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BBC Horizon - The Secret of El Dorado

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.

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.

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.

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.

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.

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?

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.

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.


NARRATOR (JACK FORTUNE): The Amazon Basin, August 2002. This is the trail of the greatest legend of the Americas, the legend that hidden in the Amazon jungle there was once a fabulous kingdom of gold: El Dorado.

DR CLARK ERICKSON (University of Pennsylvania Museum): In the late 1500s the Spanish came here looking for El Dorado. They didn't find it, but I think it's right down there.

NARRATOR: This is the story of how archaeologists have uncovered the lost civilisation behind the myth of El Dorado, but this was not a kingdom of gold. The secret of the real El Dorado was something far more valuable, something with the power to transform our world. The story begins on the banks of the Rio Negro in the Central Amazon. A party of scientists is embarking on a voyage which they hope will provide answers to a five hundred year old mystery. They are retracing the route of the very first Europeans to penetrate the Amazon Basin, a party of Conquistadors commanded by the Spaniard Francisco de Orellana.

READER: What I shall tell will be as an eyewitness, as a man whom God chose to give a part in a strange and hitherto never experienced voyage of discovery.

NARRATOR: In 1542 Orellana led an expedition deep into the heart of the Amazon. He was searching for El Dorado, the kingdom of gold that Indians said lay hidden in the jungle. For eight months he drifted through the rainforest. When he finally returned to Spain, he brought with him spellbinding tales of an unknown civilisation.

READER: There was one town that stretched for 15 miles without any space from house to house which was a marvellous thing to behold. There were many roads here that entered into the interior of the land, very fine highways. Inland from the river at a distance of six miles more or less there could be seen some very large cities that glistened in white and besides this, the land is as fertile and as normal in appearance as our Spain.

NARRATOR: But when a few years later the Spanish returned to the Amazon in search of the kingdoms Orellana had described, they found nothing. Just a few scattered Indian settlements.

DR JAMES PETERSEN (University of Vermont): Well when Orellana came down the Solimoes and up the Negro in the area where we're passing the, it seems as if there were thousands and thousands of people, extensive villages that stretched for miles and then 20 years later, 50/60 years later, after Orellana, the, no one ever saw again what Orellana had seen and described in some detail.

NARRATOR: So the mystery is: was there really once a civilisation here, an El Dorado in the Central Amazon, or was it all just a figment of Orellana's imagination, a story made up to impress the Spanish Court?

JAMES PETERSEN: The million dollar question is: were there large and complex societies here in the Central Amazon as Orellana recorded in the 1540s. In fact it's, it's the myth of El Dorado, the myth of large, complicated Amerindian cultures here in the Amazon and the answer lies out there somewhere in, in, in the forest, in the jungle.

NARRATOR: For five hundred years the myth of El Dorado has lured adventurers and explorers into the jungles of South America and beyond the Amazon they did find wonderful things - ruined Inca citadels like Machu Picchu, hidden high in the Andes, and in Central America they found the lost cities of the Maya, but in the Central Amazon itself they have never found anything: no pyramids, no temples, just jungle, so many scientists concluded long ago that the legend of El Dorado was a gigantic wild goose chase, the province of fantasists and conmen.

PROF BETTY MEGGERS (Smithsonian Institution): Don't believe these accounts unless you can verify it archaeologically because these people had other motivations than, than the truth.

NARRATOR: Betty Meggers should know what she's talking about. She's worked in the region longer than anyone. She concluded 40 years ago that an Amazonian civilisation could never have existed and she had a compelling scientific theory to prove it. It was all to do with agriculture. Agriculture lies at the heart of all the great civilisations of the world. Only intensive agriculture produces enough food to sustain large, settled populations and only large, settled populations build cities and ceremonial centres, those defining features of civilisation. Without intensive agriculture civilisation cannot exist, but in the Amazon all attempts at intensive agriculture have led to disaster. The yellow jungle soil is just too poor. Even modern techniques have simply led to ecological catastrophe with vast swathes of forest being cleared, only for the land to be abandoned.

BETTY MEGGERS: Every effort that we've made to develop sustainable, permanent agriculture, I mean millions of dollars have been spent on efforts to do that and they've all failed and so what if, if you're going to believe that the indigenous population had a secret that, that we haven't been able to discover with all our technology and so forth and so on, you know that's fine, but what is it?

