GoKawiilToday, we know that soil and nutritional conditions in the Amazonian forest are highly heterogeneous. In Acre, several local interlocutors have confirmed that the soil is fertile and suitable for maize cultivation, for example. This has currently been confirmed by a base cation concentration map, which shows that the soil in the Aquiry area is among the most nutrient rich in the entire terra firme forest of Amazonia40. Furthermore, the correspondence between semi-domesticated and domesticated trees and the presence of Amazonian earthworks has been demonstrated8,11. Brazil nut trees were among them, and our study suggests that its current western extension to the Purus River41,42,43 corresponds to the western limit of the geometric ditched enclosures. The macrofossils of the Brazil nut and the fruits of diverse palms, such as the bacaba (Oenacarpus mapora), peach palm (Bactris gasipaes var. gasipaes and var. chichagui), uricuri (Attalea phalerata) and some Astrocaryum species have been detected in excavations as important sources of protein for the local population, and those tree species are still present around earthworks8. The variety of crops cultivated in the Aquiry area is also impressive, as detailed in Spanish accounts from the sixteenth and early seventeenth centuries8. In addition, phytolith studies have revealed the ancient presence of maize (Zea mays) and squash (Cucurbita sp.) in the geoglyph sites37. Furthermore, manioc, sweet potato, chilli pepper, beans and peanuts were also probably among the most common cultigens37. Therefore, poor soil and carrying capacity do not appear to be major limiting factors for population density in this area, as has been suggested for Amazonia as a whole12. On the contrary, the members of the geoglyph-building communities were active stewards of the land, water and forest gardens, and the construction and management of the sites were clearly linked to sociocosmology and social life entangled with more-than-human beings24.
To summarize, by using airborne LiDAR technology, it has been possible to triple the area of the Aquiry civilization from the earlier 60,000 km2 (ref. 1) to approximately 183,000 km2. Although the southern, western and northern boundaries of the Aquiry are well documented by the presence of geometric ditched enclosures, the question of its southeastern corner remains open due to the lack of archaeological research on earthworks in Rondônia. The presence of 347 total identified earthworks, with 334 of those earthworks used for extrapolation, in the systematically scanned area of 2,380 km2 is much higher than expected. According to our estimation, the entire area would have approximately 24,000–30,000 earthworks, of which about two thirds belonged to the Aquiry.
Today, the state of Acre covers 164,000 km2 with a population of 0.83 million (5.1 people per square kilometre), of which about 3.5% are Indigenous44. Although different methods and datasets may yield somewhat different conclusions, our results strongly suggest that between ad 100 and 300 the population of the Aquiry was approximately 1.25–3 million (7.1–16.5 per square kilometre), based on LiDAR data, earlier satellite imagery, ethnographic information and archaeological excavations. Furthermore, population density in the 85,000 km2 core area may have reached 13.6–32.6 per square kilometre.
It seems that periodic burning of the same bamboo-rich patches over thousands of years45,46 maintained the fertility of the soil and helped to create the conditions for the emergence of complex societies in the first millennium bc, comparable with other great South American civilizations. Eventually, the density of earthwork openings in the forested landscape became such that the concept of forest urbanism47, or low-density urbanism13,48, can be adequately applied, despite the lack of consensus on the meanings attached to the term ‘urbanism’49. At the same time, different social and linguistic groups were involved in managing, planting and maintaining their subsistence systems as well as participating in hunting, fishing and gathering activities. However, unlike many other tropical civilizations16, it appears that large-scale water management projects with raised fields and causeways were not a major focus of their subsistence strategies in extensive interfluve environments, although the presence of water sources appears to be one of the key elements for the earthwork sites and their roads3. In any case, these ancient achievements constitute an important legacy that is still observable in the soil, and the composition and biodiversity of the contemporary tropical forest. This legacy should be fully considered in future studies of Amazonia, including climate modelling and sustainable land-use planning. Furthermore, if our findings are confirmed and comparable results are obtained in other regions of the wider Amazonian area, the total precolonial population density should also be re-evaluated.