Rapid radiation of humans in South America after the last glacial maximum: A radiocarbon-based study

Anthropic cut marks in extinct megafauna bones from the Pampean region (Argentina) at the last glacial maximum

The initial peopling of South America is a topic of intense archaeological debate. Among the most contentious issues remain the nature of the human-megafauna interaction and the possible role of humans, along with climatic change, in the extinction of several megamammal genera at the end of the Pleistocene. In this study, we present the analysis of fossil remains with cutmarks belonging to a specimen of Neosclerocalyptus (Xenarthra, Glyptodontidae), found on the banks of the Reconquista River, northeast of the Pampean region (Argentina), whose AMS 14C dating corresponds to the Last Glacial Maximum (21,090-20,811 cal YBP). Paleoenvironmental reconstructions, stratigraphic descriptions, absolute chronological dating of bone materials, and deposits suggest a relatively rapid burial event of the bone assemblage in a semi-dry climate during a wet season. Quantitative and qualitative analyses of the cut marks, reconstruction of butchering sequences, and assessments of the possible agents involved in the observed bone surface modifications indicate anthropic activities. Our results provide new elements for discussing the earliest peopling of southern South America and specifically for the interaction between humans and local megafauna in the Pampean region during the Last Glacial Maximum.

Morphometrics and Phylogenomics of Coca (Erythroxylum spp.) Illuminate Its Reticulate Evolution, With Implications for Taxonomy

South American coca (Erythroxylum coca and E. novogranatense) has been a keystone crop for many Andean and Amazonian communities for at least 8,000 years. However, over the last half-century, global demand for its alkaloid cocaine has driven intensive agriculture of this plant and placed it in the center of armed conflict and deforestation. To monitor the changing landscape of coca plantations, the United Nations Office on Drugs and Crime collects annual data on their areas of cultivation. However, attempts to delineate areas in which different varieties are grown have failed due to limitations around identification. In the absence of flowers, identification relies on leaf morphology, yet the extent to which this is reflected in taxonomy is uncertain. Here, we analyze the consistency of the current naming system of coca and its four closest wild relatives (the "coca clade"), using morphometrics, phylogenomics, molecular clocks, and population genomics. We include name-bearing type specimens of coca's closest wild relatives E. gracilipes and E. cataractarum. Morphometrics of 342 digitized herbarium specimens show that leaf shape and size fail to reliably discriminate between species and varieties. However, the statistical analyses illuminate that rounder and more obovate leaves of certain varieties could be associated with the subtle domestication syndrome of coca. Our phylogenomic data indicate extensive gene flow involving E. gracilipes which, combined with morphometrics, supports E. gracilipes being retained as a single species. Establishing a robust evolutionary-taxonomic framework for the coca clade will facilitate the development of cost-effective genotyping methods to support reliable identification.

Environmental conditions associated with initial northern expansion of anatomically modern humans

The ability of our ancestors to switch food sources and to migrate to more favourable environments enabled the rapid global expansion of anatomically modern humans beyond Africa as early as 120,000 years ago. Whether this versatility was largely the result of environmentally determined processes or was instead dominated by cultural drivers, social structures, and interactions among different groups, is unclear. We develop a statistical approach that combines both archaeological and genetic data to infer the more-likely initial expansion routes in northern Eurasia and the Americas. We then quantify the main differences in past environmental conditions between the more-likely routes and other potential (less-likely) routes of expansion. We establish that, even though cultural drivers remain plausible at finer scales, the emergent migration corridors were predominantly constrained by a combination of regional environmental conditions, including the presence of a forest-grassland ecotone, changes in temperature and precipitation, and proximity to rivers.