NARRATOR: Meggers's conclusion was that without advanced agriculture the people of the Amazon were simply unable to develop civilisation. The few scattered tribes were obviously a relic of an ancient way of life, nomads left behind by history.

DR MICHAEL HECKENBERGER (University of Florida): Most people have a very clear idea of what your average Amazonian looks like - small groups, more or less one with nature, what I have occasionally perhaps impolitely called Stone Age savages frozen at the dawn of time - an imagery that somehow Amazonian people represent our contemporary ancestors. They're people like us 10,000 years ago, as if they have no history.

NARRATOR: With no traces of civilisation in the Central Amazon and a compelling scientific reason for their absence, it seemed clear that Orellana must have been a liar. El Dorado could never have existed, but this traditional view of the Amazon has recently been challenged. A thousand miles from where Orellana once travelled, on the fringes of the rainforest, lie the Mojos Plains of Bolivia. What was discovered in this remote place would begin a revolution in our understanding of the prehistoric Amazon. Among those to make these discoveries was Clark Erickson, an unusual kind of archaeologist.

CLARK ERICKSON: I don't dig holes in the ground, I study landscapes. We drive over landscapes, fly over them and walk over them.

NARRATOR: Erickson has spent more than ten years studying the Mojos Plains. It's a very different environment from the Central Amazon, but just as harsh. The landscape here is dominated not by rainforest but by open grassland known as savannah. In the rainy season the savannah is flooded by several feet of water. In the summer it's dry as tinder. It makes it difficult to grow crops and hardly anyone lives here today, but then archaeologists noticed something odd about this landscape: the savannah is criss-crossed by unnatural looking straight lines and covered by mysterious striped patterns. There are also thousands of strange, isolated mounds covered in forest.

CLARK ERICKSON: A major feature on these savannahs are forest islands - the locals call them isles - and there are probably five, maybe ten thousand of these in this part of the Amazon.

NARRATOR: Erickson decided to take a closer look at these forest islands and inside every one he found the same thing: signs of human habitation.

CLARK ERICKSON: I have big debates with my natural science colleagues about the formation of these islands. They think they're natural, but I can come out here and with, usually within five minutes or so I can find a handful of pottery, or charcoal or food remains or human bone that indicate that people were here. For instance here's a, here's a fragment of, of probably a very large vessel. By the curvature here it was probably some kind of a large cooking urn, typical domestic ware you'd find on sites like this where people live.

NARRATOR: It looked to Erickson as if the thousands of forest islands were, in fact, man-made prehistoric settlements, but what kind of society had built them? Then Erickson's college William Balée took him to a site he'd found within an area of dense jungle. Hidden beneath the trees was a huge earthen mound.

CLARK ERICKSON: I remember the first time we came out here and I, we, we had no idea how big this was and it wasn't until the third or fourth day where we got the data off the computer and we could see that it was 18 metres tall. This is the highest mound in the whole area.

PROF WILLIAM BALÉE (Tulane University): Look on that palm we still have the tree tag. That's…

NARRATOR: The mound was full of ancient pottery.

WILLIAM BALÉE: …on the, on the plot as well.

CLARK ERICKSON: Well it's almost half pottery, half soil. This is the highest part of the mound and the quantity of pottery is just incredible here.

WILLIAM BALÉE: Look at this.

NARRATOR: What was striking was the size of many of the vessels.

CLARK ERICKSON: …trees around here.

WILLIAM BALÉE: Look at the size of this thing. What were they incising on the inside?

CLARK ERICKSON: Yeah this is, this is a large vessel probably about three met, a metre and a half, maybe two metres across, by the diameter of the rim here and then the grooves were placed inside when the pot, when the clay is still wet and they used this as a big grinding platform 'cos there's no stone out here.

WILLIAM BALÉE: And look, so that would have been…

NARRATOR: Vessels this large suggested this was no temporary encampment.

