The milpa is a polyculture system combining maize (Zea mays), beans (Phaseolus vulgaris), and squash (Cucurbita spp.) in the same field, with land rotation and fallow periods. The system exploits precise ecological complementarities: beans fix atmospheric nitrogen in the soil, offsetting maize's consumption; squash covers the ground with its large leaves, reducing evaporation and suppressing weeds; maize provides physical support for the bean vines. The result is a high-caloric-yield-per-hectare system that maintains soil fertility without external fertilizers. The maize-bean-squash combination also provides a complete protein diet (the essential amino acids in beans complement those in maize). The system was the food base of all major Mesoamerican civilizations and constitutes the most documented precedent of what modern agronomy calls polyculture and permaculture calls 'plant guilds'. Its origin is archaeologically placed between 2000 and 1500 BC in the Valley of Mexico and the Maya area.