The seeds of the Inca Empire were planted about 2,700 years ago when a warm spell combined with piles of llama excrement allowed maize agriculture to take root high up in the South American Andes, according to a new study.

“They were constructing fields and weeding them. And probably trading took off, made possible by llama caravans transporting goods, such as maize, coca leaves, salt and a ceremonial product called cinnabar,” Alex Chepstow-Lusty of the French Institute of Andean Studies in Lima told me Sunday in an email.

The finding is inferred by a record of pollen and mites in a core of mud taken from a small lake located at about 11,000 feet up in the Andes surrounded by agricultural terraces and next to an ancient trading route that connected tropical forest and mountain communities.