text: 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.
target: Humans lived in the Amazon rainforest much earlier than previously thought, and even helped shape its biodiversity, researchers have said.