Ancient humans may have inhabited North America nearly 30,000 years ago, a new study suggests — twice as long as archaeologists have conventionally believed.

Archeology holds that North America was populated about 15,000 years ago by people who crossed over a newly formed land bridge after the last Ice Age, who are known as the Clovis people because of the type of arrowheads they made.

But a study published Wednesday in Nature details findings in a cave high in the mountains of northern Mexico that revealed nearly 2,000 stone tools. Analysis of the artifacts in Chiquihuite Cave, as well as DNA analysis of the surrounding sediments, dates humans to between 25,000 and 30,000 years ago.

“For decades people have passionately debated when the first humans entered the Americas,” said DNA scientist Professor Eske Willerslev, of St John’s College at the University of Cambridge, in a statement. “Chiquihuite Cave will create a lot more debate, as it is the first site that dates the arrival of people to the continent to around 30,000 years ago — 15,000 years earlier than previously thought.”

The researchers identified DNA from black bears, rodents, bats, voles and kangaroo rats, said Mikkel Winther Pedersen, a geneticist from the University of Copenhagen and one of the study’s lead authors.

No human DNA was found in the cave, leading scientists to surmise that they had visited it periodically, for short periods of time.

“We think people spent part of the year there using it as a winter or summer shelter, or as a base to hunt during migration,” said Willerslev, who is also director of The Lundbeck Foundation GeoGenetics Centre at the University of Copenhagen. “This could be the Americas’ oldest ever hotel.”

The prevailing theory for decades has been that these people migrated via a land bridge between Siberia and Alaska, then were stopped by the remaining Ice Age glaciers and settled in Beringia for several thousand years before making their way south.

However, mounting evidence suggests that hunting and other human activity was under way long before the Clovis would have arrived. Even so, the earliest date was thought to be 15,000 years ago.

Indigenous Peoples in the United States commonly refer to themselves as having been here since “time immemorial,” and Indigenous scholars have long called the Clovis-Beringia theory into question.

Moreover, a 2016 study, also in Nature, showed that the land bridge the Clovis were thought to have crossed would not have supported such migration until much later, as The Christian Science Monitor reported at the time.

“By the time the famous Clovis population entered America, the very early Americans had disappeared thousands of years before,” said Ciprian Ardelean of the University of Zacatecas in Mexico in a statement on the current study. “There could have been many failed colonizations that were lost in time and did not leave genetic traces in the population today.”

The evidence does not qualify as definitive proof of their existence, anthropology professor Tom Dillehay of Vanderbilt University told the Associated Press. Though not involved in the study, he said items like hearths, butchered bones and burned edible plant remains would be more indicative of human occupation.

The researchers agreed that these early inhabitants left scant clues.

“We don’t know who they were, where they came from or where they went,” Ardelean said in the researchers’ statement. “They are a complete enigma. We falsely assume that the Indigenous populations in the Americas today are direct descendants from the earliest Americans, but now we do not think that is the case.”

At the very least, the findings upend long-held notions, Pedersen said.

“The location of Chiquihuite Cave definitely rewrites what has conventionally been taught in history and archeology and shows that we need to rethink where we look for sites of the earliest people in Americas,” he said.