Study Areas

Study area 2 spans the Bolivian-Brazilian border, encompassing the geologically-controlled forest-savanna mosaic of the easternmost LM, clear-water (nutrient-poor) rivers, and interfluvial terra firme rainforest of the Pre-Cambrian Shield (PCS). A 200 km2 LiDAR survey has revealed 20 circular or elliptical trench systems (‘ring ditches’), some of which cover 200 ha, hidden beneath rainforest. The high density and monumental scale of these earthworks suggest a stratified society. Initial excavations of several ring ditches reveal a complex spatial-temporal pattern of discrete single occupation layers that together attest to human occupation from ca. 300 to 1500 CE. This spatially shifting pattern of late Holocene occupation contrasts with Study Area 1, as well as the Monte Castelo (MC) shell mound (6 m high, 1.5 ha in area) in the river Guaporé savanna flood-plain of this study area. At MC, recent test excavations reveal a continuous >8,000 year occupation sequence. Evidence from seeds, starch grains and phytoliths demonstrates cultivation of maize, manioc, sweet potato, rice, squash and beans as early as 2500 BCE, while ceramics dating to 3,200 BCE are among the oldest in the Americas. Deeper deposits have yielded lithic remains, hearths and other cultural materials/structures which, based upon preliminary work in the 1980s, likely pre-date 6,000 BCE. The MC site contains abundant human bones in mortuary contexts extending to 2,500 BCE, providing excellent potential to determine, through stable isotope analyses, how the contribution of maize to the diet may have changed through time – in particular, whether maize was consumed as part of a mixed subsistence diet or became the main agricultural staple.