Mysterious Earthen Rings Predate Amazon Rainforest

A series of square, straight and ringlike ditches scattered throughout the Bolivian and Brazilian Amazon were there before the rainforest existed, a new study finds.

These human-made structures remain a mystery: They may have been used for defense, drainage, or perhaps ceremonial or religious reasons. But the new research addresses another burning question: whether and how much prehistoric people altered the landscape in the Amazon before the arrival of Europeans.

"People have been affecting the global climate system through land use for not just the past 200 to 300 years, but for thousands of years," said study author John Francis Carson, a postdoctoral researcher at the University of Reading in the United Kingdom. (See Images of the Ancient Amazonian Earthworks)

For many years, archaeologists thought that the indigenous people who lived in the Amazon before Christopher Columbus arrived in the Americas in 1492 moved across the area while making barely a dent in the landscape. Since the 1980s, however, deforestation has revealed massive earthworks in the form of ditches up to 16 feet (5 meters) deep, and often just as wide.

These discoveries have caused a controversy between those who believe Amazonians were still mostly gentle on the landscape, altering very little of the rainforest, and those who believe these pre-Columbian people conducted major slash-and-burn operations, which were later swallowed by the forest after the European invasion caused the population to collapse.

Carson and his colleagues wanted to explore the question of whether early Amazonians had a major impact on the forest. They focused on the Amazon of northeastern Bolivia, where they had sediment cores from two lakes nearby major earthworks sites. These sediment cores hold ancient pollen grains and charcoal from long-ago fires, and can hint at the climate and ecosystem that existed when the sediment was laid down as far back as 6,000 years ago.

An examination of the two cores - one from the large lake, Laguna Oricore, and one from the smaller lake, Laguna Granja - revealed a surprise: The very oldest sediments didn't come from a rainforest ecosystem at all. In fact, the Bolivian Amazon before about 2,000 to 3,000 years ago looked more like the savannas of Africa than today's jungle environment.

The question had been whether the early Amazon was highly deforested or barely touched, Carson said.

"The surprising thing we found was that it was neither," he told Live Science. "It was this third scenario where, when people first arrived on the landscape, the climate was drier."

The pollen in this time period came mostly from grasses and a few drought-resistant species of trees. After about 2,000 years ago, more and more tree pollen appears in the samples, including fewer drought-resistant species and more evergreens, the researchers report today (July 7) in the journal Proceedings of the National Academy of Sciences. Charcoal levels also went down, indicating a less-fire-prone landscape. These changes were largely driven by an increase in precipitation, Carson said.

The earthworks predate this shift, which reveals that the diggers of these ditches created them before the forest moved in around them. They continued to live in the area as it became forested, probably keeping clear regions around their structures, Carson said.

"It kind of makes sense," he said. "It's easier to stomp on a sapling than it is to cut down a big Amazonian tree with a stone ax." (Gallery: Biodiversity of the Amazon (Photos))

The discovery that the human activity came before the forest answers some questions, like how Amazonian people could have built in the rainforest with no more than stone tools (they didn't have to), how many people would have been necessary to construct the structures (fewer than if clear-cutting had been required), and how the population survived (by growing maize).

The study also has wider implications for the modern day, Carson said. The question of how to preserve the Amazonian rainforest is difficult to answer; some people say humans need to get out, and others believe people and the forest can coexist. Ancient history could provide a guide, as well as a greater understanding of how the forest has recovered from earlier perturbations. (The Amazon also drives climate as well as responds to it, thanks to its ability to take up carbon from the atmosphere.)

The new study suggests that the modern forest is a coproduction between humans and nature, Carson said. Natural cycles drove the rainforest to sprout, but humans stayed on-site for 1,500 years afterward, he said.

"It's very likely, in fact, that people had some kind of effect on the composition of the forest," Carson said. "People might favor edible species, growing in orchards and things like that, altered the soils, changing the soil chemistry and composition, which can have a longer-lasting legacy effect."

Those long-range changes are next for Carson and his colleagues to investigate. "This kind of study has only just started in Amazonia," Carson said.

More from LiveScience:

Gallery: Aerial Photos Reveal Mysterious Stone Structures Image Gallery: Amazing 'Fairy Circles' of the Namib Desert Images: One-of-a-Kind Places on Earth Copyright 2014 LiveScience, a TechMediaNetwork company. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

Article originally appeared on LiveScience.

Shown here, a ring ditch next to Laguna Granja in the Amazon of northeastern Bolivia.

