Read Story Transcript

The Amazon rainforest may have once been much more densely populated than previously believed.

Archeologists from the University of Exter have discovered the remnants of 81 villages interconnected by roads deep in the tropical forest of what is now Brazil — long before the arrival of European colonizers.

The villages are likely just a fraction of the settlements stretched across 1,800 kilometres of the Amazon between the Juruena and Aripuana rivers — an area that may have been home to one million people, according to the study published Tuesday in the journal Nature Communications.

So how did such a vast network of communities go unnoticed before?

"There is very little research in many areas of the Amazon. The Amazon is huge," archeologist Jonas Gregorio de Souza told As It Happens host Carol Off.

"Many of the sites have only been discovered recently due to the advance of the agricultural and cattle ranching practices in the Amazon. So it's kind of ironic that thanks to modern deforestation, we are actually being able to find the sites now."

Cows graze on deforested Amazon rainforest next to another tract recently cleared and burned near the city of Novo Progresso, Brazil, in 2013. Agriculture in the Amazon has facilitated archaeological discoveries in recent years. (Nacho Doce/Reuters)

The settlements are deep in the forest, far from any major rivers where human activity is usually found.

Their existence contrasts with the long-held belief among archeologists that the inner-areas of rainforest were largely untouched by human activity in pre-Columbian times.

But the findings do match up with historical accounts, de Souza said. In fact, that's what brought them to the region.

"Travellers described this region of Aripuana river as being full of large villages connected by roads," he said.

"So we were hoping to find also the archeological correlates of these villages."

Who were they?

The area appears to have been continuously occupied between 1250 and 1500, the study found.

The presence of a dark and fertile soil that's formed only through long-term human occupation suggests these were permanent communities — not temporary settlements created by nomadic peoples.

Jonas Gregorio de Souza stands in a ditch discovered in the Amazon rainforest — which archaeologists say was far more densely populated in pre-Columbian times than previously believed. (Jose Iriarte/University of Exeter)

De Souza said they likely lived in homes made from straw and leaves, similar to those of modern Amazonian tribes, which explains why no remnants of structures were found.

These early occupants probably farmed a variety of crops, including cacao trees, sweet potatoes and maize, de Souza said.

Their villages were separate and likely diverse, with a mosaic of people speaking varied languages, the researchers say.

Mysterious geoglyphs​

One thing the villages have in common is the presence of "earthworks" called geoglyphs — ditches dug into the shapes of squares, circles and hexagons.

The study shows there are 1,300 of these geoglyphs across 400,000 square kilometres of the Amazon. The smallest is 30 metres across, while the largest is almost 400 metres.

Aerial surveys revealed geoglyphs in the Amazon forest. The geometric shapes are trenches carved into the ground. (Jose Iriarte/University of Exeter)

It's not clear what they were for, but de Souza suggested some may have been used for protection.

"These sites are fortified settlements," he said.

"If the structures that we found, the ditches around the settlements, are the fences, then it's very likely that there might have been a large degree of warfare."

What happened to them?

De Souza said it's very likely these populations would have already been suffering from outbreaks of disease before Europeans in the area.

"Populations that had contact with the Europeans were spreading the diseases to those that still hadn't," de Souza said.

"When the Portuguese finally arrived in the region, then you have direct violence and genocide and slavery to a population that probably was already dwindling as a consequence of diseases for which they had no antibodies."

Written by Sheena Goodyear. Interview produced by Imogen Birchard.