Taguatagua 3: A new late Pleistocene settlement in a highly suitable lacustrine habitat in central Chile (34°S)

We present the results of the excavations and analyses of the diverse and exceptional archaeological assemblage of Taguatagua 3, a new late Pleistocene site located in the ancient Tagua Tagua lake in Central Chile (34°S). The anthropogenic context is constrained in a coherently dated stratigraphic deposit which adds new information about the mobility, subsistence strategies, and settlement of the early hunter-gatherers of southern South America. The age model constructed, as well as radiocarbon dates obtained directly from a combustion structure, indicate that the human occupation occurred over a brief time span around 12,440-12,550 cal yr BP. Considering taphonomic, geoarchaeological, lithic, archaeobotanical, and zooarchaeological evidence, as well as the spatial distribution combined with ethnographic data, we interpret Taguatagua 3 as a logistic and temporary camp associated mainly with gomphothere hunting and butchering. Nevertheless, several other activities were carried out here as well, such as hide and/or bone preparation, small vertebrate and plant processing and consumption, and red ochre grinding. Botanical and eggshell remains suggest that the anthropic occupation occurred during the dry season. Considering the contemporaneous sites recorded in the basin, we conclude that the ancient Tagua Tagua lake was a key location along the region's early hunter-gatherer mobility circuits. In this context, it acted as a recurrent hunting/scavenging place during the Late Pleistocene due to its abundant, diverse, and predictable resources.

Earliest directly dated rock art from Patagonia reveals socioecological resilience to mid-Holocene climate

The timing for the evolution of the capacity to inscribe the landscape with rock art has global relevance. While this was an in-built capacity when Homo sapiens first colonized the Americas, the heterogeneous distribution of rock art shows that it was a facultative behavior arising under unknown socioecological conditions. Patagonia was the last region to be explored by humans. While its rock art is globally important, it remains largely undated by absolute methods. We report the earliest set of directly radiocarbon-dated rock art motifs from the archaeological site Cueva Huenul 1 (northwestern Patagonia, Argentina), starting at 8.2 thousand years before the present (ka B.P.), predating previous records by several millennia, and encompassing over 3 ka (~130 human generations). This mid-Holocene "rock art emergence" phase overlaps with extremely arid conditions and a human demographic stasis. We suggest that this diachronic rock art emerged as part of a resilient response to ecological stress by highly mobile and low-density populations.

Towards understanding human-environment feedback loops: the Atacama Desert case

The overall trajectory for the human-environment interaction has been punctuated by demographic boom-and-bust cycles, phases of growth/overshooting as well as of expansion/contraction in productivity. Although this pattern has been explained in terms of an interplay between population growth, social upscaling, ecosystem engineering and climate variability, the evoked demographic-resource-complexity mechanisms have not been empirically tested. By integrating proxy data for population sizes, palaeoclimate and internal societal factors into empirical modelling approaches from the population dynamic theory, we evaluated how endogenous (population sizes, warfare and social upscaling) and exogenous (climate) variables module the dynamic in past agrarian societies. We focused on the inland Atacama Desert, where populations developed agriculture activities by engineering arid and semi-arid landscapes during the last 2000 years. Our modelling approach indicates that these populations experienced a boom-and-bust dynamic over the last millennia, which was coupled to structure feedback between population sizes, hydroclimate, social upscaling, warfare and ecosystem engineering. Thus, the human-environment loop appears closely linked with cooperation, competition, limiting resources and the ability of problem-solving. This article is part of the theme issue 'Evolution and sustainability: gathering the strands for an Anthropocene synthesis'.

Ancient mitogenomes from the Southern Pampas of Argentina reflect local differentiation and limited extra-regional linkages after rapid initial colonization

OBJECTIVE

This study aims to contribute to the recovery of Indigenous evolutionary history in the Southern Pampas region of Argentina through an analysis of ancient complete mitochondrial genomes.

MATERIALS AND METHODS

We generated DNA data for nine complete mitogenomes from the Southern Pampas, dated to between 2531 and 723 cal BP. In combination with previously published ancient mitogenomes from the region and from throughout South America, we documented instances of extra-regional lineage-sharing, and estimated coalescent ages for local lineages using a Bayesian method with tip calibrations in a phylogenetic analysis.

RESULTS

We identified a novel mitochondrial haplogroup, B2b16, and two recently defined haplogroups, A2ay and B2ak1, as well as three local haplotypes within founder haplogroups C1b and C1d. We detected lineage-sharing with ancient and contemporary individuals from Central Argentina, but not with ancient or contemporary samples from North Patagonian or Littoral regions of Argentina, despite archeological evidence of cultural interactions with the latter regions. The estimated coalescent age of these shared lineages is ~10,000 years BP.

