Humans May Have Reached Americas 15,000 Years Earlier Than Previously Thought

Archaeologists collecting samples for ancient DNA analyses at Chiquihuite Cave. Pictured from left, Dr. Juan I. Macías-Quintero, Martín Martínez-Riojas, Prof. Eske Willerslev and Dr. Mikkel Winther Pedersen. (Image: Devin A. Gandy)
Archaeologists collecting samples for ancient DNA analyses at Chiquihuite Cave. Pictured from left, Dr. Juan I. Macías-Quintero, Martín Martínez-Riojas, Prof. Eske Willerslev and Dr. Mikkel Winther Pedersen. (Image: Devin A. Gandy)

The arrival of humans in the Americas may predate the peak of the last ice age, new research shows.  Humans may have reached the Americas over 30,000 years ago, new research from international teams of scientists shows — a period 15,000 years earlier than previously thought.

However, the human impact on now-extinct large mammals (megafauna) occurred much later, when populations significantly increased. These findings — published across two papers in Nature Today — are based on archaeological research at the Chiquihuite Cave in central Mexico and statistical modeling of dates from 42 archaeological sites, including from Chiquihuite Cave. Dr. Lorena Becerra-Valdivia, a postdoctoral research fellow at UNSW Sydney who completed this research, said:

The archaeological team discovered nearly 2,000 stone tools and other stone pieces at Chiquihuite Cave, which has been under excavation for almost a decade. The artifacts belong to a type of material culture never seen in the Americas, suggesting a previously unknown lithic (or stone) industry.

A stone tool found below the Last Glacial Maximum rock layer, belonging to a material culture never seen in the Americas. (Image: Dr Ciprian Ardelean)

A stone tool found below the Last Glacial Maximum rock layer, belonging to a material culture never seen in the Americas. (Image: Dr. Ciprian Ardelean)

The scientists used radiocarbon dating to find the ages of evidence from the site, like bone, charcoal, and sediment DNA. The luminescence dating method — which measures the light emitted from energy in certain rocks and soils — was also used on some samples. More than 50 dates were pulled from the excavation site, with the oldest cultural evidence dating to the Last Glacial Maximum (LGM) — the peak of the Ice Age — 26,000 to 18,000 years ago.

Located in the mountains, Chiquihuite Cave is unusually high compared to other archaeological sites in the Americas. (Image: Thomas L.C. Gibson)

Located in the mountains, Chiquihuite Cave is unusually high compared to other archaeological sites in the Americas. (Image: Thomas L.C. Gibson)

But statistical modeling estimates that humans occupied the site even earlier than that. Dr. Becerra-Valdivia, who is an archaeological scientist and radiocarbon dating expert at UNSW’s Chronos 14 Carbon-Cycle Facility, explained:

A failed colonization

The Chiquihuite Cave is a high-altitude site located in the Astillero Mountains in northern-central Mexico. Sitting at 2,750 meters above sea level, its height is unusual compared to other archaeological sites in the Americas: most are open sites, megafauna kill-sites, or shallow rock shelters.

Dr Ciprian Ardelean, who led the Chiquihuite Cave archaeological excavations, suggests the people of Chiquihuite represent a ‘failed colonisation’, bearing no genetic resemblance to today's First Americans populations. (Image: Devin A. Gandy)

Dr. Ciprian Ardelean, who led the Chiquihuite Cave archaeological excavations, suggests the people of Chiquihuite represent a ‘failed colonization,’ bearing no genetic resemblance to today’s First Americans populations. (Image: Devin A. Gandy)

The location of the cave — and its antiquity — challenge commonly held models in First Americans research. The University of Zacatecas’ Dr. Ciprian Ardelean, who led the archaeological excavations, said:

Dr. Jean-Luc Schwenninger, senior co-author and head of the Luminescence Dating Laboratory at the University of Oxford, who has been dating the site for almost a decade, said:

A population boom

Dr. Becerra-Valdivia and Prof. Tom Higham, also from the University of Oxford, used the Chiquihuite Cave findings to build a detailed chronological framework of the arrival and dispersal of humans into North America. They statistically combined the 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. Dr. Becerra-Valdivia said:

Dr Lorena Becerra-Valdivia (centre) sampling material for radiocarbon dating alongside fellow researchers Prof. Joaquín Arroyo-Cabrales and Dr Juan I. Macías-Quintero (from left to right). (Image: Devin A. Gandy)

Dr. Lorena Becerra-Valdivia (center) sampling material for radiocarbon dating alongside fellow researchers Prof. Joaquín Arroyo-Cabrales and Dr. Juan I. Macías-Quintero (from left to right). (Image: Devin A. Gandy)

The researchers hope that more archaeological investigations in Central and South America will allow for continent-wide models to be developed. Prof. Higham said:

Uncovering new stories

The chronological framework dates the start of human occupation across North America, as well as the beginnings of three distinct stone tool traditions. So far, the earliest evidence for cultural occupation is at the Chiquihuite Cave. Prof. Higham said:

To develop their chronological framework, the researchers used a statistical approach known as Bayesian age modeling, which was performed on software (OxCal) developed in Oxford by Prof. Christopher Bronk Ramsey. They combined the dates from Chiquihuite Cave with hundreds of dates found in archaeological sites across North America and Beringia, the ancient land bridge connecting the continent to Asia.

Dr Lorena Becerra-Valdivia working near the entrance of Chiquihuite Cave. (Image: Thomas L.C. Gibson)

Dr. Lorena Becerra-Valdivia working near the entrance of Chiquihuite Cave. (Image: Thomas L.C. Gibson)

The framework shows that although humans were likely present in the region before, during, and after the LGM, widespread human occupation probably began much later, during a period of abrupt global climate warming. Dr. Becerra-Valdivia, who was also a co-author on the Chiquihuite Cave archaeological paper, said:

The disappearance of now-extinct megafauna, like mammoths and types of horses and camels, happened at broadly the same time as human expansion during this warmer period — that is, between 14,700 and 12,900 years ago. The authors suggest that the increase in the human population seems to be linked to a significant impact on the catastrophic decline of these large megafauna.

Provided by: University of New South Wales [Note: Materials may be edited for content and length.]

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