Humans may have reached the Americas 30,000 years ago – some 15,000 years earlier than previously thought – according to new research from two international teams of scientists.
However, they suggest that human impact on now extinct large mammals (megafauna) occurred much later, when populations had significantly increased.
The findings – published in two papers in the journal Nature – are based on archaeological research at the Chiquihuite Cave in central Mexico and statistical modelling of dates from 42 sites.
The cave is at altitude – 2750 metres above sea level in the Astillero Mountains – which is unusual for the Americas.
The archaeological team, which was led by Ciprian Ardelean from Mexico’s Universidad Autónoma de Zacatecas, excavated nearly 2000 stone tools from the cave, along with plant remains and environmental DNA. The artefacts belong to a type of material culture never seen in the Americas, they say, suggesting a previously unknown stone industry.
Lorena Becerra-Valdivia, now at Australia’s University of NSW, and Thomas Higham, from the University of Oxford, UK, then used radiocarbon and luminescence ages from 42 sites in North America and Beringia – the ancient land bridge connecting America to Asia – to determine patterns of human dispersal.
They produced a statistical model that reveals a robust signal of human presence dating to at least the Last Glacial Maximum (26,000 to 19,000 years ago) and immediately after.
“Human presence occurs before an archaeological site is created,” says Becerra-Valdivia.
“Using the archaeological evidence and Bayesian age modelling – a powerful tool that incorporates dates and archaeological evidence through statistics – we can estimate humans arrived at Chiquihuite Cave as early as 33 to 31,000 years ago.”
The findings do not neatly fit with a scenario in which humans first entered North America from Asia via Beringia, before heading south and developing the Clovis (stone tool) culture.
The new dates are pre-Clovis and suggest, the researchers say, that humans may have first entered the continent via a route along the Pacific Coast.
“The finds at Chiquihuite Cave are extremely exciting,” says Ardelean. “The archaeology is older than anything we have seen before and the stone tools are of a type that is unique in the Americas.
“Human-made flaked stone artefacts are there by the thousands, embedded in layered sedimentary deposits that are now well-dated.
“It is curious that the site was occupied so much earlier than others. It seems likely to us that the people of Chiquihuite represent a ‘failed colonisation’, one which may well have left no genetically detectable heritage in today’s First Americans populations.”
Becerra-Valdivia and Higham say their chronological framework shows that while humans were likely present in the region before, during and after the Last Glacial Maximum, widespread occupation probably began much later, during a period of abrupt global climate warming.
“It was only around 14,700 years ago that those people became more highly visible in the archaeological record,” says Becerra-Valdivia. The disappearance of now extinct megafauna, including mammoths and types of horses and camels, happened at about the same time.
To develop the framework, the researchers combined dates with stratigraphic – or rock layer – information to estimate the start and end of human occupation at each of the sites. The dates were then plotted spatially across the continent.
“This approach involves taking a step back and looking at the entire picture to better understand what happened in the past,” says Becerra-Valdivia.