Stable isotope evidence for pre-colonial maize agriculture and animal management in the Bolivian Amazon

Biochemical analysis of the bone collagen of humans and muscovy ducks reveals the significance of maize to diets at the same time as significant socioeconomic complexity.
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Over the past two decades, there have been dramatic changes in archaeological perceptions of the Amazon. Recent evidence shows that instead of a Counterfeit Paradise, where environmental conditions limited agricultural potential and human development (Meggers 1971), the Amazon was a hub for plant domestication. Evidence for experimentation with plant cultivation extends as far back as 10,000 years ago (Lombardo et al. 2020; Watling et al. 2018), meanwhile, maize appears in the archaeological record around 4,500 BCE and has been argued to have played a major role in socioeconomic changes in different parts of the Amazon Basin (Lombardo et al. 2020). Despite the plentiful evidence of domesticates, their contribution to past diets across space and time is still a largely debated topic as archaeological plant remains suffer from inherent survival biases, particularly in tropical rainforest environments. Here, stable carbon (δ13C) and nitrogen (δ15N) isotope analysis from human bone becomes a key tool, as it can directly determine long-term human reliance on different resources and dietary change across space and time through the principle of ‘you are what you eat’ (Tykot 2004). 

Our paper is inserted into a larger project, aimed to contribute to understandings of subsistence strategies across the Amazon basin by using stable carbon and nitrogen evidence from human and faunal bone collagen recovered from various archaeological sites (Hermenegildo 2022). This presented quite a challenge, as the combination of heat and humidity of the equatorial forests creates one of the most hostile environments for the preservation of any biological remain, let alone for centuries. The prior state of stable isotope research in the Amazon highlights this issue, with only 16 human results published before the start of the study. Indeed, during the analysis, we learned the hard way why the data is currently so limited. Many of the bone remains showed poor collagen preservation and high levels of contamination, with one area of study providing no usable data at all. 

The surprise came with the study of the bone remains from the sites of Salvatierra and Mendoza, the focus of our new paper published in Nature Human Behaviour (Hermenegildo et al. 2024). These sites are located in the Bolivian Llanos de Mojos (Figure 1) and belong to the Casarabe culture, which has made recent headlines as a result of LiDAR work that has revealed vast earthworks, extensive networks of causeways and monumental constructions (Prümers et al. 2022). Critically for our work, these sites also have one of the largest known assemblages of human remains in the whole Amazon basin. They also demonstrated exceptional organic preservation. Of the 89 individuals collected, only one did not have enough collagen and two showed signs of contamination. 

Figure 1: The Llanos de Mojos (left) and the studied sites (right). Each triangle represents a monumental site

Figure 1. The Llanos de Mojos (left) and the studied sites (right). Each triangle represents a monumental site.

Importantly, the isotope results pointed to an overall dominance of maize in the diets for most of the sites’ occupation, between around 700 and 1400 CE (Figure 2). Given the volume of carbonized kernels recovered at Salvateirra (Bruno 2010), we expected maize to have made a noticeable contribution to the human stable isotope record, however, not to the extent that it was a core dietary staple. Our data indicates for the first time that, in certain contexts, maize agriculture was a viable subsistence strategy in ancient Amazonia. Moreover, maize likely had a central role in the expansion of the vast network of Casarabe sites and their monumental constructions. The early evidence of maize in the north Llanos de Mojos (Lombardo et al. 2020) and the evidence of the agricultural Casarabe culture at the end of the sequence suggests that the Llanos de Mojos was a fundamental area in the adaptation and expansion of maize into the Amazon. Recent genetic evidence supports this idea, placing the southwestern Amazon as a secondary improvement centre for maize, from where it later expanded and diversified into new varieties (Kistler et al. 2018). 

The archaeological evidence from the Amazon, however, highlights the incredible variety of plant domesticates utilized by Indigenous societies, many of which are fundamental crops in present times such as sweet potatoes and manioc (Iriarte et al. 2020). On the other hand, local domestication of animals is a rare occurrence, mostly reserved to the Andean region (llamas, alpacas and guinea pigs). Muscovy duck (Cairina moschata) is the only locally domesticated species to be endemic to the South American lowlands but there is overall little archaeological evidence of its presence in the region and its history of management has remained uncertain. However, the fauna assemblage from Salvatierra shows a sizeable quantity of muscovy duck remains. Here, remarkably, our stable isotope data demonstrated a similar – if not higher – maize consumption to the humans. This indicates ducks were likely intentionally fed maize, or even possibly domesticated, since at least 800 CE. Such evidence of management has not been previously reported in the archaeological record of the South American lowlands.

