In recent years, scientists’ understanding of the Amazon rainforest has been increasingly informed by a body of research that suggests the landscape was actively, intensively shaped by indigenous peoples before the arrival of Europeans. Some studies ascribe the tree species that now dominate the forest to prehistoric human management and landscape engineering. Other work posits that when colonizers from Europe caused massive losses to indigenous Amazonians with disease, slavery and warfare, the sudden interruption in landscape-scale manipulation resulted in so much forest regrowth that it caused a global drop in atmospheric carbon dioxide that brought about a climactic shift that is known as the ‘Little Ice Age.’

Prehistoric human influence

Now a new study led by Smithsonian researchers, published in the journal Proceedings of the National Academy of Sciences, suggests that for at least the past 5,000 years, large areas of the rainforest in western Amazonia located away from the fertile soils near rivers were not periodically cleared with fire or subject to intensive land use by the indigenous population before the arrival of Europeans. The study, led by Smithsonian senior scientist emerita Dolores Piperno of the National Museum of Natural History and the Smithsonian Tropical Research Institute in Panama, is the latest entry in a nearly decade-long scientific debate over prehistoric human influence in the world’s largest rainforest.

‘Far from implying that complex, permanent human settlements in Amazonia had no influence over the landscape in some regions, our study adds substantially more evidence indicating the bulk of the indigenous population’s serious impact on the forested environment was concentrated in the nutrient-rich soils near rivers, and that their use of the surrounding rainforest was sustainable, causing no detectable species losses or disturbances, over millennia,’ Piperno says. To explore the extent and scale of indigenous modification of the Amazon, Piperno and her co-authors collected and analyzed a series of 10 roughly 3-foot-long soil cores from three sites in the remote northeastern corner of Peru.

Study sites

The three sites were located at least a half-mile (about 1 kilometer) away from river courses and floodplains, known to researchers as terra firme forests. These forests comprise more than 90% of the Amazon’s land area and is therefore crucial to determining the extent of indigenous influence on the landscape, precisely because most major settlements identified by archaeologists thus far are near rivers.

Piperno and her co-authors used the soil cores to create timelines of plant life and fire history at each location going back some 5,000 years. To do this, the team extracted long-lasting microfossil particles of dead plants called phytoliths and looked for traces of fire such as charcoal. Fire, in a landscape that receives nearly 10 feet of rain annually, is nearly always human in origin and would have been instrumental in clearing large areas of land for human uses, such as agriculture and settlement.

Phytoliths

The team identified which plant type each phytolith belonged to by comparing them with a comparative reference library of modern plants and used radiocarbon dating to reveal how long ago the plants lived. The dating of both phytoliths and charcoal determined the age of the plant fossils and any remnants of fire found in a core. Finally, the researchers also conducted surveys of the modern forests found around each core. These forest inventories evinced the dizzying diversity of the region, yielding 550 tree species and 1,300 other species of plants.

All the analyses pointed in the same direction. There was no evidence for crop plants, slash and burn agriculture, forest clearing or the establishment of forest gardens. “These are very similar to results from other terra firme regions of Amazonia, suggesting wholesale alterations of forest across vast areas of Amazonia did not occur in prehistory,’ says co-author Crystal McMichael from the UvA Institute for Biodiversity and Ecosystem Dynamics.

Resilience to past disturbance

Instead, the researchers saw a rainforest ecosystem that remained relatively stable for thousands of years and is much like the ones still standing in similarly undisturbed regions today. Piperno and McMichael state ‘This means that ecologists, soil scientists and climatologists looking to understand this region’s ecological dynamics and capacity for storing carbon can be confident that they’re studying forests that haven’t been heavily modified by people.’

But Piperno says it also means we should not assume the forests were once resilient in the face of significant past disturbance,’ and added that this has important implications for good sustainable land use and conservation policies because such policies require adequate knowledge of past anthropogenic and natural impacts on the Amazonian ecosystem together with its responses. In light of these results, the research team also finds the idea that reforestation following the arrival of Europeans triggered the Little Ice Age implausible. ‘Without significant forest clearing in these and other regions studied by our team and others it appears unlikely that there was sufficient forest regeneration to have affected global carbon dioxide after European contact,’ Piperno says.

Future research

More work still needs to be done in other yet unstudied regions away from riverbanks and floodplains to obtain a wider view of the vast Amazon and that the team’s results do not imply that no form of indigenous forest management occurred in the region, just that it was not intensive enough to show up in the soil cores.

‘To me, these findings don’t say that the indigenous population wasn’t using the forest, just that they used it sustainably and didn’t modify its species composition very much,’ Piperno says. ‘We saw no decreases in plant diversity over the time period we studied. This is a place where humans appear to have been a positive force on this landscape and its biodiversity over thousands of years.’

Funding information

Funding and support for this research were provided by the Smithsonian, the National Science Foundation, the Wenner-Gren Foundation for Anthropological Research, the European Research Council and the Gordon and Betty Moore Foundation.

Publication details

Dolores R. Piperno, Crystal H. McMichael, Nigel C. A. Pitman, Juan Ernesto Guevara Andino, Marcos Ríos Paredes, Britte M. Heijink, and Luis A. Torres-Montenegro: ‘A 5,000-year vegetation and fire history for tierra firme forests in the Medio Putumayo-Algodón watersheds, northeastern Peru,’ in PNAS (7 June 2021). DOI: https://doi.org/10.1073/pnas.2022213118

Contact person