DISCUSSION

The history of the human populations in the Southern Pampas is temporally deep, exhibiting long-term continuity of mitogenome lineages. Additionally, the identification of highly localized mtDNA clades accords with a model of relatively rapid initial colonization of South America by Indigenous communities, followed by more local patterns of limited gene flow and genetic drift in various South American regions, including the Pampas.

Craniovascular variation in four late Holocene human samples from southern South America

Craniovascular traits in the endocranium (traces of middle meningeal vessels and dural venous sinuses, emissary foramina) provide evidence of vascular anatomy in osteological samples. We investigate the craniovascular variation in four South American samples and the effect of artificial cranial modifications (ACM). CT scans of human adult crania from four archeological samples from southern South America (including skulls with ACM) are used for the analyses. The craniovascular features in the four samples are described, skulls with and without ACM are compared, and additionally, South Americans are compared to a previously analyzed sample of Europeans. Of the four South American samples, the Southern Patagonian differs the most, showing the most distinct cranial dimensions, no ACM, and larger diameters of the emissary foramina. Unlike previous studies, we did not find any major differences in craniovascular features between modified and non-modified skulls, except that the skulls with ACM present somewhat smaller foramina. South Americans significantly differed from Europeans, especially in the anteroposterior dominance of the middle meningeal artery, in the pattern of sinus confluence, in the occurrence of enlarged occipito-marginal sinuses, and in foramina frequencies and diameters. Craniovascular morphology is not affected by the cranial size, even in skulls with ACM, indicating a minor or null influence of structural topological factors. Concerning the samples from distinct geographic and climatic environments, it must be evaluated whether the craniovascular morphogenesis might be partially influenced by specific functions possibly associated with thermoregulation, intracranial pressure, and the maintenance of intracranial homeostasis.

Changes in projectile design and size of prey reveal the central role of Fishtail points in megafauna hunting in South America

Fishtail projectile points are the earliest widespread projectile type in South America, and share chronology and techno-morphology with Clovis, the oldest North American projectile type. Both were temporally associated with late Pleistocene megafaunal extinctions. Although the elusive direct evidence of human exploitation of megafauna in South America had kept Fishtails out of the extinction debate, a recent paper showed a strong relationship between the temporal density and spatial distribution of megafauna and Fishtail projectile points, and proposed that this weapon was designed and used for megafauna hunting, contributing to their extinction. If so, this technology must be distinctly different from post-FPP technologies (i.e., early Holocene projectile points), used for hunting smaller prey, in terms of distribution and functional properties. In this paper, we explore the changes in projectile point technology, as well as the body mass of potential megafaunal prey, and show that Fishtails were strongly related to the largest extinct megafaunal species.

Human Y chromosome sequences from Q Haplogroup reveal a South American settlement pre-18,000 years ago and a profound genomic impact during the Younger Dryas

The settlement of the Americas has been the focus of incessant debate for more than 100 years, and open questions regarding the timing and spatial patterns of colonization still remain today. Phylogenetic studies with complete human Y chromosome sequences are used as a highly informative tool to investigate the history of human populations in a given time frame. To study the phylogenetic relationships of Native American lineages and infer the settlement history of the Americas, we analyzed Y chromosome Q Haplogroup, which is a Pan-American haplogroup and represents practically all Native American lineages in Mesoamerica and South America. We built a phylogenetic tree for Q Haplogroup based on 102 whole Y chromosome sequences, of which 13 new Argentine sequences were provided by our group. Moreover, 1,072 new single nucleotide polymorphisms (SNPs) that contribute to its resolution and diversity were identified. Q-M848 is known to be the most frequent autochthonous sub-haplogroup of the Americas. The present is the first genomic study of Q Haplogroup in which current knowledge on Q-M848 sub-lineages is contrasted with the historical, archaeological and linguistic data available. The divergence times, spatial structure and the SNPs found here as novel for Q-Z780, a less frequent sub-haplogroup autochthonous of the Americas, provide genetic support for a South American settlement before 18,000 years ago. We analyzed how environmental events that occurred during the Younger Dryas period may have affected Native American lineages, and found that this event may have caused a substantial loss of lineages. This could explain the current low frequency of Q-Z780 (also perhaps of Q-F4674, a third possible sub-haplogroup autochthonous of the Americas). These environmental events could have acted as a driving force for expansion and diversification of the Q-M848 sub-lineages, which show a spatial structure that developed during the Younger Dryas period.