The cocoa frog is one of six new frog species that were recently found in a rainforest-dominated mountainous region of southeastern Suriname. “At a time when so many frog species are declining and undergoing extinctions worldwide, it is particularly uplifting to discover so many new frogs in a single area,” Trond Larsen, a tropical ecologist and director of the Rapid Assessment Program at Conservation International, told Discovery News. He and his team found about 60 new species in the South American country.

The tiny Lilliputian beetle measures just 2.3 mm long and could be the smallest dung beetle in the entire Guiana Shield and among the smallest in the world. “Dung beetles act as a cleanup crew in the forest,” Larsen said. “By burying dung, they not only control parasites and disease, including those that affect people, but also disperse seeds and recycle nutrients that enable forest regeneration.”

Leeanne Alonso, director of Global Biodiversity Exploration for Global Wildlife Conservation, went on the expedition with Larsen. She thinks this beetle might be small and red to look like a seed stuck in poo, thereby fooling predators. “Dung beetles in forest areas are a good indicator of mammal diversity,” she added. Without mammals providing their food source, there would be few such insects.

The collection of new animals includes 11 species of fish that are probably new to science. “Small, brightly-colored tetras similar to this one are popular in the aquarium trade, and sustainable exports of wild species could provide financial support to local communities and incentives to conserve the species’ natural habitat,” Larsen said.

Alonso added that, as new species go, fish are relatively rare. “It’s amazing that so many were found in this region, which I believe has the world’s best and most beautiful and pristine forests in the world.” She loved it so much that she took her family there on a vacation after the research work ended.

Bats are another “good indicator of habitat quality,” Alonso said. She explained that, in this case, the bat thrives on fruit, so the region must support plenty of healthy fruit-producing trees.

Genuine coral snakes are highly venomous, but this false coral snake’s name is itself somewhat misleading, as the researchers found out the hard way. Alonso said that a helicopter pilot transporting the scientists was bitten by one. “His arm really swelled up,” she said, explaining that all such snakes have sharp teeth and venom, just not as poisonous as the “real” coral snake this species resembles.

This extraordinary new insect displays waxy fronds at the end of its body that was built for jumping among plants. “Maybe the fronds are meant to resemble anthers of a flower, helping with camouflage?” Alonso said, admitting that nature sometimes works in still-mysterious ways. She continued, “So little is known about insects from this region, so this was a real find.”

Top-level predatory big cats, such as this margay, are yet another sign of healthy habitat. More of them generally means there are more prey animals to feast upon. “Margays love to sleep and hide in caves at the site,” Alonso added.

Six new katydids, including this one, were discovered. Larsen described it as a “gangly species with oversized, spiny hind legs.” The newly discovered katydids "are indicative of the pristine, healthy forests of the Upper Palumeu Watershed," Larsen said, "and the forests in turn help to ensure continued flows of clean, plentiful water used by people throughout the rest of the country.”

“Despite their generally diminutive size, water beetles can be useful indicators of water quality, and also help to filter and keep water clean,” Larsen said. “Many of the 26 new water beetle species discovered on this survey are probably restricted to isolated habitats, especially in the mountains of southeastern Suriname, and may occur nowhere else.”

Eleven new fish species were found in the region, dubbed a "tropical Eden" by the researchers. Larsen said, “This new sucker-mouthed armored catfish was rare, and only encountered in the narrow, upper reaches of the Palumeu River.

“This delicate slender opossum is really cute,” Alonso said. “It’s hard to find small mammals like this, which are indicative of primary forest.”

Coprophanaeus lancifer

is the largest of all South American dung beetles, Larsen shares. Despite its name, this species feeds more frequently on dead animals than on dung. A highly unusual case in the Animal Kingdom, both males and females of this species possess a long horn on their head, which they use during intense battles with other individuals of the same sex. The vast difference in adult body size seen here is primarily determined by how much food was available to the developing larva. This species is capable of rapidly burying large animal carcasses, providing an important ecological service that sustains rain forest health.

“Given the beautiful coloration, high visibility and popularity of frogs in the poison dart frog family (Dendrobatidae), most species in this group are relatively well known,” Larsen said. “Therefore, the discovery of this species potentially new to science is particularly exciting. The toxic secretions of poison dart frogs hold great potential to yield new medicines that could greatly benefit the world -- yet with frogs declining globally, their protection in the wild is essential.”

The researchers could have just scratched the new species surface in southeastern Suriname, given that other animals, fish, insects and more unknown to science could be found there. The region’s human population is currently small -- only about 500,000 -- but it’s growing and there is a threat of future habitat-destroying activities, such as mining and logging. Alonso hopes that the wilderness can be protected, with money-generating activities such as ecotourism allowing both humans and amimals to thrive there.