The Milpa system of western Mesoamerica: historical depth,evolutionary dynamics and dispersal routes to South America

The core of the pre-ceramic food system in western Mesoamerica could have been structured with the ancestral wild populations of maize, beans and squash; in turn, incipient agricultural management of these species, year after year, at the same site, could have given rise to the Milpa agri-food system. This paper attempts to answer the following questions: 1. How and when did the Milpa agri-food system originate?, 2. Which human groups could have done it?, 3 Could the domestication of the ancestral wild populations of maize, squash and beans have been possible at several sites in the Americas?, 4. What were the dispersal routes? To answer these questions, paleoecological, archaeological, ethno-botanical, and molecular genetic evidence was collated. The significance of domestication and the importance of studying the evolutionary dynamics of the wild-domesticated population complex of the species grown in the Milpa system were analyzed, and the challenges, threats and opportunities for its genetic resources were identified. The Milpa agri-food system was structured with species from the tropical deciduous forest about 9,000 years ago. Maize could have spread to Panama, Ecuador and Peru from 7,400 to 6,500 years ago. Wild ancestral populations of squash and beans also could have been domesticated in both Mesoamerica and South America. In plants, domestication creates human dependence; for humans, it generates a high social cost that is presently unvalued. The evolutionary dynamics of the Milpa population complexes helps us to develop strategies for conservation and restoration, and to assess the escape risks of domestic and trans-genes. Social and economic changes brought about by globalization of markets pose a major threat to Milpa genetic resources. However, they are a key tool for tackling climate change.