Scientists unearth secrets of Bolivian Amazonia

 The enigmatic ‘forest islands’ set amidst the grasslands of Bolivian Amazonia have yielded the earliest evidence of human habitation in the region. Previously thought to be relict landforms cut away by shifting rivers, or long-term bird rookeries or termite mounds, these piles of freshwater snails, animal bones and charcoal are now known to have been built up over millennia, starting from at least 10,400 years ago, by ancient hunter-gatherers.

Researchers say that Isla de Tesoro tells us that early South Americans moved across a wider variety of landscapes than previously thought, and adapted their ways of life to cope in these challenging environments [Credit: University of Wollongong]

Using novel approaches drawn from archaeology, geomorphology and geochemistry, an international team of researchers, led by Dr Umberto Lombardo of the University of Bern, has conducted detailed excavations of a large mound known locally as Isla del Tesoro (Treasure Island).

Isla del Tesoro tells us that from over 10,000 years ago, hunter-gatherers were moving across the grasslands hunting a variety of mammals, catching fish and birds, and gathering large quantities of freshwater snails [Credit: University of Wollongong]

Associate Professor Katherine Szabó from the University of Wollongong’s Centre for Archaeological Science and Dr Jan-Hendrik May from UOW’s School of Earth and Environmental Sciences were among the international team members.

Cross-section transect of the shell midden SM1: Dashed lines and grey arrow highlight the onion-like growth of the midden reflected in the 14C dates. The black triangles above mark the coring locations and the white triangle the excavation site [Credit: Public Library of Science]

Distinctive chemical signatures of human presence were recorded at high levels throughout the mound sediments, and studies of the animal bones and shells indicate they are the remains of ancient human meals. Team members said that Isla del Tesoro tells us that from over 10,000 years ago, hunter-gatherers were moving across the grasslands hunting a variety of mammals, catching fish and birds, and gathering large quantities of freshwater snails.

Details of recovered burnt earth, shells and bone remains from excavations at SM1. A) Thin section from Unit VI; aragonitic and micritic shell fragments cemented together; a bone is visible in the upper left corner (cross polarized light, XPL); b) Pomacea shells found at a depth of 110 cm; c) Impact scar between refitted fragments of a Blastocerus dichotomus tibia found at 160 cm. Mineral dendrites covering the edges of bones and surface damage indicate a percussive blow; d) Mandibular fragment of Mazama sp. found at a depth of 70–75 cm; e) Fragment of burnt earth found at a depth of 140 cm with incised parallel lines, probably culturally-modified; f) Units IV and V as observed in the excavations; Unit V is a layer of well cemented shells surrounded by loose fragments that form Unit IV [Credit: Public Library of Science]

Over time, the refuse of these hunting and gathering forays built up forming mounds, which sat elevated above the floodplain. These refuse or ‘midden’ mounds in turn provided a habitat for local plants and animals, transforming them into the forest islands so recognisable in the landscape today. It is highly likely that many more midden mounds lie buried beneath the metres of silts under the current savannah, according to the researchers.

Material retrieved from the uppermost 30 cm of SM1 : Unit I, corresponding to the late-Holocene occupation. A) Fragmented pottery; b) Bone tools; c) Fragment of human skull; d) Biogenic burnt earth, probably a wasp chamber [Credit: Public Library of Science]

Regularly flooded savannah landscapes such as those surrounding Isla del Tesoro have long been thought to be an inhospitable environment for early hunter gatherers. The densities of animal prey are lower and less predictable than in coastal areas, near stable watercourses or in forested areas where early South American archaeological sites are typically found. Dr Lombardo and colleagues’ work at Isla de Tesoro tells us that early South Americans moved across a wider variety of landscapes than previously thought, and adapted their ways of life to cope in these challenging environments.