2.1: Reading/Media

The Bolivian Amazon. On the seasonally flooded forest-savanna landscape in Bolivian Amazon, the ancient Casarabe culture (500-1400 CE) developed water drainage and storage infrastructure to cultivate maize monoculture. (Photo by Dirk Embert/WWF. Wikimedia.)

Maize Monoculture on Drained Savannas in Pre-Hispanic Amazonia, 700 BP

Excerpts from "Lidar reveals pre-Hispanic low-density urbanism in the Bolivian Amazon" by H. Prümers, C. Betancourt, J. Iriarte, et al. Nature 606, 325–328 (2022). CC BY

Excerpts from "Maize monoculture supported pre-Columbian urbanism in southwestern Amazonia" by U. Lombardo, L. Hilbert, M. Bentley, et al. Nature 639, 119–123 (2025). CC BY 4.0

The Casarabe culture of Amazonia

Excerpt (Prümers et al.): Archaeological remains of agrarian-based, low-density urbananism have been reported to exist beneath the tropical forests of Southeast Asia, Sri Lanka and Central America. However, beyond some large interconnected settlements in southern Amazonia, there has been no such evidence for pre-Hispanic Amazonia. Here we present lidar data of sites belonging to the Casarabe culture (around AD 500 to AD 1,400) in the Llanos de Mojos savannah–forest mosaic, southwest Amazonia, revealing the presence of two remarkably large sites (147 ha and 315 ha) in a dense four-tiered settlement system. The Casarabe culture area, as far as known today, spans approximately 4,500 km², with one of the large settlement sites controlling an area of approximately 500 km². The civic-ceremonial architecture of these large settlement sites includes stepped platforms, on top of which lie U-shaped structures, rectangular platform mounds and conical pyramids (which are up to 22 m tall). The large settlement sites are surrounded by ranked concentric polygonal banks and represent central nodes that are connected to lower-ranked sites by straight, raised causeways that stretch over several kilometres. Massive water-management infrastructure, composed of canals and reservoirs, complete the settlement system in an anthropogenically modified landscape. Our results indicate that the Casarabe-culture settlement pattern represents a type of tropical low-density urbanism that has not previously been described in Amazonia.

Figure \(\PageIndex{1}\): a, Occupation of the Cotoca lidar area. b, Sites and major archaeological features revealed by lidar in the Cotoca area. c, Lidar image of the large settlement site Cotoca with cross sections A–B and C–D. m.a.s.l., metres above sea level. (From Prümers et al. 2022.)

Subsistence food cultivation of the Casarabe culture

Excerpt (Lombardo et al.): The Casarabe culture (500–1400 CE), spreading over roughly 4,500 km² of the monumental mounds region of the Llanos de Moxos, Bolivia, is one of the clearest examples of urbanism in pre-Columbian (pre-1492 CE) Amazonia. It exhibits a four-tier hierarchical settlement pattern, with hundreds of monumental mounds interconnected by canals and causeways. Despite archaeological evidence indicating that maize was cultivated by this society, it is unknown whether it was the staple crop and which type of agricultural farming system was used to support this urban-scale society. Here, we address this issue by integration of remote sensing, field survey and microbotanical analyses, which shows that the Casarabe culture invested heavily in landscape engineering, constructing a complex system of drainage canals (to drain excess water during the rainy season) and newly documented savannah farm ponds (to retain water in the dry season). Phytolith analyses of 178 samples from 18 soil profiles in drained fields, farm ponds and forested settings record the singular and ubiquitous presence of maize (Zea mays) in pre-Columbian fields and farm ponds, and an absence of evidence for agricultural practices in the forest. Collectively, our findings show how the Casarabe culture managed the savannah landscape for intensive year-round maize monoculture that probably sustained its relatively large population. Our results have implications for how we conceive agricultural systems in Amazonia, and show an example of a Neolithic-like, grain-based agrarian economy in the Amazon.

Figure \(\PageIndex{2}\): Ancient Amazon rock art. Figurative images from the Late Pleistocene show (a) fishing and (b,c,e) hunting as way of life. But whether Amazonians had developed the subsistence agriculture of a complex society in the Holocene has not been previously revealed. (From Hampson et al. Arts 13, 135, 2024.)

