Looking back, I can’t say I wasn’t warned. I’d asked an Indian chief to contact the next village by radio, and heard the threat loud and clear: ‘If the gringo comes here, he’ll be killed.’ It was my Peruvian guide, Cho, who came up with a solution: instead of passing through the hostile village, we’d inflate our rubber rafts and paddle over to a long shingle island. There, I could continue my mission to walk the entire length of the Amazon river, without fear of attack. Which is exactly what we did. Then, at the other end of the island,...