But, realistically, I couldn't expect much after all these years. Soon, I feared, I'd be on my way home again, sapped by the humidity, and none the wiser about Fawcett - just like the 13 expeditions that had been on his trail before me.
As we continued cutting through the slow brown waters of the Kuluene river, pairs of macaws rose from the riverbank trees and fish leaped clear of the water to avoid some hidden menace stirring down there in the depths. I thought back through Fawcett's unhappy tale. At 58, Percy Harrison Fawcett had surveyed for years in Bolivia and had been awarded the prestigious Founder's Medal of the Royal Geographical Society. He also had a mystical bent, and in Rio's state library he had unearthed a manuscript, dated 1753 and purporting to be an account of a ruined civilisation - a sort of Atlantis in Brazil's Mato Grosso.
On 20 April 1925, he set out in search of the lost city with his eldest son Jack and Jack's friend Raleigh Rimell, both in their twenties. They worked their way north through the Mato Grosso towards my present position - the Upper Xingu, a south-eastern tributary of the Amazon. By then, Raleigh's leg was badly infected with a tick bite. "You need have no fear of any failure," he wrote in a note given to his returning porters, as they carried on, now alone. And then, nothing.
Commander Dyott, heading the "relief" expedition three years later, was soon in danger of disappearing here himself. "We are surrounded by Indians at this place," he radioed in a distressed state. Dyott's own predicament left him in no doubt about the fate of the explorer. He reported that "the Fawcett expedition perished at the hands of hostile Indians during July 1925", before abandoning his radio to flee. The Kalapalos had told him that they'd followed Fawcett for four days beyond the Kuluene, the river I was travelling now, and seen the smoke of his camp fire. "On the fifth day there was no more smoke, and we knew that the white men had been killed by bad Indians." These must be the Suyas, or one of the apparently more aggressive groups to the east. However, there was no body, no conclusive proof.
Alternative theories came and went - he had starved, he was eaten by jaguars. Then, in 1951, it seemed the riddle had finally been solved. Orlando Villas Boas, the Brazilian champion of Indian rights, announced that the Kalapalo chief Sarari had confessed the truth. They had killed Fawcett after he had upset them by not sharing a duck that he had shot - an outrageous thing to the communally minded Indians - and had compounded the error by slapping a child.
"All the Kalapalos came to the top of the cliff by the lake and sat in a semicircle," Orlando is quoted as saying at the time. "They made me promise that no aeroplanes would come [to exact revenge], and after that they told me how the killing was done."
They had set an ambush at the Green Lagoon, and clubbed the white men to death. Fawcett was buried, but the others - attacked immediately after Fawcett, while still crossing the lagoon itself - were left to the alligators in the water where they fell.
There was no doubting Orlando's credentials - the legendary Villas Boas brothers had been instrumental in forming the Xingu Reserve, a safe haven in the fast-depleting forest. Furthermore, he had the evidence. But an end to the mystery it was not. Fawcett's surviving son, Brian, insisted the bones belonged to a shorter man. And when he cross-examined the Kalapalos himself he found the story to be "full of contradictions". Furthermore, a duplicate set of dentures kept by Fawcett's widow didn't fit the skull.
The mystery remained - and still remained last year, as I set about making a TV programme on Fawcett. I'd envisaged the ill-fated colonel as a neat symbol of the clumsiness - not to say arrogance - of the white man and his dreams. However, the various academics and authorities on the Amazon were less than enthusiastic. "The story is irrelevant," a leading historian told the BBC flatly.
I could see his point. To be brutal, Fawcett had achieved absolutely nothing of historical importance. However, there was no doubting that Fawcett's tale meant something. No coffee-table book on explorers was complete without him. He was the Livingstone of the Amazon, someone as consumed by his dream as Scott was by the Antarctic.
"Besides," we were told, "there is no mystery. His bones are sitting in Sao Paulo." These were the bones that had been uncovered by Orlando Villas Boas and were now in a laboratory awaiting DNA analysis.
Again, I had to agree. Why would the Indians lie to Orlando, their one friend - and furthermore confess to a murder that they didn't even commit? So I asked a close friend of the Kalapalo, Paulo Pinage - a Brazilian who had worked for Funai, the government Indian agency - to radio them. We would like their side of the story.
Permission eventually came back, and I'd set off with Paulo, Ruhi Hamid - my BBC producer - and a camera. They'd now gone ahead of me to Tanguru escorted by the headman, Vajavi, to once more make plain our intentions. He said it was essential - the Kalapalos are keenly aware of the encroachments of the white man. Indeed, it hadn't escaped our notice that the Kalapalos had, only a couple of years previously, held hostage 11 members of the last expedition to investigate Fawcett.
The village of Tanguru was at last ahead of me, high on the bank. I could see thatched rooftops and children washing in the river, ducking and dive- bombing each other. Soon girls were running delighted from the river, wiping the water from their faces and hoiking up my hefty luggage and leading the way. "No doubt times have just changed," I thought, following a girl who had balanced my rucksack on her head.
