Humans were living in a cave in Mexico about 30,000 years ago, archaeologists announced in a bombshell article published Wednesday in Nature. This is the latest discovery to knock the stuffing out of the “Clovis First” theory which suggests humans reached the only as late as about 15,000 years ago.
Other recent discoveries around North America have also indicated an earlier arrival date, but it’s the discovery of the stone tools and other evidence of human occupation in a cave in Mexico dating to at least 26,500 years ago that most changes the paradigm. It indicates that people reached the continent at the height of the Ice Age, not only afterward.
Separately, a statistical model of human dispersal factoring in genetic and climatic evidence, also reported in Nature on Wednesday, concluded that humans likely reached the Americas before the Last Glacial Maximum, which was about 26,000 years ago.
The bottom line is that the new discoveries do not necessarily support the scenario of people crossing the Beringia land bridge to North America from Asia, then spreading southward and establishing the Clovis culture (which is marked by distinctive fluted stone points). Possibly, the scientists postulate, they arrived via a route along the Pacific Coast.
Picky in Ice Age Mexico
The theory of pre-Clovis occupation of the Americas has been gaining credence with the discovery of multiple sites in the New World that date to well over 10,000 years. That in itself would indicate that either the migrants from Asia spread really fast, or that people arrived earlier than thought.
Just a few examples: Evidence, including shell fishhooks, indicates that Mexico’s Isla Cedros was occupied by 12,000 years ago; elsewhere in Mexico paleo-spelunkers were mining for ocher, also some 12,000 years ago.
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All this fresh evidence for pre-Clovis occupation of the Americas is augmented by a charming report published last week on coprolites (fossil feces) in caves in Oregon, identifying the ancient emissions as human and dating their arrival on the cave floor to over 12,000 years ago.
Then there’s Monte Verde in Chile, dated to about 14,500 years ago. And even that pales in comparison with sites such as Gault in Texas, where occupation may have begun around 26,000 years ago, and the Meadowcroft Rockshelter in Pennsylvania, where the indications date back 19,000 years ago.
Now in central Mexico, Ciprian Ardelean and colleagues report on stone tools, plant remains and environmental DNA in Chiquihuite Cave in Zacatecas, concluding it was occupied from about 30,000 years (or 33,000 years ago, according to the statistical paper) to 13,000 years ago.
In other words, during the Last Glacial Maximum 26,500 to 19,000 years ago, Mexico was occupied: that pushes back putative dating for human dispersal to the region to as early as 33,000 to 31,000 years ago, the authors say.
The cave in question is a big one with two main chambers and lies 2,740 meters (8,990 feet) above sea level and, more pertinently to their lifestyle, 1,000 meters above the valley floor below.
What were these early settlers doing at such heights? The megafauna they loved to eat were far below, and Pleistocene-era sites tend to be rock shelters in proximity to the habitat of prey animals. The archaeologists do not suggest an explanation. It may or may not be relevant that Denisovans conferred unto some extant Asian populations, such as the Tibetans, a tolerance for high altitude.
The archaeologists count 1,900 stone tools found in a 3-meter stratified sequence in the lofty cave, and note that the technology didn’t change much over that time – and that it was unique. “The flaked artifacts reflect a technological tradition that was previously unknown, and remains mostly unchanged over the sequence,” the team writes.
Evidently, the prehistoric denizens were picky: Of all the rock types around them, they chose to make their tools from green or blackish types of recrystallized limestone – which are actually rather rare in those parts. The raw material was not found in the cave, it had to have been brought there.
In an intriguing parallel to a (much, much older) site found in Israel, the archaeologists believe they have found evidence that the cave site served as a “school of rock” – teaching how to make stone tools. The evidence is blanks showing repeated failed blows and error repairs. This, they say, is a reliable indicator of “intentionality, expert guidance and learning.”
In any case, the stone tools attest to a unique lithic industry: nothing like it has been found in any other cultural complexes of the late Pleistocene or Early Holocene epochs in the Americas. That unique lithic technology and the unusual location of this habitat, at high altitude, suggest an unexpected level of cultural diversity.
As for the dating of this extraordinary find, it was based on radiocarbon dating of organics: bone, charcoal and in sediment. A different technology – optically stimulated luminescence – was used as a control.
As for DNA testing, the team noted a change in the plant life before and after the Late Glacial Maximum. Before and during this epoch, the area was more densely forested, including with pines. Afterward, the pines all but disappeared, though not entirely, and the dominant floral life was Agavoideae – probably a type of Joshua trees – and grasses.
They also found charcoal fragments in all the archaeological layers, though cannot distinguish between wildfires and human-made fireplaces at the entrance of the cave. Another thing found in all layers was bat and rodent DNA.
What they didn’t find was human DNA, but that isn’t unusual in such contexts – and it wasn’t the bats or rats that were flaking stone tools.
It also seems likely that the region was only sparsely inhabited, by humans at least, at this early stage. And absent genetic information on the people or much else, it is impossible to suggest at this stage what happened to these people, and if the latter-day peoples of the Americas include their descendants or whether they went extinct.
The mark of humanity: Extinction
Separately, Lorena Becerra-Valdivia of the University of New South Wales and Thomas Higham of the University of Oxford analyzed data from 42 North American and Beringian archaeological sites to produce a statistical model of human dispersal.
Based on the available genetic and climatic evidence, they conclude that humans were likely present in the Americas before the Last Glacial Maximum, albeit sparsely. More widespread occupation only began with a period of warming as the Ice Age waned, from about 14,700 years ago, they believe.
They add that during the glacial maximum, the climate south of the ice sheet was relatively temperate.
And as the humans spread, Becerra-Valdivia and Higham deduce, the megafauna died out. The two suspect this was not coincidence.
It has been separately shown that human beings ate big animals when they could, though they would not cavil at consuming littler ones. Basically, the humans desperately needed the fat that big animals have in abundance and little animals like birds and rabbits do not.
The extinction of most of the world’s megafauna, starting around 130,000 years ago and accelerating in the late Pleistocene, cannot be blamed on one factor acting alone. Climate change surely did for some species that couldn’t adapt, but there is also no question that humans were responsible for at least some cases.
In the Americas, “Human presence in the continent precedes the majority of the dates for the last appearances of extinct genera,” the authors write. These include the American camel which lasted through to the Clovis epoch, and mammoths: yes, we met them there and ate them. The timing of their extinction and human spread is hard to take as coincidental. “This raises the distinct possibility that widespread human expansion in population and space was a key factor in the extinction of large terrestrial mammals,” the authors state.
The bottom line is that the Clovis First model is dead – but so are the big animals. Also, archaeologists arguing for dates of about 20,000 years for sites in Brazil seem to be vindicated, points out Prof. Ruth Gruhn of the University of Alberta in a third article on the subject published in Nature on Wednesday.
She also suggests why no sites that old have been found north of that: because they were inundated by the rising seas as the ice sheets melted, and are now submerged and gone.