The role of grain agriculture as the subsistence base of prehistoric complex societies in both the Old and New World has been a matter of sustained debate for many decades. In Mesoamerica, the earliest evidence of maize as a staple crop dates to 4,000 calendar years before the present. The timing and nature of maize’s role as the staple crop of Andean civilizations, as seen in early historical accounts, is controversial. In Amazonia it is well established, from both archaeological and palaeoecological data, that maize has been cultivated since at least 6,850 calendar years before the present; however, to date there is no evidence of it being a staple crop. Most societies had mixed economies relying on multiple cultigens. Roosevelt proposes that the rise of social complexity in the Amazon was based on maize agriculture. However, current archaeological evidence has not been conclusive of maize cultivation being the staple crop of complex societies of the Amazon. Current archaeobotanical and palaeoecological data from Late Holocene complex societies in Amazonia indicate polyculture (mixed-cropping) agroforestry, not maize monoculture, as the basis of a subsistence economy.

Recent archaeological research has revealed evidence for low-density urbanism, social complexity and large populations in the Andean foothills of the Upano River region of Ecuador, and in the monumental mounds region (MMR) in the seasonally flooded savannahs of the Bolivian Amazon. Here in the MMR, the Casarabe people built hundreds of monumental mounds interconnected by canals and causeways across a flat forest–savannah mosaic landscape dominated by seasonally flooded savannahs, with forests restricted to non-flooded palaeo-river levées. Whereas drained fields and terraces, built on extremely fertile volcanic soils, were clearly integral to low-density agrarian urbanism of the Upano region, the type of farming system needed to sustain the Casarabe culture is still unknown. It has been proposed that the construction of drainage canals permitted cultivation of the relatively fertile sediments of the seasonally flooded savannahs of the MMR without the need for deforestation. However, no agricultural fields or other food production systems have hitherto been found in connection with such canals, leaving unanswered the question of how the Casarabe people managed to feed its relatively large population. To address this issue, we combine remote-sensing imagery with a programme of coring, test pits, radiocarbon dating and pollen and phytolith analyses on both seasonally flooded savannahs and forest.

We have identified two unreported and complementary agrotechnologies in the savannahs of the MMR: dense drainage networks and artificial farm ponds (Fig. \(\PageIndex{3}\)), in which different portions of a savannah (Fig. \(\PageIndex{3}\), top left inset), or different savannahs within the same area (Fig. \(\PageIndex{3}\), bottom right inset), have been heavily modified—into either intricate arrangements of canals or clusters of circular depressions.

Figure \(\PageIndex{3}\): Triangles represent monumental mounds; black dots, ponds; thin black lines, canals; light grey areas, forest; white areas, savannah; and dark grey areas, lakes. Inset top left, the northern side of a savannah is crisscrossed by canals but the southern side is dotted with ponds. Inset bottom right, one savannah (to the east) has been modified by the excavation of a densely packed network of drainage canals, and two other savannahs, to the west, are dotted with ponds; in the former, the network of canals drains into Lake Francia located about 4 km to the north. 690 and 695 indicate the locations of the two farm ponds sampled for this study. Inset top right, boundaries of Amazonia and the image of World Countries Generalized provided by ESRI under the ArcGIS Pro license. Scale bar, 5 km. Credit: European Commission JRC. (From Lombardo et al. 2025)

Hydraulic modification of the savanna landscape

The drainage network

In one of the savannahs under study (Fig. \(\PageIndex{4}\)), the small canals converge into larger canals that drain the whole savannah toward Lake Francia to the north. We identified three orders of drainage canals: the first order (1 in Fig. \(\PageIndex{4}\)b), the smallest, are around 4 m wide and 25 cm deep, the second order (2 in Fig. \(\PageIndex{4}\)) are around 8 m wide and 70 cm deep and the main canal (third order) that drains into the lake is 14 m wide and 1.8 m deep (3 in Fig. \(\PageIndex{4}\)), becoming 3.2 m deep about 1.5 km before reaching the lake. Overall, the drainage network drains towards the north, becoming ever deeper with respect to the general topography. Several stratigraphic profiles of the canals show that the original depth of the canal network was around 80 cm deeper than at present for the second-order canals (see profiles 667 and 671) and around 45 cm deeper for the first-order canals (for example, profile 674). The drainage network is associated with circular elevated platforms roughly 50 cm in height, resembling pre-Columbian forest islands, and with small mounds of around 2–3 m in diameter. The elevated platforms are surrounded by deep canals (profiles 666 and 677).

