Evidence of human occupation in Mexico around the Last Glacial Maximum

The initial colonization of the Americas remains a highly debated topic1 , and the exact timing of the frst 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 frst Americanpopulations2 . However, recent investigations provide reliable evidence of a human presence in the northwest region of Mexico3,4 , the Chiapas Highlands5 , Central Mexico6 and the Caribbean coast7–9 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 fndings in the Americas10–17of cultural evidence that dates to the Last Glacial Maximum (26,500–19,000years ago)18, and which push back dates for human dispersal to the region possibly as early as 33,000–31,000years ago. The site yielded about 1,900stone artefacts within a 3-m-deep stratifed sequence, revealing a previously unknown lithic industry that underwent only minor changes over millennia. More than 50radiocarbon 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

A chronometric investigation of the initial peopling of the Americas

The initial peopling of the American continent marks a major event in the expansion of humans across the planet. For most of the 20th century, it was believed that a group of big-game hunters initially entered North America approximately 13,500 years ago, through the now flooded land bridge of Beringia, between modern day Siberia and Alaska. Once across, they would have moved southward through an ice-free corridor formed between the Laurentide and Cordilleran ice sheets, which began their recession after the last Ice Age. This model, however, was effectively refuted in 1997 when archaeologists found evidence in Chile, South America, which was reliably older than the North American record suggested. Today, questions associated with this dispersal are debated in First American research, and remain largely unanswered. These include the antiquity and origin of the initial arrivals, the number of migrational episodes, and the impact of human expansion in the late Quaternary faunal extinctions. In response, considering that timing is fundamental in the study of human dispersals, this investigation employs radiocarbon dating and Bayesian age modelling to build a robust chronological framework with which to elucidate broad spatio-temporal patterns. This is accomplished by obtaining reliable radiocarbon measurements for key archaeological sites across the continent, resolving dating discrepancies using a single-amino-acid isolation method, and building largescale Bayesian age models to track human presence and frame current evidence. This involved the laboratory processing of dateable material from 18 archaeological sites, the production of 133 radiocarbon dates, and the statistical analysis of archaeo-chronometric data from 41 archaeological sites in North America and Beringia. A widely used method, called ‘14C-dates-asdata’, is assessed from a radiocarbon dating perspective, and the potentially confounding effects introduced by challenges in sample processing in the analysis are demonstrated. This approach is the only other quantitative technique apart from Bayesian age modelling that uses the chronometric record to elucidate large-scale population events. Here, however, it is argued that Bayesian age modelling is the most efficient tool in tackling human dispersals. Results obtained in this thesis have enabled the creation of a new Bayesian age model for the initial peopling of the Americas. This suggests that the earliest arrivals to mid-latitude North America were humans from eastern Asia, who most likely crossed near the end of the LGM. The synchronous commencement of Beringian, Clovis and Western Stemmed cultural traditions is also identified, and their overlap with the last appearance dates for 18 extinct genera suggests that human expansion was a key driver behind animal extinction events. Notably, these findings overturn the widespread notion that North America was first penetrated only after 16-15,000 years ago. It is hoped that, in the future, new evidence in First Americans research will increase the resolution of the models here created, and enhance the conclusions drawn. Particular attention should be paid to Central and South America, regions with more limited archaeo-chronometric data and immediate chronometric potential. Improved chronological control in these regions is key in understanding the pattern of American settlement

A chronology for the earliest human burials at Cuchipuy, central Chile

Cuchipuy is an archaeological site within the ancient Laguna de Tagua Tagua area (O’Higgins Region, central Chile; known for containing the remains of extinct fauna), with evidence for cultural activity spanning most of the Holocene, including over 50 human burials. The bulk of chronometric work at Cuchipuy was carried out in the 1980s, where a discrepancy within the radiocarbon dating results raised questions on the antiquity of the oldest funerary deposits (Stratum 4). Given the importance of both the site and area in prehistoric studies, this analysis aimed to reassess the chronology of Stratum 4 through the production of new radiocarbon dates on human remains and the application of Bayesian modelling. When combined with previously published ages, results place the commencement of Stratum 4 at 7320–7160 cal BP, within the early-mid Holocene. This is later than previous conclusions based on the discrepant dataset, refuting a temporal connection to Paleoindian deposits within the region. In addition, stable light isotope results suggest that human diet during the period was diverse, relying on both terrestrial and freshwater aquatic resources. This pattern changes with later populations, however, where the isotopic signal reflects the consumption of mainly terrestrial resources

Evidence of human occupation in Mexico around the Last Glacial Maximum

The initial colonization of the Americas remains a highly debated topic1, 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 populations2. However, recent investigations provide reliable evidence of a human presence in the northwest region of Mexico3,4, the Chiapas Highlands5, Central Mexico6 and the Caribbean coast7,8,9 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 Americas10,11,12,13,14,15,16,17of cultural evidence that dates to the Last Glacial Maximum (26,500–19,000 years ago)18, 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

THE APPLICATION OF POLLEN RADIOCARBON DATING AND BAYESIAN AGE-DEPTH MODELING FOR DEVELOPING ROBUST GEOCHRONOLOGICAL FRAMEWORKS OF WETLAND ARCHIVES

Wetland sediments are valuable archives of environmental change but can be challenging to date. Terrestrial macrofossils are often sparse, resulting in radiocarbon (14C) dating of less desirable organic fractions. An alternative approach for capturing changes in atmospheric 14C is the use of terrestrial microfossils. We 14C date pollen microfossils from two Australian wetland sediment sequences and compare these to ages from other sediment fractions (n = 56). For the Holocene Lake Werri Berri record, pollen 14C ages are consistent with 14C ages on bulk sediment and humic acids (n = 14), whilst Stable Polycyclic Aromatic Carbon (SPAC) 14C ages (n = 4) are significantly younger. For Welsby Lagoon, pollen concentrate 14C ages (n = 21) provide a stratigraphically coherent sequence back to 50 ka BP. 14C ages from humic acid and \u3e100 μm fractions (n = 13) are inconsistent, and often substantially younger than pollen ages. Our comparison of Bayesian age-depth models, developed in Oxcal, Bacon and Undatable, highlight the strengths and weaknesses of the different programs for straightforward and more complex chrono-stratigraphic records. All models display broad similarities but differences in modeled age-uncertainty, particularly when age constraints are sparse. Intensive dating of wetland sequences improves the identification of outliers and generation of robust age models, regardless of program used

Radiocarbon protocols and first intercomparison results from the chronos carbon-cycle facility, University of New South Wales, Sydney, Australia

The Chronos 14Carbon-Cycle Facility is a new radiocarbon laboratory at the University of New South Wales, Australia. Built around an Ionplus 200 kV MIni-CArbon DAting System (MICADAS) Accelerator Mass Spectrometer (AMS) installed in October 2019, the facility was established to address major challenges in the Earth, Environmental and Archaeological sciences. Here we report an overview of the Chronos facility, the pretreatment methods currently employed (bones, carbonates, peat, pollen, charcoal, and wood) and results of radiocarbon and stable isotope measurements undertaken on a wide range of sample types. Measurements on international standards, known-age and blank samples demonstrate the facility is capable of measuring 14C samples from the Anthropocene back to nearly 50,000 years ago. Future work will focus on improving our understanding of the Earth system and managing resources in a future warmer world