The Secret of El Dorado - programme summary

In 1542, the Spanish Conquistador, Francisco de Orellana ventured along the Rio Negro, one of the Amazon Basin's great rivers. Hunting a hidden city of gold, his expedition found a network of farms, villages and even huge walled cities. At least that is what he told an eager audience on his return to Spain.

"No one ever saw again what Orellana described"

The prospect of gold drew others to explore the region, but none could find the people of whom the first Conquistadors had spoken. The missionaries who followed a century later reported finding just isolated tribes of hunter-gatherers. Orellana's story seemed to be no more than a fanciful myth.

A proven liar?

When scientists came to weigh up the credibility of Orellana's words, they reached the same conclusion. As productive as the rainforest may appear, the soil it stands in is unsuited to farming. It is established belief that all early civilisations have agriculture at their hearts. Any major population centre will have connections with a system of intensive agriculture. If a soil cannot support crops sufficient to feed a large number of people, then that serves as an effective cap on the population in that area. Even modern chemicals and techniques have failed to generate significant food from Amazonian soil in a sustainable way . The thought that indigenous people could have survived in any number - let alone prospered - was dismissed by most scientists. Scientific consensus was sure that the original Amazonians lived in small semi-nomadic bands and that Orellana must have lied.

"Every effort to develop sustainable permanent [Amazon] agriculture failed"

Clues from the Bolivian savannah

Bolivia's Llanos de Mojos (Mojos Plains) are 2,000km from Orellana's route down the main channel of the Amazon. The terrain is savannah grassland with extreme seasons - floods in the wet; fires in the dry. Crops are hard to grow and few people live there. But back in the 1960s archaeologist Bill Denevan noted that the landscape was crossed with unnaturally straight lines. Large areas were also covered with striped patterns.

Recently, Denevan's work has been followed up by Clark Erickson, a landscape archaeologist. His attention was drawn to the numerous forest islands dotted across the savannah like oases. Down on the ground he found them littered with prehistoric pot sherds, a clear sign of early human habitation. Some mounds were as much as 18m high and much of the pottery was on a grand scale as well. Such huge vessels were too big for wandering nomads. Here were permanent settlements, where hundreds or even thousands of people had once gathered for huge ceremonies. To Erickson, these were signs of an advanced society - a civilisation.

"They have words for domestic plants from 2,000 years ago"

Erickson and a colleague, William Balée, needed evidence for organised farming and found help working with the region's Amerindians. Some of the mounds are still inhabited by indigenous people. The language of the Sirionó offers clues to their past. Words for staple crops like maize, as well as cotton and dye plants, hint at what may have been farmed hundreds, even thousands, of years ago.

Erickson's interpretation of the lie of the land is that the mounds were built to offer protection from floodwaters, with the most sacred buildings always at the centre of the mound on the highest level. There is historical evidence for this - a Spanish expedition of 1617 remarked on the extent and high quality of a network of raised causeways connecting villages together. Those causeways can still be seen as straight lines cutting across the savannah. Alongside them run canals, a result of their construction.

"Their work is on a par with anything the Egyptians did"

Denevan and Erickson have shown that the striped patterns are relics of a system of raised fields. From the air, the area which appears to have been turned over to such agriculture is clear. It covers thousands of square kilometres. In conjunction with the controlled irrigation a canal network might offer, it could have sustained hundreds of thousands of people. Erickson believes the Mojos Plains were home to a society which had totally mastered its environment.

If land now little suited to agriculture could once have supported hordes of people, is there a chance Orellana's mythical El Dorado has some basis in fact?

When anthropologist Michael Heckenberger met the Kuikuru tribe in the central Amazon he was impressed by the complexity of their social structure. Why, he wondered, would a tribe of just 300 people adopt such a hierarchical way of life? (Received opinion held that Amazonian tribes were small, egalitarian societies.) He found evidence that the Kuikuru had once lived in an integrated network of villages, each one many times the size of their modern-day settlements. Heckenberger believes the prehistoric Kuikuru were not the semi-nomadic wanderers of anthropological theory. Instead, they lived in large chiefdoms - the advanced society described by Orellana.

"As much as 10% of Amazonia is covered in terra preta"

The secret of the soil

The search for clues in the Amazon takes place at grass roots level - in the soil itself. Along Brazil's Tapajos River, archaeologist Bill Woods has mapped numerous prehistoric sites, some with exquisite, 2,000 year old pottery. There is a common thread: the earth where people have lived is much darker than the rainforest soil nearby. Closer investigation showed that the two soils are the same, the dark loam is just a result of adding biological matter. The Brazilians call this fertile ground terra preta. It is renowned for its productivity and even sold by local people.

Archaeologists have surveyed the distribution of terra preta and found it correlates favourably with the places Orellana reported back in the 16th century. The land area is immense - twice the size of the UK. It seems the prehistoric Amazonian peoples transformed the earth beneath their feet. The terra preta could have sustained permanent intensive agriculture, which in turn would have fostered the development of advanced societies. Archaeologists like Bill Petersen, from the University of Vermont, now regard Orellana's account as highly plausible. But if the first Conquistadors told the truth, what became of the people they described?

"The Indians may have used a 'slash and char' technique"

Tragically, the visitors brought diseases to which the Amerindians had little resistance: smallpox, influenza, measles. Orellana and his men were the first and last Europeans to set eyes on an Amazonian civilisation. They themselves may have been the ones to trigger its rapid decline.

Yet the Amazonians' greatest achievement lives on. Soil scientists analysing the terra preta have found its characteristics astonishing, especially its ability to maintain nutrient levels over hundreds of years. 20th century techniques of farming on cleared, torched rainforest - so-called slash and burn agriculture - have never been sustainable. With the vegetation burned off, the high rainfall soon leaches all the nutrients out of the soil. Research has shown that even chemical fertilisers cannot maintain crop yields into a third consecutive growing season, yet terra preta remains fertile year after year.

"The material is alive. The biology is the important thing"

Nature and nurture

Again, Orellana's accounts offer potential insight. He reported that the indigenous people used fire to clear their fields. Bruno Glaser, from the University of Bayreuth, has found that terra preta is rich in charcoal, incompletely burnt wood. He believes it acts to hold the nutrients in the soil and sustain its fertility from year to year. This is the great secret of the early Amazonians: how to nurture the soil towards lasting productivity. In experimental plots, adding a combination of charcoal and fertiliser into the rainforest soil boosted yields by 880% compared with fertiliser alone.

Yet terra preta may have a still more remarkable ability. Almost as if alive, it appears to reproduce. Bill Woods has met local farmers who mine the soil commercially. They find that, as long as 20cm of terra preta is left undisturbed, the bed will regenerate over a period of about 20 years. He suspects that a combination of bacteria and fungi is causing this effect.

Today, scientists are busy searching for the biological cocktail that makes barren earth productive. If they can succeed in recreating the Amerindians' terra preta, then a legacy more precious than the gold the Conquistadors sought could spare the rainforest from destruction and help feed people across the developing world.