Grey: eters defining the four hinge points, estimated using MCMC

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Grey: eters defining the four hinge points, estimated using MCMC

Grey: eters defining the four hinge points, estimated using MCMC

Using the broader South American radiocarbon dataset, Riris & Arroyo-Kalin propose three periods (8

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Figure 6. The dates of hinges A and D are not free parameters since they are fixed at 14 kyr BP and 2.5 kyr BP, respectively. Red: ML parameters estimated separately using the search algorithm. (Online version in colour.)

Using the broader South American radiocarbon dataset, Riris & Arroyo-Kalin propose three periods (8

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Figure 7. Credible intervals of the 3-CPL model. (a) Model PDFs using the joint parameters of 1000 samples from the joint posterior parameter distribution (black) and ML parameters (red), hinge points marked A–D. (b) The 50%, 75% and 95% credible intervals (grey) of all model PDFs (grey), and parameter values (red), sampled from the joint posterior parameter distribution. (Online version in colour.)

Using the broader South American radiocarbon dataset, Riris & Arroyo-Kalin propose three periods (8

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13. Discussion

The three phases identified in table 2 can be contextually informed by the archaeological record from dry regions in South America. While there is recent debate surrounding some earlier human occupations in the Americas [30,46–49], kyr BP represents a widely accepted range for the successful human exploration of the South American continent and of the SAAD in particular [50–52], based on both archaeological evidence and genomic data [30,53]. In the SAAD, the period extending between 14 000 and 10 821 yr BP is characterized by a remarkably high growth rate of 4.15% per 25 year generation. While current global population growth rates average ca 30% per generation (just over 1% per annum), this is a consequence of modern technological advances, and recent estimates on the prehistoric growth for human populations indicate a much smaller growth rate of 1% per generation (0.04% per annum) [54–56]. Therefore, the magnitude of population growth in this first phase is unusually high, and far greater than during any subsequent phase. This is likely due to the successful exploration and colonization of diverse and uncontested niches by early human societies [36,52,57] resulting in typical spread dynamics . At 14.6 kyr BP, the Antarctic Cold Reversal (ACR) began, resulting in colder conditions that were similar to those attributed to the North Atlantic Younger Dryas (YD) stadial [58–61]. This colder climate was accompanied by glacial advances throughout South America and higher lake levels in the Altiplano, which, based on palynological and glacial geological studies, appear to be a result of precipitation increase over the Altiplano [62,63]. The amelioration of different ecological niches and richness in resident species such as megafauna would have increased the carrying capacity of the SAAD, leading to rapid human population growth and exploration of new landscapes. This population expansion would have had a significant ecological impact. In addition to climatic changes (discussed below), the increase in predation rates and niche displacement would ultimately have contributed to the extinction of the American megafauna [50,64]. The regional pace of this anthropogenic impact on megafauna extinction remains unclear in the SAAD given its sparse evidence in some areas, such as in north-central Chile .

Table 2. Summary of the best 3-CPL model represented as ML dates of hinge points, and the growth rates of the three phases. 95% CI calculated using quantiles.

The second phase covers almost four millennia between 10 821 and 7055 yr BP and is associated with a slight population decrease (?0.05% per generation, table 2). It has previously been hypothesized that human populations experienced periodic fluctuations during the mid Holocene in response to climatic forces [2,36]. However, our analysis, using a refined dataset and improved method, does not support this hypothesis for the SAAD. Instead, the best model suggests a population that was failing to grow, despite the estimated population size relative to occupiable land still being very low (ca 200 000 people in South America) . While ethnographic and theoretical studies demonstrate how a process of alternating growth and decline offers one possible mechanism that can give the long-term appearance of a stable plateau-like population trend [66,67], we are unable to identify these hypothesized fluctuations. Indeed, neither did the modelling results presented by Goldberg et al. , which, like our results, indicate little or no change in the population size ca 9–7 kyr BP. The question, therefore, remains as to what prompted such a significant shift from a rapidly growing population to one that was stagnating. 4, 8.2 and 8.1 kyr BP) with exceptionally high frequency of climatic anomalies, which they correlate with an initial drop in relative population at and after 8.6 kyr BP and lasting until at least 6 kyr BP. Likewise, Goldberg et al. identified two mid-Holocene dips from additional SPD simulation analysis. Indeed, a relatively abrupt onset of aridity is recorded in a number of continental and erica [58,68], and specifically the SAAD [17,31,69–71]. This landscape was, therefore, remarkably different from the one experienced by the first colonizers. Almost all the megafaunal species were either extinct or going extinct , forest cover significantly decreased, surface water availability decreased and temperatures were higher . Importantly, a recent analysis at the scale of South America has identified demographic declines associated with climate change , thus substantiating the case for diverse demographic trajectories behind continent-wide patterns.

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