Dark soils are the shadow of hidden agriculture

Stains left in the soil from past farmers, and the plants they cultivated, are helping illuminate prehistoric inhabitants of the Amazon.

One of the most exciting areas to work on in archaeology at the moment is the Amazon basin. Less than twenty years ago, nearly all the attention in South American archaeology was in the Andes region. Botany, ecology and archaeology are coming together to suggest archaeologists have missed an awful lot in the Amazon.

The Amazon
Image: Canva

Archaeology is the study of remains. The bigger and more solid those remains are, the easier they are to study. In the west, there were cities of stone and an empire when the Europeans arrived. The Europeans in the west were the Spanish. Sometimes called, “God’s Civil Servants”, the Spanish were meticulous record keepers and so compiled an extensive ethnographic record of the cultures they were subjugating. The society of the Andes was recognisably a complex culture.

In the east there was forest, and that has been a lot less studied, possibly because to the Europeans the forest was wild. While the archaeology might not be spectacular, the botany is revealing something very strange has happened in the Amazon. There are about 390 billion trees in the Amazon, but half of those belong to one of 227 species. This selectivity is something ter Steege and colleagues called ‘hyperdominance’. Research published last year found that domesticated species were much more likely to be hyperdominant than wild species.

A study by Maezumi and colleagues looks at this skewed botany, and the context it sits in. In this case it’s in Amazonian dark earths. This is a dark, rich and fertile soil created by past agricultural use. Maezumi and colleagues note that forests on these soils look distinctly different when you look at the species on them. They also cite work showing that modern home gardens tend to have more useful plants in them when they’re on soils associated with complex archaeological sites. Finally, they add that these soils also tend to help grow exotic plants, which demand more nutrients than the wild Amazonian plants. The team took a closer look soil samples associated with archaeological sites and Amazonian dark earths.

What they found was that from around 4500 years ago pollen from maize Zea mays starts to appear in the samples. About 3200 years ago they see sweet potato, Ipomoea batatas. Along with a change in the structure of the soil, it shows that farming began around 2500 BC. At this early stage it looks like the people of the Amazon were already farming mixed crops.

After 500 BC the team see in increase in the number of plant species being farmed. Phytoliths, distinctly shaped microscopic lumps of silica formed by plants, show this increase in diversity. More changes in the soil show the formation of the Amazon Dark Earths, a little over 500 years ago. The note this change occurs around the expansion of the Santarém polity.

The authors conclude: “As modern deforestation and agricultural plantations expand across the Amazon Basin, coupled with the intensification of drought severity driven by warming global temperatures, these data provide a detailed history of over four millennia of anthropogenic land use that progressively intensified, in the absence of large-scale deforestation, that has a lasting legacy on the composition of modern rainforests in the eastern Amazon.”

The forest has reclaimed much of the human settlement in the forest. But while no huge monumental architecture might remain, the trees remain in the dark soils. This peculiar fertility shows that farmers of many centuries in the past still leave their mark on forest life.

Reference List

Ter Steege, H., Pitman, N. C. A., Sabatier, D., Baraloto, C., Salomao, R. P., Guevara, J. E., … Molino, J.-F. (2013). Hyperdominance in the Amazonian Tree Flora. Science, 342(6156), 1243092–1243092. https://doi.org/10.1126/science.1243092

Levis, C., Costa, F. R. C., Bongers, F., Peña-Claros, M., Clement, C. R., Junqueira, A. B., … Salomão, R. P. (2017). Persistent effects of pre-Columbian plant domestication on Amazonian forest composition. Science, 355(6328), 925–931. https://doi.org/10.1126/science.aal0157

Maezumi, S. Y., Alves, D., Robinson, M., de Souza, J. G., Levis, C., Barnett, R. L., … Iriarte, J. (2018). The legacy of 4,500 years of polyculture agroforestry in the eastern Amazon. Nature Plants, 4(8), 540–547. https://doi.org/10.1038/s41477-018-0205-y Read free, through ReadCube

Lehmann, J., Kern, D. C., Glaser, B., & Wodos, W. I. (Eds.). (2003). Amazonian Dark Earths. https://doi.org/10.1007/1-4020-2597-1