Who the first Americans were is barely known outside of the tools that they left in the archaeological record. For most of the late 20th century US researchers claimed that the first people to migrate into the Americas produced stone tools of the Clovis culture that first appear just before the Younger Dryas cold period, around 13.2 to 12.9 thousand years (ka) ago. The hallmark of Clovis culture is the finely-worked stone spear point, and its association with butchered large mammals: the Clovis people were apparently big-game hunters Despite other, albeit less convincing, signs of earlier human habitation, this notion ossified for a seemingly irrefutable reason. To reach the Americas from NE Asia on foot, these people would have had to cross the Bering Straits via the Beringia land bridge exposed as sea level fell during the Last Glacial Maximum (LGM). That would have taken them to Alaska, but an exit to the south remained blocked by the huge Laurentian ice sheet until around 13 ka. Once an ice-free route had opened, the Clovis people migrated quickly to reach the site from which they take their name in New Mexico. But other archaeological sites discovered in the last couple of decades, extending as far south as Chile, have yielded ages that clearly predate the Clovis culture (see: Clovis First hypothesis dumped, May 2008). Beneath a Clovis-bearing layer at a site in Texas excavators unearthed thousands of totally different tools reliably dated to as far back as 15.5 ka (see: Clovis first hypothesis refuted, May 2011). This opened the realistic possibility that the earliest migrants had not necessarily walked from Asia, but may have followed a marine route along the Pacific coast and spread eastwards as opportunities presented themselves.
Now Mesoamerica has convincingly verified migration more than twice as long ago as that which littered North America with Clovis tools. It emerged from the Chiquihuite Cave 2.7 km high in the Astillero Mountains of northern Mexico. Almost 2000 stone artefacts were found throughout a 3 m thick layer of sediment beneath the cave floor that spans 27 to 13 ka, (Ardelean, C.F. and 27 others 2020. Evidence of human occupation in Mexico around the Last Glacial Maximum. Nature, v. 584 p. 87–92; DOI: 10.1038/s41586-020-2509-0). The technology revealed by the tools is more primitive than that of the Clovis culture. Artefacts occur throughout the layer, which extends back in time from the Younger Dryas, through the preceding period of warming and the LGM itself. Although colder than the present equitable climate of the high mountain valleys of Northern Mexico environmental data obtained from the layer show that it was viable for occupation through the LGM. Of the 42 highly precise and accurate radiocarbon dates those from some of the stratigraphically deepest part of the layer exceed 33 ka, which the authors suggest may establish the initial human occupation of the cave. Incidentally, although the paper was published online in July 2020 it was submitted to Nature in October 2018. That is a very long time in the editorial and review process. There is no indication as to why there was such a delay: maybe an indication of some continuing defence of the Clovis First hypothesis among the reviewers …
The radiocarbon dating in the paper was carried out at the state-of-the-art accelerator mass spectrometer unit at the University of Oxford, UK, by two of the co-authors (Lorena Becerra-Valdivia and Thomas Higham). They too published a Nature paper in late July 2020, which discusses their new dating of 42 archaeological sites in North America and Siberia (Becerra-Valdivia, L. & Higham, T. 2020. The timing and effect of the earliest human arrivals in North America. Nature, v. 584, p. 93-97; DOI: 10.1038/s41586-020-2491-6). In Mesoamerica and North America (the Clovis heartland) their results suggest that, as in Chiquihuite Cave, ‘people were present in different settings before, during and immediately following the LGM’, their ranges increasing over time. These people would likely not have followed the same route suggested for the later Clovis people, i.e. across Beringia and then parallel to the topographic grain in the Western Cordillera, ice-cap melting permitting. An interesting suggestion by Becerra-Valdivia and Higham is that post-LGM expansion in numbers and range of these early American contributed to the famous extinction of the North American Pleistocene megafauna. Dating the extinctions of different genera suggests that disappearance of the megafauna may not have been a single event during the Younger Dryas, but seems to have been during at least two other episodes peaking at about 40 and 24 ka. Both the ecological devastation supposedly associated with the Clovis people and the impact theory for its cause depend on a single event.
See also: Gruhn, R. 2020. Evidence grows for early peopling of the Americas. Nature, v. 584, p. 47-48; DOI: 10.1038/d41586-020-02137-3; Rincon, P. 2020. Earliest evidence for humans in the Americas (BBC News, 22 July 2020); Keys, D. 2020. Humans reached the Americas 11,000 years earlier than previously thought, archaeologists discover (Independent, 22 July 2020)