A genomic perspective on South American human history

It has generally been accepted that the current indigenous peoples of the Americas are derived from ancestors from northeastern Asia. The latter were believed to have spread into the American continent by the end of the Last Glacial Maximum. In this sense, a joint and in-depth study of the earliest settlement of East Asia and the Americas is required to elucidate these events accurately. The first Americans underwent an adaptation process to the Americas' vast environmental diversity, mediated by biological and cultural evolution and niche construction, resulting in enormous cultural diversity, a wealth of domesticated species, and extensive landscape modifications. Afterward, in the Late Holocene, the advent of intensive agricultural food production systems, sedentism, and climate change significantly reshaped genetic and cultural diversity across the continent, particularly in the Andes and Amazonia. Furthermore, starting around the end of the 15th century, European colonization resulted in massive extermination of indigenous peoples and extensive admixture. Thus, the present review aims to create a comprehensive picture of the main events involved in the formation of contemporary South American indigenous populations and the dynamics responsible for shaping their genetic diversity by integrating current genetic data with evidence from archeology, linguistics and other disciplines.

Morphometric affinities and direct radiocarbon dating of the Toca dos Coqueiros' skull (Serra da Capivara, Brazil)

The biological variation of the earliest skeletons of South America has been intensely debated for the last two centuries. One of the major research constraints has been the limited number of available samples dating to the early Holocene. We here present the first direct radiocarbon-date for the early Holocene human skeleton from Toca dos Coqueiros (Serra da Capivara, Brazil), also known as “Zuzu” (8640 ± 30 BP; 9526–9681 cal years BP). We performed craniometric analyses using exclusively samples from Brazil, to revisit the sex of the skeleton, and to discuss the evolutionary processes involved in the occupation of the continent. The sex of the individual was estimated as a female when compared to late and early Holocene individuals, but as a male when compared only to the early Holocene series. We also found that Zuzu presents the strongest differences with the late Holocene Guajajara individuals, located nearby, and the strongest similarities with the early Holocene series from Lagoa Santa, attesting for solid biological affinities among early Holocene individuals from Brazil, as well as a moderate level of morphological variation among them. This suggests that the early individuals were part of the same heterogeneous lineage, possibly a different one from which late Holocene populations diverged.

Disentangling the last 1,000 years of human-environment interactions along the eastern side of the southern Andes (34-52°S lat.)

Understanding how people have shaped landscapes requires detailed information on past changes in climate, vegetation, fire, and land use. The environmental and human history of four sites along the eastern Andes of southern South America (34–52°S) shows the changing influence of people and climate on landscape development over the last millennia. Initially, burning by hunter-gatherers and climate variability shaped forest, shrubland, and grassland mosaics. Widespread alteration of fire regimes and vegetation ∼400 y ago is attributed to increased Native American pastoralism prior to extensive Euro-American settlement. Late-19th century ranching and logging led to broadscale changes in fire activity and vegetation across the region. These high-resolution, landscape-scale reconstructions reveal complex human–environment interactions that are often overlooked in regional-to-global syntheses.

