Widespread population decline in South America correlates with mid-Holocene climate change

Abstract

Quantifying the impacts of climate change on prehistoric demography is crucial for understanding the adaptive pathways taken by human populations. Archaeologists across South America have pointed to patterns of regional abandonment during the Middle Holocene (8200 to 4200 cal BP) as evidence of sensitivity to shifts in hydroclimate over this period. We develop a unified approach to investigate demography and climate in South America and aim to clarify the extent to which evidence of local anthropic responses can be generalised to large-scale trends. We achieve this by integrating archaeological radiocarbon data and palaeoclimatic time series to show that population decline occurred coeval with the transition to the initial mid-Holocene across South America. Through the analysis of radiocarbon dates with Monte Carlo methods, we find multiple, sustained phases of downturn associated to periods of high climatic variability. A likely driver of the duration and severity of demographic turnover is the frequency of exceptional climatic events, rather than the absolute magnitude of change. Unpredictable levels of tropical precipitation had sustained negative impacts on pre-Columbian populations lasting until at least 6000 cal BP, after which recovery is evident. Our results support the inference that a demographic regime shift in the second half of the Middle Holocene were coeval with cultural practices surrounding Neotropical plant management and early cultivation, possibly acting as buffers when the wild resource base was in flux.

Archaeogenomic Distinctiveness of the Isthmo-Colombian Area

“…It is worth mentioning, however, that the graph without admixture in Los Rieles became statistically significant (|Z|<3) when Lagoa Santa was replaced with the younger Laranjal sample (Brazil, ∼6.8 kya) (Figure S37B). This confirms our f4 -statistics (Figure 6A) and the scenario of a widespread population turnover in South American during mid-Holocene, as previously suggested (Moreno-Mayar et al, 2018; Posth et al, 2018), a finding that also correlates with climate changes in the southern continent (Riris and Arroyo-Kalin, 2019). As for the Ancient Panamanians, they are a mix between the source of SNA2 (prior to Spirit Cave) and the admixture between SNA1 and SNA2 (Figure S37A).…”

“…The Pacific coast populations (Los Rieles, Chile, ∼10.9 kya, Figure S26) exhibit greater affinity to Spirit Cave, while the ancient genomes from the Atlantic side show the same pattern as Anzick-1 when considering individuals older than ∼7 kya (Lagoa Santa, Brazil, ∼10.4 kya, Figure S27). These distinctive signals persisted up to about 7 kya, when they were probably erased by a major population turnover in South America (Moreno-Mayar et al, 2018; Posth et al, 2018), facilitated by a widespread population decline due to mid-Holocene climate changes (Riris and Arroyo-Kalin, 2019).…”

Ancient DNA reveals the lost domestication history of South American camelids in Northern Chile and across the Andes

“…Nevertheless, while human driven demographic changes are likely to have driven some of the changes observed in this data, the effect of environmental changes cannot be ignored, in particular during the middle Holocene climatic optimum (~8.2 kyr BP -4.2 kyr BP). This time span is marked by a period of strong hydroclimate shifts that led to the aridization of the Andes as rainfalls became more stochastic and temperatures increased (Riris & Arroyo-Kalin, 2019). Such aridification is likely to have changed the habitable range for the vicuña further exacerbating the demographic pressure that it was already experiencing.…”

Did pre-Columbian populations of the Amazonian biome reach carrying capacity during the Late Holocene?

“…To facilitate a broad understanding of the structure of this dataset, we visualise the calibrated dates employing two complementary techniques: Summed Calibrated Probability Distributions (SPDs) and Composite Kernel Density Estimates (CKDEs). In the case of the former (Figure 1b, Figure 2a), and in order to examine expectations derived from prior research [20,27,28], we attempted to fit our SPD time-series to exponential (reflecting population growth with unlimited resources) and logistic (reflecting a decreasing rate of population growth as resources become scarce) models. To this end, we simulated and back-calibrated n random dates, where n is the number of bins, before summing their calibrated probability distributions.…”

“…These studies, however, have either relied on very small datasets or eschewed explicit model-testing to assess whether fluctuations in the constructed SPD time series constitute demographic shifts worthy of attention [22]. While this is partly an outcome of a lower intensity of archaeological research in the region, resulting in fewer and heterogeneously distributed radiocarbon dates compared to other world regions, the importance of employing large datasets and a model-testing approach [23][24][25][26] is underscored by a more recent study [27] that identifies links between multiple phases of middle Holocene demographic downturn and high climatic variability in South America. This study demonstrates that following a middle Holocene demographic nadir, sharp population growth in South America started around the 4 th millennium BP, a finding that is consonant with suggestions that between 5.5 and 2.0 ka BP South America witnessed exponential human population growth [28].…”