At least one mystery surrounding the giant Amazonian “Earth-symbols” of Acre may finally be resolved.

Football field sized patterns of deep ditches dot the landscape of northwestern Brazil. The discovery of these huge earthworks, which date back almost 2,000 years, has triggered intense debate regarding their origins among archaeologists and ecologists.

Some believe their existence implied vast land-clearing, but evidence from a recent study suggests otherwise. Rather, the geoglyph builders of millenniums past may have relied on small-scale burning to open relatively tight spaces within carefully cultivated forests, revealing sophisticated agroforestry practices that could have serious implications for current conservation efforts in both North and South America.

The new paper contributes to the growing body of research suggesting that the popular image of the pre-Columbian Americas as “pristine landscapes” is nothing more than a myth, as journalist Charles Mann argued in his book “1491.” From widespread burning of the Eastern forest in North America, to the extensive control over watersheds in the southwest, “basically a zillion archaeological studies show all kinds of human manipulation,” he tells The Christian Science Monitor in an email.

This new thinking suggests that efforts to preserve forest ecosystems in some apocryphal "natural" state through absolute control may do more harm than good, because it neglects the fact that they never were truly wild: forests evolved together with humans.

It wasn't until the late 1970's that deforestation first revealed the geoglyphs hiding under upland rainforest. Covering more than 5,000 sq. miles in the Brazilian state of Acre, more than 450 large-scale earthworks appeared to paint a picture of massive deforestation in the pre-Columbian Amazon. Some assumed the geoglyphs' construction would have required systematic land-clearing, viewing it as evidence that the forests could prove resilient to today's slash-and-burn farming practices, as well.

Not so, according to Jenny Watling, an archaeologist at the University of São Paulo, whose new research on the geoglyphs depicts a large and vibrant culture living sustainably in the early rainforest.

“Our new evidence showing that they transformed the composition of these forests over thousands of years to encourage useful species, without degrading the forest, shows that this area supported large populations who were able to flourish here in the past,” she tells the Monitor in an email.

The team took soil samples from holes up to five feet deep at two geoglyph sites in Acre. By studying charcoal layers – a sign of long-ago fires – and microscopic plant remains called phytoliths, the researchers were able to read thousands of years of ecologic history from the dirt.

The story opens 6,000 years ago in a largely bamboo forest, resilient enough to survive the dry spells that reduced neighboring Bolivian forest to savannah. But everything changed when humans arrived on the scene 4,000 years ago, as supported by independent archeological evidence.

A relatively heavy charcoal layer suggests that the new residents cleared the bamboo forest with fire, letting the fast-growing palm trees spring up to fill the space left behind. Over the long term, one would expect the palms to be overtaken by slower-growing plants, but that’s not what the team found.

Instead palms flourished for three thousand years, likely encouraged by the human newcomers, who could use them for food and building material. Dr. Watling suspects they engaged in practices including planting seeds, transplanting saplings, and weeding out undesirable plant species, as well as light burning and farming. She calls these techniques agroforestry: “[maintaining] the forest but [changing] its species composition to make it a more livable place.”

These early agroforesters seem to have been the ancestors of the geoglyph builders, whose first earthworks appear 2,000 years ago and seem to have been used periodically for over a thousand years. Rather than haphazardly burning wide swaths of land for construction as had been presumed, the charcoal record tells a story of narrow clearings just a few city blocks wide, opened around the geoglyphs for short periods, decades at a time.

The purpose of the ditches remains mysterious, but some archeologists believe the builders comprised a number of small, independent groups that interacted often and shared a belief system, according to Watling. The geoglyphs, which show signs of ritual use but not long term habitation, may have been expressions of that system.

The builders’ tale ends with their abandonment of the earthworks 650 years ago, after which palm traces drop off suddenly.

How much early inhabitants changed these parts of the Amazon is still a “very hot debate” among experts, but Watling says her research “supports the idea that indigenous peoples have been conscious actors throughout the history of Amazonia, and that they actively transformed their environment to make it more livable.”

This under-appreciated cultivation of the land by indigenous peoples can lead to serious problems when experts try to create conservation policy based on a misunderstanding of what is, and isn’t normal.

“In many cases over long term human interaction the forests and people adjusted to each other, and the 'natural' state is one of human management,” Mr. Mann explains. But “these ideas haven't yet really percolated to ecologists and environmental activists, many of whom continue to use 1492 [the year Christopher Columbus arrived in the Americas] as a benchmark for the onset of human activity.”

As an example, he points to fire management in the Western part of the United States, where archeologists believe native peoples burned forests at a low level for thousands of years. This controlled burning kept down undergrowth and cleared out dead litter on the ground, preventing it from becoming fodder for wildfires of the massive scale we sometimes see today.

Additionally, fire helps some Western tree species reproduce better. Basically, Mann thinks we might be doing it wrong with our all or nothing approach: “A lot of archaeologists and a growing number of ecologists have argued that our current system of forest management is a mistake – it's not how these forests existed in the millennia before contact with Europeans.”

He blames a number of fires in the West on "Smokey the Bear-type policies" that don't recognize the protective effect careful management by light burning can have, if done correctly.

“Understanding what they did might help us in our efforts to keep our hemisphere as a decent place, ecologically speaking. They weren't idiots, those long-ago folks,” Mann says.

Watling agrees, and says the North American case is “very similar, and shows how human groups have co-evolved with ecosystems. In some cases, if you take the humans out of the picture, things can get ugly quite quickly.”

She sees current slash-and-burn farming, in which a forest is completely cut and burnt to the ground to open areas for farming, as destroying the forests more than using them. Indigenous societies, by contrast, were and are able to use land without destroying it, a contrast she calls “a sad paradox.”

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But she sees a way forward, hoping that her work will help validate Native practices.

“If we are ever to work towards a more 'sustainable' Amazon future, we need to learn the importance of indigenous knowledge for creating and maintaining the very biodiversity we seek to preserve.”