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Examining pan-neotropical diasporas

Periodic Reporting for period 1 - EXPAND (Examining pan-neotropical diasporas)

Reporting period: 2019-06-01 to 2021-05-31

The ExPaND (Examining Pan-Neotropical Diasporas) project integrates archaeology, paleoecology and computer modelling to test the role of climate change in the dispersal of tropical forest farmers out of Amazonia during the late Holocene. The expansion of farmers and their languages was a key process in shaping cultural and linguistic geographies across the globe. Many such expansions have recently been shown to coincide with periods of climate change, which would have offered both opportunities and constraints for human migrations. South America offers a perfect case study, with major cultural diasporas occurring over the last 5000 years - a period of increasing precipitation and expansion of forests. With 200 million people estimated to become migrants due to sea-level rise, desertification and other environmental hazards in the next 30 years, understanding human migration dynamics in response to climate change is a topic that reaches far beyond archaeology.

The objective of the project was to test whether climate-driven forest expansion during the late Holocene facilitated the spread of cultures practising polyculture agroforestry (forest farming) from the Amazon to other parts of lowland South America. This was accomplished by compiling archaeological and palaeoeclogical data that could be integrated in an agent-based model (computer simulation) of human expansions in scenarios of climate change. If late Holocene climate change had a role in the migration of South American cultures, it was expected that simulations including the environment as a constraint to human settlement should replicate the archaeological chronology more accurately than those where movement happened regardless of the environment.
Archaeological data were collected from publications, databases, unpublished theses and reports. The database, which was made available online, contains 2762 radiocarbon dates from 1023 archaeological sites in lowland South America. These data were analysed using statistical methods for ascertaining spatial patterns in the dispersal of archaeological phenomena - i.e. to understand their origin and speed of advance. Specifically, four archaeological cultures were tested, each related to the diffusion of ceramics and farming and associated with the expansion of a major language family in South America: Saladoid-Barrancoid (Arawak languages), Incised-Punctate (Karib languages), Tupi (Tupi languages) and Una (Macro-Jê languages). Space-time regressions were employed, correlating the earliest date per site with distance from hypothetical origins. The earliest sites were tested as potential origins, but iterations were also performed over all sites. Results showed high correlations and inferred speeds of expansion reasonable for demic diffusion in the case of some cultures, but not in others.

An agent-based model (computer simulation) of population growth and dispersal was developed based on a review of the literature about tropical farmers. Information was gathered on population growth, village size, how often did they move and how far. A range of values for demographic and territorial parameters could then be established. The model was written in Python and based on the architecture of published agent-based models for the expansion of the Neolithic. The model was run for the four archaeological cultures mentioned above, taking into account different simulated start dates and coordinates. For each culture, a comparison was made between simulated arrival times at different regions and the earliest respective radiocarbon dates. In order to arrive at the set of parameters that would best fit the archaeological gradients in radiocarbon dates, genetic algorithms were employed. Even in that case, only two archaeological cultures were found to be adequately modelled as demographic expansions, the first (Saladoid-Barrancoid) showing a speed of expansion comparable to that of the Neolithic in Europe (ca. 1 km/yr).

The previous model did not include a dynamic environment. To specifically test the role of climate change in human dispersal, the Tupi were chosen as a case study. The Tupi are one of the major cultural and linguistic expansions of South America, and one which has repeatedly been linked to climate change/forest expansion. The environment of the model was taken from previously published biome reconstructions based on the HadCM3 climate model. Two scenarios were tested, (1) one in which all land cells can be settled and (2) one in which only tropical moist forest cells can be settled. Simulated arrival times were compared to the empirical radiocarbon record of the Tupi sites. The results showed that including the reconstructed late Holocene vegetation as a constraint to movement greatly improves the match with the empirical radiocarbon record – giving further support to the hypothesis that climate change shaped the dispersal of forest farmers in South America.
The main contributions of the project to the state of the art of the discipline include the creation of the first unified database of archaeological sites and dates in lowland South America, and the development of new algorithms for analysis of spatial gradients in radiocarbon dates and simulation of demic-diffusion processes. The database developed by the project was later incorporated by other archaeological databases. The project also has consequences for our understanding of how climate change in the Neotropics affected human settlement in the past. In particular, the vast area connecting the Amazon and the Alantic Forest biomes has been shown to have experienced expansion and retraction of forests, shaping the migration of human cultures in the late Holocene, reinforcing the vulnerability of this region under scenarios of future climate change.
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