Researchers have long debated the degree to which Native American land use altered landscapes in the Americas prior to European colonization. Human–environment interactions in southern South America are inferred from new pollen and charcoal data from Laguna El Sosneado and their comparison with high-resolution paleoenvironmental records and archaeological/ethnohistorical information at other sites along the eastern Andes of southern Argentina and Chile (34–52°S). The records indicate that humans, by altering ignition frequency and the availability of fuels, variously muted or amplified the effects of climate on fire regimes. For example, fire activity at the northern and southern sites was low at times when the climate and vegetation were suitable for burning but lacked an ignition source. Conversely, abundant fires set by humans and infrequent lightning ignitions occurred during periods when warm, dry climate conditions coincided with ample vegetation (i.e., fuel) at midlatitude sites. Prior to European arrival, changes in Native American demography and land use influenced vegetation and fire regimes locally, but human influences were not widely evident until the 16th century, with the introduction of nonnative species (e.g., horses), and then in the late 19th century, as Euro-Americans targeted specific resources to support local and national economies. The complex interactions between past climate variability, human activities, and ecosystem dynamics at the local scale are overlooked by approaches that infer levels of land use simply from population size or that rely on regionally composited data to detect drivers of past environmental change.

p3k14c, a synthetic global database of archaeological radiocarbon dates

Archaeologists increasingly use large radiocarbon databases to model prehistoric human demography (also termed paleo-demography). Numerous independent projects, funded over the past decade, have assembled such databases from multiple regions of the world. These data provide unprecedented potential for comparative research on human population ecology and the evolution of social-ecological systems across the Earth. However, these databases have been developed using different sample selection criteria, which has resulted in interoperability issues for global-scale, comparative paleo-demographic research and integration with paleoclimate and paleoenvironmental data. We present a synthetic, global-scale archaeological radiocarbon database composed of 180,070 radiocarbon dates that have been cleaned according to a standardized sample selection criteria. This database increases the reusability of archaeological radiocarbon data and streamlines quality control assessments for various types of paleo-demographic research. As part of an assessment of data quality, we conduct two analyses of sampling bias in the global database at multiple scales. This database is ideal for paleo-demographic research focused on dates-as-data, bayesian modeling, or summed probability distribution methodologies.

Population Histories and Genomic Diversity of South American Natives

South America is home to one of the most culturally diverse present-day native populations. However, the dispersion pattern, genetic substructure, and demographic complexity within South America are still poorly understood. Based on genome-wide data of 58 native populations, we provide a comprehensive scenario of South American indigenous groups considering the genomic, environmental, and linguistic data. Clear patterns of genetic structure were inferred among the South American natives, presenting at least four primary genetic clusters in the Amazonian and savanna regions and three clusters in the Andes and Pacific coast. We detected a cline of genetic variation along a west-east axis, contradicting a hard Andes-Amazon divide. This longitudinal genetic variation seemed to have been shaped by both serial population bottlenecks and isolation by distance. Results indicated that present-day South American substructures recapitulate ancient macroregional ancestries and western Amazonia groups show genetic evidence of cultural exchanges that led to language replacement in precontact times. Finally, demographic inferences pointed to a higher resilience of the western South American groups regarding population collapses caused by the European invasion and indicated precontact population reductions and demic expansions in South America.

Deep classification of cut-marks on bones from Arroyo del Vizcaíno (Uruguay)

The earliest widely accepted presence of humans in America dates to approximately 17.5 cal kyr BP, at the end of the Last Glacial Maximum (LGM). Among other evidence, this presence is attested by stone tools and associated cut-marks and other bone surface modifications (BSM), interpreted as the result of the consumption of animals by humans. Claims of an older human presence in the continent have been made based on the proposed anthropogenic modification of faunal remains; however, these have been controversial due to the highly subjective nature of the interpretations. Here, we employ advanced deep learning algorithms to objectively increase the accuracy of BSM identification on bones. With several models that exhibit BSM classification accuracies greater than 94%, we use ensemble learning techniques to robustly classify a selected sample of BSM from the approximately 30 kyr BP site of Arroyo del Vizcaíno, Uruguay. Our results confidently show the presence of cut-marks imparted by stone tools on bones at the site. This result supports an earlier presence of humans in the American continent, expanding additional genetic and archaeological evidence of a human LGM and pre-LGM presence in the continent.