CLARK ERICKSON: These, we're talking about permanent settled people here. You don't, you don't carry these around on your back when you're, you're trekking or doing hunting/gathering. These are village folk and, and a lot of these were probably for cooking up not family meals, but cooking up meals for huge groups of people. We're talking maybe, you know, parties for thousands of people, so I think that that indicates that there's something going on beyond autonomous villages in this area. We also have some mounds, such as this one, that's much larger, much taller than most of the other mounds, which could be some kind of ceremonial centre or possibly a political capital.

NARRATOR: It was a revolutionary thought. Hidden beneath these trees Erickson had found a huge prehistory settlement and evidence that thousands of people had once gathered here for great ceremonies. These were the characteristics of civilisation. The first clues to what this society might have looked like came from another nearby mound called Iviato. Iviato is still inhabited, by an indigenous Indian tribe, the Sirionó.

CLARK ERICKSON: As an archaeologist this is an incredible place because here we have an archaeological site that's probably occupied 1,000, maybe 3,000 years. We still have people living on it and the Sirionó have been living here for quite a while and you get a sense of what this mound might have been like in the past.

NARRATOR: Iviato still retains its original terraced structure, with three different levels still visible.

CLARK ERICKSON: It's a 4-5m tall mound, its base is probably about maybe eight hectares or so in size. Typical you have the original surface and you come up a slight rise to a first platform here which is very badly eroded and then you come up farther to the second platform that we're standing on now and this would have been probably where most of the houses were located, many more than exist here today.

NARRATOR: Erickson believes that one thousand years ago the main platform of the mound would have been occupied by hundreds of houses and above them on the top tier the focus of the community, a sacred pyramid.

CLARK ERICKSON: Very characteristic on these mounds is that not, usually not in the centre, but at one side of the flat second terrace, or third terrace, is a pyramid-like structure of earth and today you see that the Sirionó church is on the top of here. Well we assume that in the past there probably was a temple on top, or some kind of priest's house.

NARRATOR: Here was compelling evidence that there had once been large, permanent settlements where today only a few people live, but that didn't seem to make sense. Large settlements meant intensive agriculture, something that was supposed to be impossible, but Erickson's college William Balée has found tantalising clues that long ago the Sirionó were cultivating a variety of important crops.

WILLIAM BALÉE: The Sirionó language has some words for domesticated plants that appear to be derived from a language spoken 2,000 years ago and that was associated with agricultural society.

(TO VILLAGER) How do you say maize?


WILLIAM BALÉE: These words include the word for achiote which is a red dye plant.


WILLIAM BALÉE: It includes the word for cotton.


WILLIAM BALÉE: They had these words not because they borrowed them from some other people, but because they have retained the cultivation of these plants through time.

NARRATOR: Like the Sirionó today, it seems the people who once lived here were settled farmers. They cultivated colourful dyes, they grew and span cotton for their clothes, they lived on carefully engineered mounds above the savannah's seasonal flood waters and ate staple crops like maize. Unlike with Orellana in the Central Amazon, there's a credible Spanish account of the area. Here Erickson found more evidence of a sophisticated society.

CLARK ERICKSON: The Solis de Holguin expedition came in in 1617 and they described entering these towns on causeways that were, they said they could ride four horses abreast, and as grand as, as the cities in, in Spain and so they were remarkably impressed with these roads, their straightness and the engineering.

NARRATOR: The Spanish account is backed up by hard evidence on the ground. One of these roads is still there.

CLARK ERICKSON: Doesn't look like much, but this is actually a prehistoric causeway that was constructed some time in the past before the arrival of the Spanish and it's still in use today. It's about 10, maybe 15m across and about 2m tall and very well preserved today. You can see the pathway going along it and they would construct these. They took the soil out from one side or both sides of the causeway and in doing so they created a canal to interconnect these areas between the mound.

NARRATOR: Settlements, roads, canals. Just how big was this society on the Mojos Plains? It was time to take to the air. At last it was clear just what the mysterious straight lines were: they were the remains of a vast network of raised roads and canals running between the settlements.

CLARK ERICKSON: So in this forest island in Baures you can see 50-100 causeways radiating out connecting it to other forest islands.

NARRATOR: And the strange striped patterns? These were also prehistoric earthworks, fields deliberately raised up a few feet above the savannah floor. Erickson believes that this was how crops were grown, where agriculture is so difficult today. Raised fields would have stayed dry in the wet season and in the dry season they were surrounded by shallow canals, a source of year-round irrigation.

