The discovery in recent years of mysterious circular earthworks in western Amazonia is one of the most important – certainly one of the most unexpected – archaeological discoveries of modern times. These strange geoglyphs, sometimes as many as six enclosures to a single site, range in diameter from 100m to 300m, with ditches up to 11m wide and about 4m deep.
They came to light following the mass deforestation that hit the region in the 1980s as part of the Amazon Colonisation Project. The sheer number of these circles is astonishing: more than 450 geoglyph sites have been mapped in an area of about 13,000km2 in the state of Acre, Brazil, and more are being reported all the time. Indeed, wherever rainforest is cleared, geoglyphs appear. What are they? And why are they so numerous?
Though investigations by Brazilian and Finnish archaeologists over the past 15 years have brought us closer to understanding these strange earthworks, questions remain.
We know that most geoglyphs were constructed between 1,000 and 2,000 years ago, during the Amazonian Formative Period, a time when populations in other distinct regions of Amazonia were also creating conspicuous marks on the landscape in the form of mounds, ditches, agricultural earthworks, and expanses of anthropogenic soils. While the geoglyphs are part of this boom, they are unique sites, unlike any others throughout the Amazon basin.
Archaeological excavation suggests the enigmatic rings are not ancient village sites: the areas enclosed by the ditches are almost always devoid of cultural material. The rare finds that are recovered are typically found in the ditches, and sometimes include what appear to be decorated ceramic vessels that have been deliberately smashed and deposited near the site entrances, reminiscent of similar deposits in Neolithic causewayed enclosures.
It seems more plausible that these formulaic architectural monuments – mainly circles and squares, and many perfectly geometric – were used sporadically for ceremonial purposes. Archaeologists Denise Schaan and Sanna Saunaluoma, who have worked extensively with the geoglyphs, suggest groups probably gathered at such sites to celebrate important annual events such as fruiting seasons, or social events such as births, marriages, and deaths; and that the smashed vessels found near the entrances may have contained food and drink consumed by the participants, before the ceramics were left as some kind of offering.
If these geoglyphs were ritual sites, where did their builders live? The simple answer is, we don’t yet know. To add to their mystery, we are yet to find evidence of associated settlements for local populations, and thus our knowledge about the societies who built these earthworks is extremely limited. It could be that the geoglyph builders belonged to a network of local and fairly autonomous groups, who were connected though a strong shared belief system. What is certain, however, is that the discovery of the geoglyphs challenges the longheld belief that Amazonia was a cultural backwater and pristine wilderness before Europeans arrived in the continent.
For years, it was argued that Amazonia was not a suitable environment for the development of large, complex human groups: the soil has limited agricultural potential, protein sources are sparse, and ethnographic studies reported only small groups of modern indigenous societies. This version of an uninhabited Amazonia perpetuated the romantic view that its forests represent near virgin ecosystems.
Now a wealth of archaeological evidence recovered over the last few decades shows large, complex societies did indeed exist and, in some regions, permanently transformed their environment to make it more productive. The best-known examples are found along major river floodplains and areas rich in aquatic resources.
However, the extent to which interfluvial upland forests, which make up over 90% of Amazonia, were transformed by human action remains hotly debated, for this is where soils are the poorest, the rainforest the thickest, and aquatic resources the farthest away. Which begs the question: who would have wanted to live there?
Well, the geoglyph builders did. With most of Acre’s geoglyphs situated 180-230m above sea level, and dozens of kilometres away from large rivers, they are impressive remnants of societies who once inhabited ecosystems that are, at least to our Western eyes, uninhabitable.
So now we have to ask ourselves: did the geoglyph builders transform the natural landscape to build these earthworks? Was the region forested at the time the geoglyphs were constructed and, if so, what was the scale of deforestation and burning practised to build and use them? How did people make a living in this supposedly hostile environment? What happened to the landscape once the geoglyphs were abandoned, and what does this imply for the long-term resilience of Amazonian forests in the face of human activity?