Lidar reveals pre-Hispanic low-density urbanism in the Bolivian Amazon

The research question focuses on understanding the settlement patterns and societal organization of the pre-Hispanic Casarabe culture in the Bolivian Amazon. Previous research had identified large monumental sites (lomas) and extensive canal systems in the Llanos de Mojos region, suggesting a complex society. However, the lack of detailed mapping of the sites, especially those hidden beneath dense forest cover, hindered a comprehensive understanding of the Casarabe culture's urbanism. This study aimed to address this gap by employing airborne LiDAR technology to map the extent and details of Casarabe settlements, testing the hypothesis that the culture exhibited a form of low-density urbanism comparable to those observed in other parts of the world but previously unknown in the Amazon. The importance of this study stems from its potential to significantly advance our understanding of the complexity and diversity of pre-Columbian societies in Amazonia, challenging existing narratives of sparse population and limited social organization in the region. Understanding the Casarabe culture's urban development is crucial for reconstructing the history of human interaction with the Amazonian environment and for broadening the understanding of global urban patterns.

The paper reviews previous research on low-density urbanism in other tropical regions, citing examples from Southeast Asia, Sri Lanka, and Central America, where LiDAR has been instrumental in revealing hidden urban landscapes. It notes the existing knowledge about the Casarabe culture in the Llanos de Mojos, including previous remote sensing and ground-based archaeological surveys that had identified numerous sites but lacked the resolution to fully understand their complexity. The literature review highlights the existing debate about the nature and extent of pre-Columbian settlement in Amazonia, with some studies suggesting sparse populations and others pointing towards more complex social organization. The authors point to previous archaeological studies that had already documented aspects of Casarabe culture including their agriculture, diet, and ceremonial practices but lacked a complete understanding of the settlement hierarchy and regional organization due to the challenges in mapping large areas of dense tropical forest.

The study employed airborne LiDAR (Light Detection and Ranging) to map a total of 204 km² across six areas containing known concentrations of Casarabe settlements. A Riegl VUX-1 scanner was used, mounted on a helicopter, flying at an altitude of 200 m above ground level. The raw LiDAR data underwent post-processing to correct for time offsets and height discrepancies between adjacent tracks. The resulting point cloud data (approximately 18 million points per km²) allowed for the creation of high-resolution digital elevation models (DEMs) with a mean point spacing of 0.3 m. These DEMs were then analyzed using various visualization techniques in ArcMap, including hillshade and slope analysis, to identify and map archaeological features. Volume calculations were performed for the major built structures at three key sites using ArcGIS, to estimate the scale of construction efforts. Radiocarbon dating was employed, using data from previously published studies and 144 new dates from this study for a total of 144 radiocarbon dates, which were analyzed using Bayesian methods with OxCal v.4.4.2 and the SHcal20 calibration curve, to determine the chronological timeframe of Casarabe occupation. The study also used a four-tiered classification system for sites, based on platform dimensions, architectural elaboration, presence of enclosures, number of causeways, and the scale of water management infrastructure.

LiDAR revealed two exceptionally large Casarabe settlement sites: Cotoca (147 ha) and Landívar (315 ha). These sites, along with 24 smaller sites (15 previously unknown), formed a four-tiered settlement hierarchy. The large sites featured elaborate civic-ceremonial architecture, including stepped platforms, U-shaped structures, rectangular platform mounds, and conical pyramids up to 22 m tall. They were surrounded by concentric polygonal banks and connected to lower-ranked sites by straight, raised causeways extending several kilometers. Massive water management infrastructure, including canals and reservoirs, was also evident. The volume of earth moved in constructing the Cotoca site’s core area (570,690 m³) is ten times greater than that of Akapana, the largest structure in Tiwanaku. The density of settlements across the 4,500 km² Casarabe cultural area averages 10 sites within a 10 km radius, indicating a highly integrated and dense settlement system. Higher densities were found in the eastern sector. Sites were spatially clustered, interconnected by causeways and canals, forming clusters ranging from 100 km² to over 500 km². A clear correlation was observed between the height of civic-ceremonial architecture and the size of the base platform. The large sites acted as central nodes for the region, but centrality was not essential for site location. Secondary centers could function as central nodes if a primary center was absent. The sophisticated water management system showcased the Casarabe ability to control water resources and manage the landscape for agriculture and other purposes.

The findings challenge the notion of sparse pre-Hispanic populations in western Amazonia. The scale and complexity of the Casarabe settlement system, characterized by monumental architecture, extensive infrastructure, and a highly integrated network of settlements, suggest a sophisticated level of social organization and resource management. The Casarabe culture’s unique form of tropical, agrarian low-density urbanism is unparalleled in South America and requires revision of existing models of pre-Columbian Amazonian societies. The scale and monumentality of Casarabe sites surpass those found in southern Amazonia, which lack monumental civic-ceremonial architecture. The findings provide new insights into the diversity of early urban development globally and highlight the significant contribution of LiDAR to uncovering hidden archaeological landscapes.

This study provides compelling evidence of a previously unknown form of low-density urbanism in pre-Hispanic Amazonia. The Casarabe culture's sophisticated settlement system, featuring monumental architecture and extensive water management, reveals a higher level of social complexity than previously understood. Future research should focus on detailed chronological studies to further understand the evolution of the Casarabe settlement system and the relationship between its different tiers, as well as investigate the social organization and cultural practices of the inhabitants.

The study's reliance on LiDAR data for mapping means that subsurface features are not directly observable. Further research including excavation work is necessary to provide more information about settlement organization and the daily lives of the people who lived there. While the authors address potential errors in the LiDAR data processing, some uncertainty remains regarding the precision of measurements, particularly regarding volumetric calculations.