CLARK ERICKSON: Raised fields are an ingenious way of cultivating areas that have generally poor soils and also very deep water during the rainy season.

NARRATOR: Erickson has found that the remains of the raised fields stretch for thousands of square kilometres. They could have sustained a huge population. Hundreds of thousands, perhaps even a million people could have lived in this remote corner of the Amazon Basin.

CLARK ERICKSON: There are times when I've been flying over this landscape and you look down and we see the beginnings of some of the raised fields. We're going to fly for sometimes 15-20 minutes, continuous engineered landscape, literally from horizon to horizon in some areas. It's truly spectacular the scale and immensity of this transformation.

NARRATOR: Finally it was clear to Erickson just what he was looking at: a society that had totally mastered its environment, a civilisation of builders and engineers as sophisticated as any in the Ancient World.

CLARK ERICKSON: When you look at the amount of labour that went into building these earthworks, the amount of earth moved, person hours involved with altering rivers' courses, building these channels connecting rivers, raising the roadways it's on par with anything the Egyptians did in terms of their pyramids or cities.

NARRATOR: These discoveries in the Bolivian Amazon raised an obvious question: if people here could overcome the limitations of their environment perhaps Orellana's claims were not so outlandish after all. Perhaps there had been an El Dorado all those hundreds of miles away in the Central Amazon and sure enough when scientists began to look more closely at the Indian tribes of the Amazon they found evidence that didn't fit with the idea that these were people who had always been stuck in the Stone Age. A few years ago anthropologist Mike Heckenberger came to work with the Kuikuru, a classic small Amazonian tribe of just 300 people. He assumed he would be dealing with a simple society, but what he found was altogether more puzzling.

MICHAEL HECKENBERGER: There are several aspects of their social structure that are very, very similar to complex or hierarchical societies elsewhere, for instance the existence of chiefly individuals who are fundamentally different at birth than non-chiefly individuals.

NARRATOR: To Heckenberger's surprise, he found that this tiny tribe had a fully-fledged aristocracy. The differences in status were expressed mainly through ritual.

MICHAEL HECKENBERGER: Some of the flutes, for instance, are only used for the commemoration of chiefly individuals.

NARRATOR: It seemed strange. Why should such a tiny group have such a complex hierarchy, but then Heckenberger found signs that the Kuikuru had once been very different. Near to the present-day village he unearthed the remains of a prehistoric settlement. MICHAEL HECKENBERGER: How's it going guys?

NARRATOR: What was significant was its size.

MICHAEL HECKENBERGER: The plaza was more or less the size of a contemporary village, but the residential areas surrounding it were significantly larger, well over ten times the size of a contemporary village.

NARRATOR: The prehistoric village would have dwarfed the present-day one and what's more, just as in the Bolivian Amazon, this large village was one of many.

MICHAEL HECKENBERGER: Where today there's one village, in the past in prehistoric times this village was integrated with other villages as close as three or four kilometres away.

NARRATOR: It all suggests that the prehistoric Kuikuru were not a tribe of Stone Age semi-nomads caught at the dawn of time. Instead they, and perhaps other Central Amazonian tribes, had once lived in large, settled societies, exactly the sort of societies described by Orellana. So archaeologists have recently begun again to explore the rainforest Orellana travelled searching for his lost civilisation and as he explored Brazil's Tapajos river, Bill Woods identified a subtle but important clue.

PROF WILLIAM WOODS (Southern Illinois University): It's, it's somewhat difficult to see, but near the top of this low bluff along the Tapajos we have very dark soil. This is just a very good example of what covers tens of thousands of hectares in the local region.

NARRATOR: This black soil, or terra preta as the Brazilians call it, is dotted all over the Amazon jungle, but what intrigued archaeologists is what it contains.

JAMES PETERSEN: You'll see all kinds of things scattered over the ground here. Many of them look like rocks or stones, but in fact they're all artefacts, mostly pottery sherds, busted up jars made by the Indians one to two thousand years ago. It's a, it's a, it's a very dense concentration, rather remarkable in all senses. After just a minute or two I was able to pick up several handfuls of really dramatic pottery sherds. if we can imagine what the whole jars would look like we'd be rather surprised by these fine works of art.

