Rich soils and plentiful rainfall have made the Yungas, which run along the eastern slopes of the Andes, an agriculture centre. Criss-crossed with ancient trading routes once plied by llama caravans, the region was a breadbasket for the Inca and earlier empires such as the Tiwanaku. This tradition continues today. As I hiked along a centuries-old trail towards the Río Coroico, I passed hillside terraces planted with coffee, bananas, cassava, guavas, papayas and citrus fruits. There were also bushy plants with slender branches, oval-shaped leaves and reddish berries: coca.