The earliest evidence for humans in the Americas

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.

Archaeologists found thousands of stone tools that suggest 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 North American archaeologists, the Clovis people had been the first to arrive in the Americas, about 11,500 years ago.

The Clovis were thought to have crossed a land bridge linking Siberia with Alaska during the last ice age.

This land bridge 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 crumble.

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 site in central Texas.

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.

“This is a unique site, we have never seen anything like this before,” said Professor Higham.

“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.

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.”

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 Clovis settlers 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.

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