Pieces of limestone from a cave in Mexico may be the oldest human tools ever found in the Americas, and suggest people first entered the continent up to 33,000 years ago – much earlier than previously thought.
The findings, published Wednesday in two papers in the journal Nature, which include the discovery of the stone tools, challenge the idea that people first entered North America on a land bridge between Siberia and Alaska and an ice-free corridor to the interior of the continent.
Precise archaeological dating of early human sites throughout North America, including the cave in Mexico, suggests instead that they may have entered along the Pacific coast, according to the research.
Ciprian Ardelean, an archaeologist with the Autonomous University of Zacatecas in Mexico, the lead author of one of the papers, said the finds were the result of years of careful digging at the Chiquihuite Cave in north-central Mexico.
The steeply-inclined cave is high on a mountainside and filled with crumbling layers of gravel: “The deeper you go, the higher the risk for the walls to collapse,” he said.
The excavations paid off with the discovery of three deliberately-shaped pieces of limestone — a pointed stone and two cutting flakes — that may be the oldest human tools yet found in the Americas.
They date from a time when the continent seems to have been occupied by only a few groups of early humans – perhaps “lost migrations” that left little trace on the landscape and in the genetic record, Ardelean said.
The tools were found in the deepest layer of sediment they excavated, which dates from up to 33,000 years ago – long before the last Ice Age, which occurred between 26,000 and 19,000 years ago.
The commonly accepted time for the arrival of the first people in North America is about 16,000 years ago, and recent studies estimate it happened up to 18,000 years ago. But the latest discoveries push the date back by more than 10,000 years.
Ardelean said even reaching the cave was a challenge for the team.
“You have to live there and cook there, because it takes you a whole day to go back and forth from the town, and it’s a five-hour climb,” he said. “It is a logistical nightmare.”
More tools were found in sediments laid down during and after the Ice Age, and indicate the cave was occupied for short periods over thousands of years, maybe by nomadic people who knew of it from ancestral legends.
“I think it was a refuge used occasionally and periodically,” Ardelean said. “Even if you never saw the site before, your grandparents had told you about it and there were indications when you got there.”
The presence of stone tools from the Ice Age – known to archaeologists as the Last Glacial Maximum, or LGM – suggested people occupied the cave even before that.
Much of North America was then covered with thick ice sheets that would have made migrations impossible, he said: “If you have people during the LGM, it is because they entered the continent before the LGM.”
Lorena Becerra-Valdivia, an archaeological scientist at the University of Oxford and the University of New South Wales, and Thomas Higham, a radiocarbon dating specialist at the University of Oxford, compared the dates from the cave sediments with other archaeological sites in North America.
Their research indicates very small numbers of humans probably lived in parts of North America before, during and immediately after the last Ice Age, but the human population grew much larger after a period of abrupt global warming that began about 14,700 years ago.
The study also suggested some people had entered the Americas before 29,000 years ago, possibly along the Pacific coast, when the land bridge between Siberia and Alaska was completely or partially submerged, Becerra-Valdivia said.
Ardelean hopes archaeologists will now look for evidence of human occupation from the earliest dates proposed in the new studies.
He thinks stone tools and perhaps ancient human DNA from that time could be hidden beneath traces of the larger human population that lived in North America after the Ice Age.
“Instead of stopping digging when you reach a certain layer… you need to go as deep as possible, because there are things down there,” he said.
Other scientists are cautious about the implications of the new research.
Anthropologist Matthew Des Lauriers of California State University, San Bernardino, who was not involved in the studies, said they “pushed the boundaries” of knowledge about the earliest human arrival in the Americas.
But he questioned how ancient people who had been in the Americas for more than 25,000 years could have remained “archaeologically invisible” for over 10,000 years.
He said that archaeologists in Australia and Japan, for example, had no difficulty finding evidence of human occupation from that time.
“Archaeologists in the Americas have either been doing things very wrong for the last 90 years, or we have here [an] anomaly that must be accounted for,” he said.