NARRATOR: The pottery they have found is exquisite and much of it dates from the time of Christ, long before the coming of the Europeans. It was the first hard proof that there had once been an advanced culture in the heart of the rainforest and when they dug down into the terra preta scientists made the most revealing discovery of all. Not only was the black soil full of pottery, but it was almost exactly the same composition as the yellow jungle soil around it, except it had been mixed with organic waste. That meant the terra preta had to be man-made.

DR EDUARDO NEVES (University of Sã';o Paulo): We know that, that this terra preta here formed with the soil, so they look very different and they are very different in a way, but that's the matrix for that. We have to have human action interfering in the yellow soil in order to create the terra preta.

NARRATOR: It was the key revelation. It meant that wherever you find terra preta there people had once lived, so scientists have started mapping the black soil and wherever Orellana reported seeing settlements there they have found it. All along the banks of the Amazon, up the Rio Negro and down the Tapajos they are finding the terra preta. In all, a massive area, twice the size of Britain.

JAMES PETERSEN: Some have estimated perhaps that as much as 10% of Amazonia's covered with this Amazonian dark earth or terra preta. Its widespread distribution if linked to culture, which the vast majority of contemporary scientists believe, suggests that native cultures are not only widespread but in some cases phenomenally numerous.

NARRATOR: But to convince everyone that there really had been a large prehistoric population in the Central Amazon the archaeologists still had to explain how these people had achieved what we cannot. How had they fed themselves on the poor Amazonian soil? The answer again seems to lie in the terra preta.

ADILSON De S SANTOS: The soil is easy to work and very fertile. We plant papaya, we plant banana, corn, beans and manioc in terra preta. Whatever you plant in terra preta does exceptionally well.

NARRATOR: Terra preta is so fertile that it's been prized by Brazilian farmers for centuries. Somehow the prehistoric Amazonians had transformed the world's worst soil into some of the best.

JAMES PETERSEN: Unintentionally perhaps, maybe intentionally, the native people enriched the soil in and around where they lived and this turn enabled them to intensify their agriculture which then, in turn, enabled their numbers to grow and become complex and Orellana and the things he described become more than plausible, very likely in this scenario.

NARRATOR: So here is the truth behind the myth of El Dorado. The prehistoric people of the central Amazon transformed the very earth beneath their feet. From their black soil sprang a civilisation that lasted for over one thousand years. They fashioned works of art to rival those of the Mayas and Incas. They built towns and even cities that spread across the jungle. Eventually they settled the Amazon Basin with millions and millions of people. It seems Orellana was telling the truth after all, but if there really had been a great society here, what had become of it? How could it have disappeared so suddenly and so completely? The most likely answer is tragically simple.

DR CHARLES CLEMENT (Institute for the Study of the Amazon): When the Europeans arrived here they found a population with a lot of susceptibility to disease and so disease took off like a wildfire in dry straw and over two hundred years the population just crashed.

NARRATOR: Smallpox, influenza, measles - these were the agents of the rapid and complete destruction of Amazonian civilisation. Orellana was both the first and last European to set eyes on it. The disease which followed in his wake destroyed in a few decades what had taken centuries to create and so a people who once dominated this landscape disappeared almost without leaving a trace. Today the jungle has reclaimed the places where there used to be towns and cities, but that is not the end of their story. There is one final twist to this tale and it's to do with the Indians' mysterious black earth. Today the Amazon rainforest is under threat as never before. Millions of acres have been wiped out every year as farmers slash and burn their way across the jungle in a largely futile attempt to turn it into farmland. The problem is once the forest has been cleared the rain just washes all the goodness out of the soil. Often, after just a few years, the farmers have to move on leaving only wasteland behind. It's one of the great environmental disasters, but now scientists have started to wonder. If millions of prehistoric Indians once lived sustainably in the Amazon then perhaps we can learn from them. What is the secret of the terra preta? Why is it so fertile year after year? There may be a clue in Orellana's account of that first journey down the Amazon.

READER: We entered the dominion of Aparia on St John's Day and already the Indians were beginning to burn over their fields.