Soil cores were collected from several locations both inside the canals and between them. Phytolith analysis shows a high abundance of phytoliths derived from the cob glumes and leaves of Zea mays in almost all canal soil profiles, with sporadic presence of Cucurbita spp. (666 and 677), Manihot sp. (677), Calathea sp. (674) and Lagenaria sp. (667) phytoliths. We cannot exclude the possibility that Cucurbita was cultivated in greater amounts than implied by the phytolith assemblage, because some domesticated Cucurbita varieties may lack scalloped phytoliths.

The majority of Oryza phytoliths are concentrated in the upper levels of all the profiles. Most upper-level glume phytoliths were from domesticated plants, whereas those from lower levels in the soil profiles (30–50 cm depth) were classified as wild species. These results are not surprising, because some of these fields are currently being used to grow modern Asian rice. The low production of diagnostic wavy-top rondel phytoliths in maize and the high abundance of these, relative to the sporadic presence of phytoliths of other cultivars, indicate that maize was by far the principal cultivar in these savannahs. We attribute the absence of maize phytoliths in the uppermost 20–25 cm of the canal soil profiles to sedimentary fill from adjacent fields over recent decades or centuries—an inference corroborated by our soil phytolith data from fields between the canals. Here, maize phytoliths appear in only one of the three profiles, suggesting that the cultivated area is likely to have been established along the canals, probably on elevated rims that have since eroded into the canals. It is probable that, while in use, the original depth of the canals was maintained by their periodic re-excavation and redistribution of canal sediment fill along the canal margins, where maize was then planted, mimicking what has been proposed for raised-field agriculture in other regions of the Llanos de Moxos.

Figure \(\PageIndex{4}\): a, The complete drainage network. Numbered dots indicate the location of phytolith profiles. b, Detail of the digital elevation model of the northern part of the drainage network. Dashed lines represent the forest–savannah boundary, showing that a great deal of the drainage network is presently covered by forest. Scale bars, 1 km (a), 500m (b). (From Lombardo et al. 2025.)

The forest

Forest in the study region grows on elevated surfaces, mostly fluvial levees, that remain above the water level during the rainy season. Four soil profiles were dug and sampled across the forest, along a transect from the savannah to a large 15-ha monumental mound, to reveal to what extent the forest was cleared for agriculture. No charcoal or any other evidence of fire was visible in any of the profiles. Phytolith profiles are all similar and do not show any obvious stratigraphic change, apart from a slight reduction in Arecaceae (palms) and an increase in Poaceae (grass) phytoliths, which could indicate a slight opening of the forest canopy. No cultivar phytoliths were found in any of the profiles. Although our data cannot show the extent to which the forest was used for agroforestry, wood harvesting, hunting or cultivation of medicinal plants, the absence of charcoal does show that slash-and-burn agriculture did not take place here.

The clusters of farm ponds

A large portion of the savannahs in the MMR contains clusters of circular depressions of 10–100 m in diameter. They are often connected, either by canals or directly by adjacency. These ponds are similar to natural depressions called gilgais, an Aboriginal Australian name for water holes, that form in vertisols because of repeated expansion and contractions of the clay. To understand their genesis and use, we sampled two ponds in two different savannahs. Profiles 690 and 689 were excavated and cored, respectively, in a large pond of roughly 100 m in diameter, with its central depression about 60 cm below the surroundings. Profile 695 was excavated in a pond of roughly 30 m in diameter, and with a central depression currently 40 cm below the surrounding savannah. Pond profile 690 exhibits a very irregular, sharp contact between the organic sediment fill and the grey, inorganic clay below, and shows no evidence of shear surfaces (slickensides), suggesting that the pond was excavated and is not a gilgai. The anthropogenic origin of these depressions is further supported by their size, which is far larger than the 15–20-m-diameter gilgais, and by their clustered linear distribution. Sediment profiles from both ponds show the continuous presence of maize phytoliths and pollen throughout, with phytoliths of Cucurbita sp. present in only two adjacent samples in profile 690 at around 40 cm depth, and a pollen grain of Manihot at approximately 50 cm depth. No other cultivars were detected. The chronology of pond profile 690 indicates that this system was in use around 1250–1550 calendar years CE.

