Swansea University's Dr Neil Loader and Emeritus Prof Alayne Street-Perrott, are among a team who have found ancient earthworks, possibly 2,000 years old.
The discoveries were made in Acre state in the western Brazilian Amazon.
Researchers from the universities of Exeter and Reading, and Brazil's SÃ£o Paulo, BelÃ©m are also part of the team.
Their research investigated ditched enclosures which were concealed for centuries by bamboo-dominated rainforest until modern deforestation allowed the discovery of more than 450 large geometrical "geoglyphs".
The team said the function of these mysterious sites is still little understood.
They are unlikely to be villages, since archaeologists have recovered very few artefacts during excavation, and their layout does not suggest they were built for defensive reasons.
Instead it is thought they were used only sporadically, perhaps as ritual gathering places, similar to the Maya pyramids of Central America, or Britain's own Stonehenge.
Although Dr Loader - who has analysed soil samples from the geoglyphs - said the surroundings in which they were built were very different to other ritual sites around the world.
He looked at phytoliths - a type of microscopic plant fossil made of silica - to reconstruct ancient vegetation; charcoal quantities, to assess the amount of ancient forest burning; and carbon stable isotopes, to indicate the type of vegetation growing there in the past.
"The indications are that the geoglyphs were constructed amongst taller vegetation. So, unlike the towering Maya pyramids of Central America, they were likely not visible above the forest canopy, and this raises questions about their purpose," he explained.
It had been assumed prior to the arrival of Europeans in the early 16th Century, the Amazonian forests had been pristine ecosystems, free from human influence.
But the new research indicates a wide variety of plant species spread over 6,000 years, which could only have been artificially brought together by humans.
It suggested instead of burning large tracts of forest - either for geoglyph construction or agricultural practices - people transformed their environment by concentrating on economically valuable trees such as palms.
The team have likened it to a form of "prehistoric supermarket" of useful forest products.
They said there is "tantalizing evidence" to suggest the biodiversity of some of Acre's remaining forests may retain a strong legacy of these ancient "agroforestry" practices to this day.

Banbury's Horton General has been trying "very hard" since April to recruit new obstetricians.
But Victoria Prentis, Banbury MP, said the "contingency" midwife plan, which she is to discuss with health secretary Jeremy Hunt, was "terrifying".
Campaigners staged a rally earlier against the changes.
"Our bottom line in Banbury is we want our maternity unit to be the best it can be," Mrs Prentis said.
Oxford University Hospitals NHS Foundation Trust has told Horton staff the plan could take effect from the end of September if it did not successfully recruit the obstetricians, who are specially-trained maternity doctors.
Paul Brennan, director of clinical services, said the Oxford trust was trying "very hard" to fill the posts, which have been advertised since April.
He described the changes as "contingency arrangements in case recruitment continues to prove unsuccessful".
But Sophie Hammond, who was among campaigners at the rally, said "lives would be put at risk".
"The thought that these key consultants and this access to emergency care and surgery could be taken away from here doesn't bear thinking about," she said.
Mr Brennan said they had worked to make the obstetrician posts "more attractive to applicants".
"This includes enhanced pay and additional training at the John Radcliffe Hospital," he explained.
"We believe we have got a good offer for doctors wanting a role that offers some responsibility and also some variety, and are again re-advertising the posts."
According to the trust, women who do not want to give birth at the unit will be offered alternatives at other hospitals.
A decision is expected in late August.
Humans lived in the Amazon rainforest much earlier than previously thought, and even helped shape its biodiversity, researchers have said.
[NEXT_CONCEPT]
A hospital's maternity services may become midwife-led because it is struggling to recruit doctors, an NHS trust has admitted.