Persistent Early to Middle Holocene tropical foraging in southwestern Amazonia

Abstract

The Amazon witnessed the emergence of complex societies after 2500 years ago that altered tropical landscapes through intensive agriculture and managed aquatic systems. However, very little is known about the context and conditions that preceded these social and environmental transformations. Here, we demonstrate that forest islands in the Llanos de Moxos of southwestern Amazonia contain human burials and represent the earliest settlements in the region between 10,600 and 4000 years ago. These archaeological sites and their contents represent the earliest evidence of communities that experienced conditions conducive to engaging with food production such as environmental stability, resource disturbance, and increased territoriality in the Amazonian tropical lowlands.

Fig. 1. Location of the Llanos de Moxos and the excavated forest islands.

(A) Location of known shell midden sites in relation to paleosol cores containing stratigraphic evidence of buried paleosols. Topographic maps showing the excavation units at (B) Isla del Tesoro (SM1), (C) La Charca (SM3), and (D) San Pablo (SM4). Photo credit: (A) Base image: World Imagery, Esri.

Fig. 2. Stratigraphic diagrams and calibrated radiocarbon dates in relation to weight frequencies of analyzed shell, bone, and burnt earth specimens from the study sites.

Note the pattern of initial increase and subsequent decrease in the abundance of shell remains, while bones and burnt clay fluctuate over time.

Fig. 3. Stratigraphy of cores and excavations carried out at SM4.

SM4 is placed above fluvial sands that overlay clay deposits (A). Both the sand and clay show high levels of hydromorphism: red and yellow staining due to the formation of iron oxides. These hydromorphic conditions are typical of tropical soils subjected to repeated cycles of humid and dry conditions, as it happens in regions with strong seasonality. However, currently, these sediments are found below a wetland (B), suggesting that these iron oxides are a relict feature of the past, when the average water table was meters below its current depth. A view from the north (C) shows how today the forest island is surrounded by tall marsh vegetation and connected to the mainland by a modern causeway. Photo credits: (B) Source image: DigitalGlobe, Google Earth, and (C) José M. Capriles, PSU.

Fig. 4. Burials documented at the Llanos de Moxos site SM3 forest island.

(A and B) Burial 1 and (C and D) burial 2 during excavations and after cleaning bones encrusted with carbonates. Photo credit: José M. Capriles, PSU.

Fig. 5. Chronology of the Llanos de Moxos forest island occupations.

Comparison of sum probability distribution of calibrated radiocarbon dates from the shell midden sites and regional paleosols (see table S2).