NARRATOR: If Orellana's observation is accurate then the Indians were using fire, but clearly not in our slash and burn fashion. Detailed analysis of the terra preta has shown it to be full of burnt plant material, but in a special form: charcoal.

DR JOHANNES LEHMANN (Cornell University): charcoal is in the area here made largely with earthen kilns where organic material, like these logs, are piled up and earth mounds are built around them and under partial exclusion of oxygen you get this charcoal.

NARRATOR: Charcoal is made when you only partially burn the trees and plants. This makes it different to slash and burn where the plant life and all the nutrients it contains are completely reduced to ash. This can be swiftly washed away by the rain, but charcoal can last in the soil for hundreds of years.

JOHANNES LEHMANN: So one of the hypothesis is that the Amerindian populations actually used some sort of slash and char technique as a soil fertility enhancer.

NARRATOR: Inspired by the Ancient Amazonians, Johannes Lehmann's student, Christoph Steiner, decided to find out exactly what effect ancient slash and char methods could have, so he has planted a series of experimental plots, some with added charcoal, some without. The experiment is still not finished, but already the results have been amazing.

CHRISTOPH STEINER (University of Bayreuth): On this plot we see what happens if we follow the traditional slash and burn technique. After the first harvest already there's nothing growing anymore and we have here now the third harvest. Here on this plot we applied mineral fertiliser, but that is not very satisfying. If you look on this there's almost no yield, almost no grain: a family couldn't live on this. That is not satisfying yield. In comparison though a plot where we, where we applied additional charcoal. Here we can see that the yield is much bigger, so there is corn and this is a plot where we applied charcoal and mineral fertiliser and this combination last harvest we had an increase in crop production of 880% in comparison to mineral fertiliser without charcoal.

NARRATOR: An 880% increase in yield is almost miraculous. Charcoal seems to hold the nutrients in the soil preventing them from being washed away by the trains. It's a simple trick, but one that Steiner believes could be the key to breaking the destructive cycle of slash and burn and so reduce the pressure on the rain forest.

CHRISTOPH STEINER: Increased soil fertility means bigger crop production and people can use the same piece of land for more time for more crop production, don't, are not forced to clear a new piece of primary intact tropical rainforest.

NARRATOR: And scientists now believe that terra preta holds yet one more secret. It seems to have another unique property, something that may once have helped it to spread across the Amazon and could now help it to spread across the world. The discovery has been made by Bill Woods. A few years ago he came across a place where terra preta was being mined and then sold on to local gardeners.

WILLIAM WOODS: What we have here is a material that is so valuable that, that people are coming in trucks and buying it. Absolutely unexpected.

NARRATOR: With decent soil so scarce in the Amazon, selling the terra preta seems an odd thing for a farmer to do, but here they've been doing it for 20 years because it appears that the black earth just keeps growing back.

FARMER: After digging the soil that's left will grow deeper. It's because it's being fed by the leaves that fall on it. You can see it happening over there and in there now.

WILLIAM WOODS: The situation is he mines it, he leaves 20cm, he allows it to rest. Then after a 20 year period the depth of this dark soil is the same as it was before the mining operation. This is extremely important in that it strongly suggests that the material is alive and that the biology of this material is the important thing that we're looking at.

NARRATOR: If Woods is right the terra preta can, in some mysterious way, reproduce itself just like a living thing. Scientists are now working to find out how it does it. There are literally tens of thousands of species of bacteria and fungi in the soil and they suspect that somewhere among them must be unique micro-organisms that allow the terra preta to grow. The hope is that if they can unlock its secrets we could reproduce terra preta anywhere. Then we could boost food production throughout the developing world. We could bring a halt to slash and burn. It sounds too good to be true, but the Indians' black earth could one day help to feed the world and save the rainforest.

WILLIAM WOODS: Here now we have, we have people that, that really have problems feeding themselves and so if then we can understand the legacy of, of the terra preta I think really that, that's our El Dorado. This would be important for the world.

NARRATOR: So there is a true irony to the story of the hunt for El Dorado. There was once a great civilisation in the Amazon, one the Europeans destroyed even as they discovered it, but the Amazonians may have left us a legacy far more precious than the gold the Conquistadors were seeking. That black earth, the terra preta, may mean a better future for us all.




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