A massive haul of stone tools discovered in a cave in Mexico provide evidence that people occupied the area more than 30,000 years ago, suggesting that humans arrived in North America at least 15,000 years earlier than had been previously thought. The discovery is backed up by a separate statistical analysis incorporating data from sites in North America and Siberia.

But some researchers are not convinced. They question the age of the tools, and whether the artefacts are tools or rather created by natural processes. Data from caves are “notoriously troublesome” to interpret, says archaeologist François Lanoë from the University of Arizona in Tucson.

First Humans in America

The first humans in the Americas came from East Asia, but when the date of their arrival remains still debatable. Some researchers suggest that it could have been as early as 130,000 years ago, but lacking the archaeological evidence, this theory is disputed. Many stone artefacts are so simple that sceptics say they were probably produced by natural geological processes and not by humans.

The mainstream consensus is that the people ff the Americas began about 15,000 or 16,000 years ago — based on genetic evidence and artefacts found at sites including the 14,000-year-old Monte Verde II in Chile.  The latest discoveries, published on 22 July in Nature, question that consensus.

New evidence from Chiquihuite Cave in Mexico joins other sites across the Americas where scientists have found signs of early human occupation (kyr, a thousand years ago) / Nature

Since 2012, a team led by Ciprian Ardelean at the Autonomous University of Zacatecas in Mexico has been excavating Chiquihuite Cave in Astillero Mountains. The researchers found 2,000 stone tools, 239 of which were embedded in layers of gravel that have been carbon dated to between 25,000 and 32,000 years old.

Caves Occasional Camps

There are some tools that Ardelean thinks suggest the site could have been visited only occasionally, perhaps as a refuge every few decades, during severe winters. At the height of the last ice age, 26,000 years ago, North America would have been a dangerous place. “There must have been horrible storms, hail, snow,” he says. He adds that the Chiquihuite Cave is well insulated and could have provided shelter to any humans who were around to witness the blizzards.

Other controversial studies claim that humans reached Americas 100,000 years earlier than thought but the analysis was disputed pointing out that it purposely omitted information from the most controversial sites, to make its case stronger. If there were people in North America so early, it’s unclear what happened to them.

“There continues to be no convincing genetic evidence of a pre-15,000-years-ago human presence in the Americas,” says geneticist David Reich at Harvard Medical School in Boston, Massachusetts

However, Ardelean says there is a simple reason why genetic studies suggest that humans spread across the Americas only relatively recently, and early groups such as the one he thinks was present at Chiquihuite Cave didn’t survive to contribute to modern gene pools. “I definitely advocate for the idea of lost groups,” he says.