Ancient mitochondrial genomes from the Argentinian Pampas inform the early peopling of the Southern Cone of South America

The Southern Cone of South America (SCSA) is a key region for investigations about the peopling of the Americas. However, little is known about the eastern sector, the Argentinian Pampas. We analyzed 18 mitochondrial genomes-7 of which are novel-from human skeletal remains from 3 Early to Late Holocene archaeological sites. The Pampas present a distinctive genetic makeup compared to other Middle to Late Holocene pre-Columbian SCSA populations. We also report the earliest individuals carrying SCSA-specific mitochondrial haplogroups D1j and D1g from Early and Middle Holocene, respectively. Using these deep calibration time points in Bayesian phylogenetic reconstructions, we suggest that the first settlers of the Pampas were part of a single and rapid dispersal ∼15,600 years ago. Finally, we propose that present-day genetic differences between the Pampas and the rest of the SCSA are due to founder effects, genetic drift, and a partial population replacement ∼9,000 years ago.

Approximate Bayesian Computation of radiocarbon and paleoenvironmental record shows population resilience on Rapa Nui (Easter Island)

Examining how past human populations responded to environmental and climatic changes is a central focus of the historical sciences. The use of summed probability distributions (SPD) of radiocarbon dates as a proxy for estimating relative population sizes provides a widely applicable method in this research area. Paleodemographic reconstructions and modeling with SPDs, however, are stymied by a lack of accepted methods for model fitting, tools for assessing the demographic impact of environmental or climatic variables, and a means for formal multi-model comparison. These deficiencies severely limit our ability to reliably resolve crucial questions of past human-environment interactions. We propose a solution using Approximate Bayesian Computation (ABC) to fit complex demographic models to observed SPDs. Using a case study from Rapa Nui (Easter Island), a location that has long been the focus of debate regarding the impact of environmental and climatic changes on its human population, we find that past populations were resilient to environmental and climatic challenges. Our findings support a growing body of evidence showing stable and sustainable communities on the island. The ABC framework offers a novel approach for exploring regions and time periods where questions of climate-induced demographic and cultural change remain unresolved.

Summed probability distributions of radiocarbon dates can be used to estimate past demography, but methods to test for associations with environmental change are lacking. Here, DiNapoli et al. propose an approach using Approximate Bayesian Computation and illustrate it in a case study of Rapa Nui.

A Bayesian approach for fitting and comparing demographic growth models of radiocarbon dates: A case study on the Jomon-Yayoi transition in Kyushu (Japan)

Large sets of radiocarbon dates are increasingly used as proxies for inferring past population dynamics and the last few years, in particular, saw an increase in the development of new statistical techniques to overcome some of the key challenges imposed by this kind of data. These include: 1) null hypothesis significance testing approaches based on Monte-Carlo simulations or mark permutations; 2) non-parametric Bayesian modelling approaches, and 3) the use of more traditional techniques such as correlation, regression, and AIC-based model comparison directly on the summed probability distribution of radiocarbon dates (SPD). While the range of opportunities offered by these solutions is unquestionably appealing, they often do not consider the uncertainty and the biases arising from calibration effects or sampling error. Here we introduce a novel Bayesian approach and nimbleCarbon, an R package that offers model fitting and comparison for population growth models based on the temporal frequency data of radiocarbon dates. We evaluate the robustness of the proposed approach on a range of simulated scenarios and illustrate its application on a case study focused on the demographic impact of the introduction of wet-rice farming in prehistoric Japan during the 1st millennium BCE.

Pleistocene Mammals from Pampean Region (Argentina). Biostratigraphic, Biogeographic, and Environmental Implications

The Pampean Region contains sedimentary sequences with abundant mammal fossil records, which constitute the chronological outline of the Plio–Pleistocene of South America. These classic localities have been used for more than a century to correlate with other South American regions. Throughout this time, a series of misinterpretations have appeared. To understand the stratigraphic significance of these localities and the geochronological situation of each unit referring to the Pleistocene, a critical historical study of the antecedents was carried out, evaluating the state of each unit. The biostratigraphic studies of the Pampean Region’s mammalian faunas improved the understanding of biogeographic changes taking into account the environmental fluctuations of the Pleistocene.

