Paleo-Americans?

It is no secret that I have long been on skeptical side of the debate around whether modern humans were present in the Americas outside the Beringian land bridge between Siberia and Alaska until the tail of of the ice age that produced the Last Glacial Maximum about 26,000 years ago. The pre-Clovis arrival of members of the Founding population of the Americas to North and South America via a Pacific route became part of the paradigm long ago. 

But evidence of a hominin presence in the Americas during or before the Last Glacial Maximum ice age has relied on very small numbers of objects that are only arguably stone tools at various sites, with equivocal attempts at dating them, and no human remains. 

Two new articles in the leading scientific journal Nature may tip the balance, although I am still not unequivocally convinced, in part, because the studies fail to address by this pilot wave founding population would have had so slight an archaeological and ecological impact.

An account at NBC News explains in an article that is mostly correct and makes only subtle errors where it is not quite right:

Pieces of limestone from a cave in Mexico may be the oldest human tools ever found in the Americas, and suggest people first entered the continent up to 33,000 years ago – much earlier than previously thought. 

The findings, published Wednesday in two papers in the journal Nature, which include the discovery of the stone tools, challenge the idea that people first entered North America on a land bridge between Siberia and Alaska and an ice-free corridor to the interior of the continent.

Precise archaeological dating of early human sites throughout North America, including the cave in Mexico, suggests instead that they may have entered along the Pacific coast, according to the research. . . . 

The commonly accepted time for the arrival of the first people in North America is about 16,000 years ago, and recent studies estimate it happened up to 18,000 years ago. But the latest discoveries push the date back by more than 10,000 years. 

The NBC article linked about says this about the first article discussing an excavation deep into a cave high on a mountain in Mexico: 

Ciprian Ardelean, an archaeologist with the Autonomous University of Zacatecas in Mexico, the lead author of one of the papers, said the finds were the result of years of careful digging at the Chiquihuite Cave in north-central Mexico. 

The steeply-inclined cave is high on a mountainside and filled with crumbling layers of gravel: “The deeper you go, the higher the risk for the walls to collapse,” he said. 

A shaped limestone point, one of the stone tools found at the Chiquihuite Cave in central Mexico that archaeologists think dates from around 30,000 years ago, before the last Ice Age.

The excavations paid off with the discovery of three deliberately-shaped pieces of limestone — a pointed stone and two cutting flakes — that may be the oldest human tools yet found in the Americas. 

They date from a time when the continent seems to have been occupied by only a few groups of early humans – perhaps “lost migrations” that left little trace on the landscape and in the genetic record, Ardelean said. 

The tools were found in the deepest layer of sediment they excavated, which dates from up to 33,000 years ago – long before the last Ice Age, which occurred between 26,000 and 19,000 years ago. . . . 

“You have to live there and cook there, because it takes you a whole day to go back and forth from the town, and it’s a five-hour climb,” he said. “It is a logistical nightmare.”

More tools were found in sediments laid down during and after the Ice Age, and indicate the cave was occupied for short periods over thousands of years, maybe by nomadic people who knew of it from ancestral legends. 

The Chiquihuite Cave is high on a mountain, at an altitude of above 8,800 feet, and the interior is very steep. . . . 

“I think it was a refuge used occasionally and periodically,” Ardelean said. “Even if you never saw the site before, your grandparents had told you about it and there were indications when you got there.” 

The presence of stone tools from the Ice Age – known to archaeologists as the Last Glacial Maximum, or LGM – suggested people occupied the cave even before that. 

Much of North America was then covered with thick ice sheets that would have made migrations impossible, he said: “If you have people during the LGM, it is because they entered the continent before the LGM.” 

The first article and its abstract are:

The initial colonization of the Americas remains a highly debated topic, and the exact timing of the first arrivals is unknown. The earliest archaeological record of Mexico—which holds a key geographical position in the Americas—is poorly known and understudied. Historically, the region has remained on the periphery of research focused on the first American populations. 

However, recent investigations provide reliable evidence of a human presence in the northwest region of Mexico, the Chiapas Highland, Central Mexico and the Caribbean coast, during the Late Pleistocene and Early Holocene epochs. Here we present results of recent excavations at Chiquihuite Cave—a high-altitude site in central-northern Mexico—that corroborate previous findings in the Americas of cultural evidence that dates to the Last Glacial Maximum (26,500–19,000 years ago), and which push back dates for human dispersal to the region possibly as early as 33,000–31,000 years ago. 

The site yielded about 1,900 stone artefacts within a 3-m-deep stratified sequence, revealing a previously unknown lithic industry that underwent only minor changes over millennia. More than 50 radiocarbon and luminescence dates provide chronological control, and genetic, palaeoenvironmental and chemical data document the changing environments in which the occupants lived. 

Our results provide new evidence for the antiquity of humans in the Americas, illustrate the cultural diversity of the earliest dispersal groups (which predate those of the Clovis culture) and open new directions of research.

Ardelean, C.F., Becerra-Valdivia, L., Pedersen, M.W. et al. "Evidence of human occupation in Mexico around the Last Glacial Maximum." Nature (July 22, 2020). https://doi.org/10.1038/s41586-020-2509-0 (citations in abstract omitted).

The NBC article linked about says this about the second article:  

Lorena Becerra-Valdivia, an archaeological scientist at the University of Oxford and the University of New South Wales, and Thomas Higham, a radiocarbon dating specialist at the University of Oxford, compared the dates from the cave sediments with other archaeological sites in North America. 

Their research indicates very small numbers of humans probably lived in parts of North America before, during and immediately after the last Ice Age, but the human population grew much larger after a period of abrupt global warming that began about 14,700 years ago. 

The study also suggested some people had entered the Americas before 29,000 years ago, possibly along the Pacific coast, when the land bridge between Siberia and Alaska was completely or partially submerged, Becerra-Valdivia said. . . . 

Anthropologist Matthew Des Lauriers of California State University, San Bernardino, who was not involved in the studies, said they “pushed the boundaries” of knowledge about the earliest human arrival in the Americas. 

But he questioned how ancient people who had been in the Americas for more than 25,000 years could have remained “archaeologically invisible” for over 10,000 years.

He said that archaeologists in Australia and Japan, for example, had no difficulty finding evidence of human occupation from that time. 

The second article and its abstract are: 

The peopling of the Americas marks a major expansion of humans across the planet. However, questions regarding the timing and mechanisms of this dispersal remain, and the previously accepted model (termed ‘Clovis-first’)—suggesting that the first inhabitants of the Americas were linked with the Clovis tradition, a complex marked by distinctive fluted lithic points—has been effectively refuted. 

Here we analyse chronometric data from 42 North American and Beringian archaeological sites using a Bayesian age modelling approach, and use the resulting chronological framework to elucidate spatiotemporal patterns of human dispersal. We then integrate these patterns with the available genetic and climatic evidence. 

The data obtained show that humans were probably present before, during and immediately after the Last Glacial Maximum (about 26.5–19 thousand years ago) but that more widespread occupation began during a period of abrupt warming, Greenland Interstadial 1 (about 14.7–12.9 thousand years before AD 2000). 

We also identify the near-synchronous commencement of Beringian, Clovis and Western Stemmed cultural traditions, and an overlap of each with the last dates for the appearance of now-extinct faunal genera. Our analysis suggests that the widespread expansion of humans through North America was a key factor in the extinction of large terrestrial mammals.

Lorena Becerra-Valdivia, Thomas Higham, "The timing and effect of the earliest human arrivals in North America" Nature (July 22, 2020) DOIhttps://doi.org/10.1038/s41586-020-2491-6 (citations in abstract ommitted).