Over a few days we slowed to the rhythm of the village - in the cool of morning, boys lobbing their toy spears, in the evening men returning with their day's fish catch and sharing it out. In some ways the village was much as Fawcett would have known it. Though there was now a rudimentary school, and the men wore Nike shorts and rode bicycles when going to the river to wash, the Kalapalos still secluded their children at puberty to learn the duties of adulthood. Everyone still lived in molocas - the huge cool, barn-like family buildings.
On the third day, the meeting was called. At last we would hear the Kalapalo side of the story in their own words. In the men's house, a half-built structure with a flapping tarpaulin to block out the noon sun, a hundred men were scrutinising me, and beyond them a further ring of women with their arms folded.
I reminded the Kalapalo that Orlando Villas Boas had reported to the world that they had confessed to the killing of Fawcett. Were these just stories? It was 50 years ago, even the elders were only children at the time. Headman Vajavi, an affable man with a paunch and thick glasses, talked for the others. "In the beginning," he said, mainly addressing his friend Paulo, "Orlando went to all the villages in the Mato Grosso and asked if anyone had seen the Englishman."
"Had anyone killed him?" interrupted Paulo, unable to wait any longer.
"Nobody knew - because the truth is, nobody killed him. Then Orlando came here to the Kalapalos and offered some beads to Vajavi's uncle and asked: `Who killed the Colonel?' And the uncle said: `Nobody killed the Colonel... Nobody knows.' " Vajavi looked at me hard. "We in the Upper Xingu are not fighters. We never killed anybody. We were accused by the Villas Boas brothers."
Accused? By their only friends? "Why?" asked Paulo. Like me, a lifelong admirer of the Villas Boas brothers, he was confused, even a little shaken. The Kalapalos mulled over their response together for a while. Flies buzzed. A white egret rose from the river and cut through the heat overhead.
"So," said Vajavi at last, "we don't know why they accused us. They had no reason to. Because, like you said, they were our friends." He repeated: "The Villas Boas brothers didn't defend us, they accused us."
By now, it was sinking in. The Kalapalos were accusing their hero of betrayal. I did not believe that this could be true. But why were they saying it?
But there was more. We listened as the Kalapalos now went on to claim that the bones that were given to Orlando were actually the bones of Vajavi's grandfather. "The only person to gain from this story was Orlando," the headman said, rather despondently. "He gained fame while we were seen as criminals. And that's the true story as we understand it."
The next day, as we were still trying to take this in, Vajavi took us by boat through the early mists to the Green Lagoon, the supposed scene of the crime. This might clarify the story, we all felt. Despite promisingly sinister signs - alligators swirled away through the mud - the lagoon banks were totally overgrown and all access to Fawcett's so-called "grave" impossible.
This was, however, the place where Fawcett met the Kalapalos.
"They met Sarari, the chief," explained Vajavi. "They met here and they stayed for a day. The chief advised him that it was not a good idea to continue and that he ought to head back. But the Colonel said it was no problem, he would carry on."
"I understood from yesterday," Paulo said, "that, after five days, smoke was seen from far away. So the Kalapalos followed and found his trail?"
"We found where they had stopped for lunch," Vajavi continued in his no-nonsense way. "After that we could find nothing... untouched forest."
He told us that the bones of his grandfather, who was tall - almost the height of Fawcett - "were dug up and and reburied in this area". We asked Vajavi about Fawcett slapping the child and being killed by a furious father.
"They are just stories," replied Vajavi, looking to the forest floor. "And that's why, even today we are saddened by this story."
We got up stiffly from the damp leaves and left the ant armies, the throbbing cicadas, the unfound remains of Fawcett. Soon we were leaving the Kalapalos with much that was new and exciting regarding Fawcett - they had even supplied fresh details, speaking of Fawcett demonstrating a distress flare and the party using them when they got in trouble. They even suggested he might have been clubbed by a bellicose party of passing Iaruna indians. So, coming away in that canoe I reflected that I could now say beyond reasonable doubt that Fawcett had died four or five days east, at the hands of another tribe. But the details now seemed of little relevance. More important - certainly to the Kalapalos - was how they now claim to have been betrayed themselves, and not by Fawcett.
Since the trip, a year ago, numerous people have checked that we quoted the indians correctly. Clearly the Kalapalos believed all they were saying, and the neighbouring Kuikurrus corroborated their story. Moreover, the oral tradition of the Indians should not be taken lightly. The spoken word is their entire history. However in all communities stories are inevitably changed over time.
What to conclude from all this? Their story of betrayal should be put in context. The very existence of the forest was in question in Orlando's day, and if conjuring up the bones of a minor explorer was going to help their cause, who can blame the indians?
So why accuse Orlando? Perhaps to the Kalapalos, he had become the one white man within reach to strike out at. Their Xingu forests are now but an island surrounded by charcoaled pastures.
Sadly, we must take seriously the first-hand account of a man of Orlando's integrity. The truth is we are all responsible for what has happened to the indians. I had been expecting to use a machete-wielding Englishman as a symbol of white arrogance. But instead Orlando, their greatest defender, is being made to carry the burden.
Benedict Allen's four-part series, `The Bones of Colonel Fawcett', begins at 7.30pm on Friday 24 September on BBC2
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