The discovery of a cave in the state of Zacatecas, in northern Mexico, gave a surprising turn to what was known about the time when the first human beings arrived in America.

Until now it was estimated that the Clovis, considered the first inhabitants of the continent, had reached it about 13,500 years ago.

But the evidence found in the so-called Chiquihuite cave, more than 2,700 meters above sea level, suggests that it was occupied by people approximately 30,000 to 13,000 years ago.

A multidisciplinary study, led by the Autonomous University of Zacatecas and published this Wednesday in the magazine Nature, based his conclusions on the analysis of the remains of hundreds of tools made with stone, animal bones, plants and sediments.

"This is a unique site, we have never seen anything like this before. The evidence from stone tools is very, very convincing. And the dating, which is my job, is solid"he said to BBC News Tom Higham, a professor at the British University of Oxford and a participant in the study.

However, some scientists questioned whether the tools found were actually made by humans and not created by some natural process.

Hundreds of stone artifacts

The archeologist Ciprian Ardelean, principal investigator of the study, arrived in 2010 at this cave after years of walking kilometers of mountains in the Concepción del Oro region in search of human evidence.

Located 1,000 meters above the valley floor, the cave has two interconnected chambers over 50 meters wide and 15 meters high each, with a floor full of stalagmites.

The team excavated a three-meter-deep section and found some 1,900 stone artifacts manufactured over thousands of years and showing one size and very unusual workFor example showing wider than long shapes.

It was thanks to two scientific techniques that the experts managed to determine the age of microscopic remains of bones, coal and sediments associated with the stones in which pollen and phytoliths were preserved.

One of them was by radiocarbon. The other method used was optically stimulated luminescence (measuring the last time the sediments were exposed to light).

Using two different techniques "added a lot of credibility and strength, particularly to the older part of the chronology, "according to Higham.

David meltzer, a professor at Southern Methodist University in Dallas, United States, who was not involved in the research, called the findings "interesting" but also raised questions.

"It is not enough to argue that stone samples could be cultural (artifacts), one has to demonstrate that they are not natural," he told The Scientist website, as natural processes can mimic some types of stone tools.

In addition, he also questioned whether the tradition of that stone tool making spanned so many years, "one hopes that it would have been much more widespread in the region, raising the question of why that technology has not been seen elsewhere. "

Greater "cultural diversity"

According to the National Institute of Anthropology and History (INAH) of Mexico – three of whose researchers participate as co-authors of the study -, the discoveries not only provide "conclusive evidence" about the greater antiquity of the human presence in America.

The evidences, highlights the organism, also indicate the "cultural diversity" of the first groups that were scattered across the continent.

Thus, Ardelean affirms that the relevance of the unknown work in discovered stone points to the fact that each human group followed its routes and faced the environment with particular responses and their own styles, highlights the INAH based on an expert interview published in Nature.

The scientific article highlights how the material selectivity when making the tools – 90% are recrystallized limestone – reflects the knowledge that these humans had of the values ​​of the stone within their reach and how they made their decisions based on it.

So far, cores, flakes, blades, scrapers, tips and adzes, among other elements, have been identified.

The researchers believe that the temperature inside the cave -about 12 degrees throughout the year, that the inhabitants would take advantage of especially to take shelter in winter- and that its main entrance was sealed by a collapse at the end of the Pleistocene contributed to the good conservation of the materials.

This made it possible to recover environmental DNA, which Ardelan defines as "DNA molecules dissolved in the earth from pollen, urine, hair, or dead cells."

Laboratory analyzes allowed to identify remains of plant species, charcoal and DNA from animals such as bats, rodents, marmots, goats and sheep. Remains of bones from bears and other species were also extracted.

What was thought until now?

The discovery in Zacatecas questions the hitherto existing consensus that the Clovis had been the first town to arrive in America approximately 11,000 or 13,000 years ago.

According to the most widespread theories, these humans crossed a land bridge known as Beringia that linked Siberia with Alaska during the last ice age and that disappeared under water when the ice melted.

Also to the Clovis – in whom DNA studies found similarities with modern Native Americans – are attributed having contributed to the extinction of large mammals in the region like the mammoth and the mastodon due to their hunting.

However, the BBC Science editor recalls, Paul RinconThis theory began to be questioned at the end of the last century.

"In the 1980s, solid evidence emerged of a 14,500-year-old human presence in Monte Verde, Chile," he says.

"And since the 2000s, other pre-Clovis settlements have been widely accepted, including 15,500-year-old Buttermilk Creek in central Texas," adds Rincon.

Be that as it may, the findings in Zacatecas they could just be the start of new discoveries on when humans actually arrived in America, according to experts.

"In Brazil, there are several settlements where you have stone tools that seem robust to me and date back to 26,000 or 30,000 years ago, dates similar to those of the Chiquihuite cave," Higham explained.

"This could be an important discovery that could stimulate new work to find other settlements in the Americas dating from this period," he concluded. (I)

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