in the sediments of the cave.
But they couldn’t find a strong enough signal.
Previous DNA evidence has shown that the Clovis settlers shared many similarities with modern Native Americans.
And scientists will now want to understand how these older populations relate to later human groups that inhabited the continent.
In the same issue of Nature, Professor Higham and Lorena Becerra-Valdivia, also of Oxford, describe how they used ages from 42 archaeological sites in North America and Beringia (the name of the region that once connected Siberia and America) to explore how humans extended.
The results reveal a signal of a pre-Clovis human presence, dating back to the 19,000-26,000 year period known as the Last Ice Maximum, when the ice caps were at their greatest extent.
Follow Paul on Twitter.