Environmental impact of geometric earthwork construction in pre-Columbian Amazonia

Abstract

There is considerable controversy over whether pre-Columbian (pre-A.D. 1492) Amazonia was largely "pristine" and sparsely populated by slash-and-burn agriculturists, or instead a densely populated, domesticated landscape, heavily altered by extensive deforestation and anthropogenic burning. The discovery of hundreds of large geometric earthworks beneath intact rainforest across southern Amazonia challenges its status as a pristine landscape, and has been assumed to indicate extensive pre-Columbian deforestation by large populations. We tested these assumptions using coupled local-and regional-scale paleoecological records to reconstruct land use on an earthwork site in northeast Bolivia within the context of regional, climate-driven biome changes. This approach revealed evidence for an alternative scenario of Amazonian land use, which did not necessitate labor-intensive rainforest clearance for earthwork construction. Instead, we show that the inhabitants exploited a naturally open savanna landscape that they maintained around their settlement despite the climatically driven rainforest expansion that began ∼2,000 y ago across the region. Earthwork construction and agriculture on terra firme landscapes currently occupied by the seasonal rainforests of southern Amazonia may therefore not have necessitated large-scale deforestation using stone tools. This finding implies far less labor-and potentially lower population densitythan previously supposed. Our findings demonstrate that current debates over the magnitude and nature of pre-Columbian Amazonian land use, and its impact on global biogeochemical cycling, are potentially flawed because they do not consider this land use in the context of climate-driven forest-savanna biome shifts through the mid-to-late Holocene.paleoecology | Amazonian archaeology | human-environment interactions | Anthropocene | Amazon rainforest

Impact of pre-Columbian “geoglyph” builders on Amazonian forests

“…This hypothesis was formerly criticized in light of findings that many earthworks in northeast Bolivia were constructed in nonforested landscapes (26), but our data indicate that, even in an archaeologically rich area that remained forested during the mid-to late Holocene, pre-Columbian deforestation was on a more localized scale than previously thought. Despite the number and density of geoglyphs, we did not find any pre-Columbian parallel for the length and extent of modern-day forest clearance in Acre.…”

“…Was bamboo forest also dominant before the geoglyphs, as some have suggested (36)(37)(38)? Or did people exploit and maintain a more open landscape afforded by dryer climatic conditions of the mid-Holocene (8000-4000 BP) (39), as recently found to be the case for pre-Columbian earthworks <1,000 y old in the forest-savanna ecotone of northeast Bolivia (26,40)? (ii) What was the extent of environmental impact associated with geoglyph construction?…”

“…Also known as "agroforestry," this type of land use is thought to have been common in pre-Columbian times, but its detection in the paleoecological record is often problematic (15) and studies based on modern distributions of useful species lack demonstrable time depth of forest modifications (23). Terrestrial paleoecology programs are essential for a better understanding of these issues, which have strong implications for the resilience of Amazonian forests to human impact and, subsequently, their future conservation (24)(25)(26).…”

Reply to Silva: Dynamic human–vegetation–climate interactions at forest ecotones during the late-Holocene in lowland South America

“…Regarding records of late-Holocene climate change, Silva (2) claims that the climate records discussed in our report (1) are all located in the high Andes, and questions the degree to which they represent lowland precipitation. As we argue in our report, these regions receive the majority of their precipitation from the Amazonian lowlands in the Holocene, and are therefore broadly representative of lowland precipitation history.…”

“…We are pleased that the publication of our recent study (1) has stimulated further discussion of the complexities of past humanvegetation-climate interactions in the Neotropics, and that Silva (2) considers that our study "fundamentally changes our understanding of the magnitude and nature of pre-Columbian land use in the Amazon region." However, we wish to address several of the points raised by Silva. Regarding ecotonal shifts in the South American lowlands, we acknowledge that ecotonal shifts have not been limited to the southern Amazon.…”