Ancient and modern mitogenomes from Central Argentina: new insights into population continuity, temporal depth and migration in South America

The inverted triangle shape of South America places Argentina territory as a geographical crossroads between the two principal peopling streams that followed either the Pacific or the Atlantic coasts, which could have then merged in Central Argentina (CA). Although the genetic diversity from this region is therefore crucial to decipher past population movements in South America, its characterization has been overlooked so far. We report 92 modern and 22 ancient mitogenomes spanning a temporal range of 5000 years, which were compared with a large set of previously reported data. Leveraging this dataset representative of the mitochondrial diversity of the subcontinent, we investigate the maternal history of CA populations within a wider geographical context. We describe a large number of novel clades within the mitochondrial DNA tree, thus providing new phylogenetic interpretations for South America. We also identify several local clades of great temporal depth with continuity until the present time, which stem directly from the founder haplotypes, suggesting that they originated in the region and expanded from there. Moreover, the presence of lineages characteristic of other South American regions reveals the existence of gene flow to CA. Finally, we report some lineages with discontinuous distribution across the Americas, which suggest the persistence of relic lineages likely linked to the first population arrivals. The present study represents to date the most exhaustive attempt to elaborate a Native American genetic map from modern and ancient complete mitochondrial genomes in Argentina and provides relevant information about the general process of settlement in South America.

Late Pleistocene South American megafaunal extinctions associated with rise of Fishtail points and human population

In the 1970s, Paul Martin proposed that big game hunters armed with fluted projectile points colonized the Americas and drove the extinction of megafauna. Around fifty years later, the central role of humans in the extinctions is still strongly debated in North American archaeology, but little considered in South America. Here we analyze the temporal dynamic and spatial distribution of South American megafauna and fluted (Fishtail) projectile points to evaluate the role of humans in Pleistocene extinctions. We observe a strong relationship between the temporal density and spatial distribution of megafaunal species stratigraphically associated with humans and Fishtail projectile points, as well as with the fluctuations in human demography. On this basis we propose that the direct effect of human predation was the main factor driving the megafaunal decline, with other secondary, but necessary, co-occurring factors for the collapse of the megafaunal community.

Archaeogenomic distinctiveness of the Isthmo-Colombian area

The recently enriched genomic history of Indigenous groups in the Americas is still meager concerning continental Central America. Here, we report ten pre-Hispanic (plus two early colonial) genomes and 84 genome-wide profiles from seven groups presently living in Panama. Our analyses reveal that pre-Hispanic demographic events contributed to the extensive genetic structure currently seen in the area, which is also characterized by a distinctive Isthmo-Colombian Indigenous component. This component drives these populations on a specific variability axis and derives from the local admixture of different ancestries of northern North American origin(s). Two of these ancestries were differentially associated to Pleistocene Indigenous groups that also moved into South America, leaving heterogenous genetic footprints. An additional Pleistocene ancestry was brought by a still unsampled population of the Isthmus (UPopI) that remained restricted to the Isthmian area, expanded locally during the early Holocene, and left genomic traces up to the present day.

Directly modelling population dynamics in the South American Arid Diagonal using 14C dates

Large anthropogenic ¹⁴C datasets are widely used to generate summed probability distributions (SPDs) as a proxy for past human population levels. However, SPDs are a poor proxy when datasets are small, bearing little relationship to true population dynamics. Instead, more robust inferences can be achieved by directly modelling the population and assessing the model likelihood given the data. We introduce the R package ADMUR which uses a continuous piecewise linear (CPL) model of population change, calculates the model likelihood given a ¹⁴C dataset, estimates credible intervals using Markov chain Monte Carlo, applies a goodness-of-fit test, and uses the Schwarz Criterion to compare CPL models. We demonstrate the efficacy of this method using toy data, showing that spurious dynamics are avoided when sample sizes are small, and true population dynamics are recovered as sample sizes increase. Finally, we use an improved ¹⁴C dataset for the South American Arid Diagonal to compare CPL modelling to current simulation methods, and identify three Holocene phases when population trajectory estimates changed from rapid initial growth of 4.15% per generation to a decline of 0.05% per generation between 10 821 and 7055 yr BP, then gently grew at 0.58% per generation until 2500 yr BP. This article is part of the theme issue 'Cross-disciplinary approaches to prehistoric demography'.