When Spanish ships first reached the Americas in 1492, they had no idea of the size of the lands on which they’d set foot, nor did they realise that they were twin continents in their own right. Columbus died believing he’d found an alternative route to the ‘Indies’, because although the early explorers new the world was a sphere, they thought it was much smaller than it turned out to be. Only eight years after Columbus’s first landing, the mouth of the Amazon was discovered, and the river’s size could be guessed by the extent of the ‘sweet water’ that it produced, changing the colour of the Atlantic Ocean across more than two million square kilometres. But the Amazon basin remained unknown to Europeans for 50 more years, and then its discovery was an accident.

It is commonly supposed that before the Conquest the Americas were sparsely inhabited. Yet Charles Mann argues in the book that in the US was called ‘1491’, that it is possible that the total population at the time was actually greater than Europe’s. There is an ongoing controversy about this, not least because only a few of the more advanced civilisations left written records and these are not easily decoded. Part of the controversy centres on an area – the Amazon – where there were no written records at all and few if any permanent structures. John Hemmings, in ‘Tree of Rivers’, judges there to have been 4-5 million people in the Amazon basin before the Spaniards and Portuguese arrived, pitching his estimate between the extremes of those that have been put forward by different scientists.

The first European to get an inkling of the size of settlements that clustered around the Amazon and its tributaries was Francisco Orellana, who unknowingly set off down the river system at the end of 1541, and is the hero of River of Darkness. He was second in command of an expedition that had set out from Quito (now capital of Ecuador) in February of that year. It was led by Gonzalo Pizarro, whose brother Francisco had defeated the Inca Emperor, Atahualpa, nine years earlier, setting up Spain’s second colony in the Americas. Gonzalo was determined to earn riches of his own, and his expedition was pursuing rumours of regions in which there were bountiful cinnamon trees and, of course, the key Spanish obsession – gold. His huge expeditionary force of soldiers, slaves, horses, pigs and hunting dogs quickly got bogged down in the forested valleys of what, unknown to Pizarro, were the Amazon’s headwaters. They found very little food, and resorted to eating their own animals and then, by now half starved, built a boat to continue their journey eastwards, reaching the River Napo. Still finding no food, Orellana was dispatched with 57 men to continue downriver and return ‘within 12 days’. They never came back. Pizarro eventually returned to Quito in the following June, having endured ‘the worst march ever in the Indies’.

Orellana and his men had been quickly swept past a point of no return, still without finding food. Orellana knew enough about the river system to be convinced that if they continued they would eventually reach coasts that were known to his countrymen. What he did not know was that the ocean was thousands of kilometres away and it would take him until August 1542 to get there, and until September to reach a Spanish outpost in what is now Venezuela.

So Orellana was to be the first European to grasp the size of the world’s biggest river system, and to encounter its many tribal peoples and settlements. Fortunately he lacked the Pizarro brothers’ cruelty and seems to have been a natural linguist, quickly learning to communicate in the various languages they encountered. Although the boats (they built a second) were often attacked, equally often they were welcomed by tribes who provided them with food. In the middle part of the journey, they passed continuous settlements on the banks of the river, giving some inkling of the numbers of people the Amazon basis sustained.

In one sense the journey ended happily: almost all his men survived, and Orellana himself was able to return to Spain to successfully answer Pizarro’s charges of treason. Gonzallo later became a traitor to the crown and, in 1548, he was executed. But by then Orellana was already dead: he’d sought the king’s permission to return with an expeditionary force to the Amazon, this time approaching it from the Atlantic, but his earlier luck did not hold. He perished before he made any significant progress back up the river that he had been the first European to navigate. A curious sequel to the failed journey occurred just eight years later, when a Spanish town in the highlands of Peru was astonished to be visited by about three hundred Indians who had travelled by canoe all the way up the river from its estuary, a feat as remarkable in its way as Orellana’s.

Unlike Pizarro, Orellana seems to have made little use of the ‘requerimiento’: the infamous passage read out (in Spanish, of course) to tribal peoples, requiring them to accept the Christian god and the Spanish emperor as their ruler. Or at least, when he used it and received the usual bemused response, he didn’t turn on his listeners and start to butcher or to enslave them. Nevertheless, those Indians who received Orellana with hostility rather than hospitality may have realised the threat which he represented, even if they could have had no idea of its scale. Apart from massacre and enslavement, the conquerors brought disease: one estimate is that one in five of the world’s population died in the epidemics unleashed by contact between the two hemispheres, of which the overwhelming majority were in the Americas.

Orellana chronicled his journey by appointing his friar as his diarist. While he may have been partly motivated by the potential significance of exploration he was undertaking, it was also an act of self-protection, as he realised soon after leaving Gonzalo that there would be competing accounts of the circumstances in which they had separated. Like Orellana, present-day travellers still face an enormous challenge if they attempt to navigate the Amazon river system from the Andes to the Atlantic. What they no longer find is plentiful evidence of indigenous peoples living as they did before the Conquest. Even the very few tribes that still eschew contact have imported plants like the banana and use steel tools unknown to their forebears. Buddy Levy’s book about Orellana’s voyage, based in part on the friar’s account, is a vivid reconstruction of a wondrous land that was about to be changed for ever by the colonisation, exploitation and destruction that Orellana began.