Today, the majority of these ponds hold water for most of the year, maintaining wet soil until the very end of the dry season. If this hydrological balance is representative of the past, these ponds would have provided sufficient water for maize cultivation around their margins throughout the dry season. This is not dissimilar to the k’hochas in the Bolivian altiplano, where yields are up to four times higher than for regional rainfed production. Similar pond-based farming systems have been described in Bangladesh and India, where ponds provide an integrated production system that includes fish farming, poultry and cultivation of pond dykes. A very similar system could have been in place in the MMR, where swamp eels (Synbranchus marmoratus) were an important part of the diet of the Casarabe culture; the Muscovy duck (Cairina moschata), the only known domesticated animal in the Amazon, was probably kept. Bones of S. marmoratus were found in pond 695 at a depth of 70 cm. During the dry season, these farm ponds would have served as ‘watering holes’, attracting game.

Water management for intense farming

The combination of these two types of landscape engineering—drainage canals and farm ponds—is unique to the MMR. We argue that it was a highly innovative agricultural strategy that enabled the Casarabe culture to substantially increase the cultivation period for maize, as well as providing easy access to fish, birds and game. Through the sophisticated system of drainage canals, some savannah wetlands were converted into drained fields suitable for intensive maize monoculture in the wet season, whereas the construction of clusters of farm ponds in other savannahs provided a reservoir of water that allowed pot irrigation, which enabled the continuation of maize agriculture throughout the dry season. The combination of these two water management systems would have allowed at least two harvests of maize per year. The lack of any evidence of cultivation and fire in the nearby forested areas suggests that slash-and-burn agriculture was unlikely to have been practised. Instead, this pre-Columbian Casarabe culture probably preserved the spatially limited, and hence highly valuable, forest resource for other key ecosystem services, such as firewood, building materials, medicinal plants and probably polyculture agroforestry.

Our results overturn the assumption that the seasonally flooded savannah of southwestern Amazonia is suitable only for cattle ranching and intensive Asian rice agriculture, and unsuitable for nutrient-demanding crops such as maize. These findings have implications for our understanding of pre-Columbian subsistence economies across Amazonia and beyond. They indicate that, during the late Holocene, alongside intensive polyculture (mixed-cropping) agroforestry on Amazonian Dark Earths, other agricultural systems such as drained fields and farm ponds in the Llanos de Moxos were primarily focused on the cultivation of maize. These practices bear similarities to agricultural strategies observed in later Andean states and chiefdoms. Collectively, as long argued, intensive cultivation of maize has had a major role in supporting the economy of some of the most complex societies in the Americas. The Casarabe people demonstrated the ability to establish a highly intensive monoculture farming system on the savannahs based on maize, maintaining the surrounding forest cover and supporting one of the most complex pre-Columbian societies in lowland South America. The Casarabe culture of the MMR provides a clear example of when the rise of social complexity is linked to intensive food production and, more specifically, to maize monoculture. It also confirms the role of grain agriculture as the main driver for increasing social complexity and, probably, inequality.

Key Geologic Concepts:

Drainage networks; sedimentation; sediment core analysis; wetland.

Supplemental Materials:

Youtube: Fire and Water: Pre-Columbian landscape management in the Southwestern Amazon by N. Duncan, University of Central Florida.

Smithsonian Magazine: "Lost Cities of the Amazon Discovered From the Air" by B. Handwerk. May 22, 2022.

University of Reading "Ancient Amazonians’ water wisdom kept families fed all year". 29 January 2025.