Tools excavated from a cave in central Mexico are strong evidence that humans were living in North America at least 30,000 years ago, some 15,000 years earlier than previously thought, scientists said Wednesday.

The artefacts, including 1,900 stone tools, showed human occupation of the high-altitude Chiquihuite Cave over a 20,000 year period, they reported in two studies published the journal Nature

"Our results provide new evidence for the antiquity of humans in the Americas," Ciprian Ardelean, an archaeologist at the Universidad Autonoma de Zacatecas and lead author of one of the studies, told AFP. "There are only a few artefacts and a couple of dates from that range," he said, referring to radiocarbon dating results putting the oldest samples at 33,000 to 31,000 years ago. "However, the presence is there."

No traces of human bones or DNA were found at the site. "It is likely that humans used this site on a relatively constant basis, perhaps in recurrent seasonal episodes part of larger migratory cycles," the study concluded.

The saga of how and when Homo sapiens arrived in the Americas -- the last major land mass to be populated by our species -- is fiercely debated among experts, and the new findings will likely be contested. "That happens every time that anybody finds sites older than 16,000 years -- the first reaction is denial or hard acceptance," said Ardelean, who first excavated the cave in 2012 but did not discover the oldest tools until 2017.

Until recently, the widely accepted storyline was that the first humans to set foot in the Americas crossed a land bridge from present-day Russia to Alaska some 13,5000 years ago and moved south through a corridor between two massive ice sheets.

Archaeological evidence -- including uniquely crafted spear points used to slay mammoths and other prehistoric megafauna -- suggested this founding population, known as Clovis Culture, spread across North America, giving rise to distinct native American populations. 

But the so-called Clovis-first model has fallen apart over the last two decades with the discovery of several ancient human settlements dating back two or three thousand years before earlier.

Moreover, the tool and weapon remnants at these sites were not the same, showing distinct origins. "Clearly, people were in the Americas long before the development of Clovis technology in North America," said Gruhn, an anthropology professor emerita at the University of Alberta, in comments on the new findings.