The question of when humans first set foot in the Americas is one of the most controversial and contested debates in anthropology. Now, two new studies have weighed in with evidence that people were living on the North American continent as far back as 33,000 years ago.

This is much earlier than previously assumed. Until recently, the commonly held view was that the earliest inhabitants in the Americas were a single group known as the "Clovis culture” that settled in the continent around 15,000-13,000 years ago. While that date has been pushed back by a number of archaeological finds in the past few decades, the currently popular date stands at around 16,000 years ago. However, the two new studies offer up evidence that pushes this date back again, this time much further.

In the first of the two studies published in Nature, archaeologists detail excavations at Chiquihuite Cave in Zacatecas, central Mexico, which contains an array of some 2,000 stone tools, plant remains, and environmental DNA. Dating of the site suggests the cave was inhabited by humans seasonally 25,000-33,000 years ago.

"These early visitors didn't occupy the cave continuously, 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 Professor Eske Willerslev of St John's College, University of Cambridge. "This could be the Americas' oldest ever hotel." 

The second study used radiocarbon from 42 archaeological sites in North America and Beringia, a land bridge that formerly linked northern Asia and North America until it became covered by sea some 11,000 years ago. Together with genetic and climatic evidence, a mathematical model showed that humans were most likely present across America before the Last Glacial Maximum about 26,000 to 19,000 years ago. 

“The First Americans are popularly believed to have arrived in the continent between 16,000 and 13,000 years ago,” said lead author Dr Lorena Becerra-Valdivia. “Our findings show evidence of humans around 15,000 years before then.”

Not only does the new research challenge when humans first arrived in America, but it also challenges many ideas about how they reached the continent. It’s long been postulated that humans reached America by crossing the Beringia land bridge from modern-day Russia. However, the presence of early human settlements by the coast suggests that people migrated along the shores of the Pacific Ocean. If this is the case, it's fair to speculate that even older archaeological sites may have once existed along the coast, but have since become submerged by water following the rise in sea level at the end of the last Ice Age. 

The new studies are almost certainly going to prove controversial to many archeologists. Some of the data used in the second study come from a number of Brazilian archaeological sites in the state of Piauí dated to around 20,000 years ago. Although these sites have been expertly analyzed, they are hotly disputed and many argue they are too old to be considered valid.

“[The] suggestion that the initial entry date was as far back as 33,000 years ago, which is more than double the currently popular date of around 16,000 years ago, will be very hard for most archaeologists specializing in early America to accept,” Ruth Gruhn, Professor Emerita in the Department of Anthropology at the University of Alberta, who was not involved in the research, writes in an accompanying article published in Nature.

“There will undoubtedly be challenges to this interpretation and close examination of the site data.”

Regardless of how these studies will go down, it’s certainly clear that the peopling of America was long before we previously believed. How long before, however, remains unclear.