Figure 2. Top: δ13C and δ15N values from Salvatierra and Mendoza humans as well as fauna from Salvatierra divided according to dietary niches. Bottom: Bayesian inferred ellipse of the same values. Snake samples were not included in Bayesian inferred ellipse analysis as the sample size (n=2) is too small to draw reliable inferred valeus Groups: Ungulates (deer [Mazama sp.] and tapir [Tapirus terrestris]), rodents (agouti [Dasyprocta sp.] and capybara [Hydrochoerus hydrochaeris]), armadillos (Dasypus novemcinctus and Euphractus sexcinctus), muscovy duck (Cairina moschata), and riverine (eels [Lepidosiren paradoxa and Synbranchus spp.] and caimans [Caiman sp.]). Standard ellipse area of isotopic niches represents an estimated 40% of the population.

Added to the recent LIDAR evidence detailing the extent and complexity of the low-density urban settlements of the Casarabe culture (Prümers et al. 2022), the stable isotope evidence of maize staple diets and muscovy duck management places the Llanos de Mojos as a unique context in the Amazon basin. This evidence of sedentary agricultural societies completely confronts perspectives on ancient Indigenous peoples of the Amazon, for decades believed by archaeologists to be largely comprised of non-agricultural, small and scattered groups (Meggers 1971). Furthermore, the stable isotope evidence adds to a growing body of studies showing the antiquity and importance of maize in the Amazon (Lombardo et al. 2020; Kistler et al. 2018), contributing to unravelling an unknown history of maize and, most importantly, the people who flourished with it.

Comparison of our data from these remarkable sites with other work undertaken during my PhD in the eastern Amazon Basin highlights that diets varied considerably across the Amazon (Hermenegildo 2022). It is clear that Indigenous societies relied on a great diversity of resources including manioc, sweet potatoes and squash, as well as wild plant and animal resources, with maize being just one of the many elements of diversified subsistence strategies where multiple domesticated and managed plant resources were consumed, without a reliance on a single staple crop. Future work will continue to explore the dietary strategies that sustained different societies across this vast area through space and time, providing insights into the economic and ecological adaptations that sustained perhaps as many as 20 million people across the Amazon Basin at the time of European colonial invasion (Koch et al., 2019).

Paper
Hermenegildo, T., Prümers, H., Jaimes Betancourt, C., Roberts, P., O'Connell, T. C. Stable isotope evidence for pre-colonial maize agriculture and animal management in the Bolivian Amazon. Nat Hum Behav (2024). https://doi.org/10.1038/s41562-024-02070-9

References

Meggers, B.J. Amazonia: Man and Culture in a Counterfeit Paradise (Smithsonian Institution Press, Washington DC) (1971).

Lombardo, U. et al. Early Holocene crop cultivation and landscape modification in Amazonia. Nature 581, 190- 193 (2020).

Watling, J. et al. Direct archaeological evidence for Southwestern Amazonia as an early plant domestication and food production centre. PloS One, 13, e0199868 (2018).

Tykot, R. Stable isotopes and diet: you are what you eat. In Physics methods in archaeometry (IOS press) (2004).

Hermenegildo, T. Fields and Forests: A Stable Isotope Perspective on the Subsistence Strategies of Past Amazonian Peoples [Apollo - University of Cambridge Repository] (2022). 

Prümers, H. et al. Lidar reveals pre-Hispanic low-density urbanism in the Bolivian Amazon. Nature, 606, 325-328 (2022).

Bruno, M. Carbonized plant remains from Loma Salvatierra, Department of Beni, Bolivia. ZAAK, 3, 151-206 (2010).

Kistler, L., et al. Multiproxy evidence highlights a complex evolutionary legacy of maize in South America. Science 362, 1309-1313 (2018).

Iriarte, J., et al. The origins of Amazonian landscapes: Plant cultivation, domestication and the spread of food production in tropical South America.  Sci. Rev.248, 106582. (2020).

Koch, A., et al. Earth system impacts of the European arrival and Great Dying in the Americas after 1492.  Sci. Rev.207, 13-36 (2019).

 

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