The Xihuatoxtla Shelter is located in the Rio Balsas valley in the state of Guerrero, Mexico, and it has evidence of the earliest maize agriculture to date, as long ago as 9,000 cal BP. The Rio Balsas valley is thought to be the original homeland of teosinte, the progenitor crop for domesticated maize. The rockshelter was used repeatedly over 7,000 and more years, by small groups of people who stayed for intervals of some seveal weeks or longer.
The shelter was formed beneath an enormous boulder, with a floor space of approximately 75 square meters. Five stratified archaeological deposits are located within the one meter soil deposit inside the rockshelter.
Layer A (1240-1000 cal BP), pre-columbian potsherds, obsidian blade fragment, some modern bottle glass
Layer C (Archaic: 5590-5320 cal BP) preceramic, scrapers, borer, flake knife, handstones and a possible milling stone
Layer D (Early Archaic: 8990-8610 cal BP) chopper, scraper, graver, knives, biface, milling stone bases, handstones
Layer E (Paleoindian or Early Archaic, >8990 cal BP) stemmed, indented point, Pedernales point base, lanceolate point, spokeshave, graver, handstones
Nearly all of the handstones and milling stones showed evidence of usewear and had starch grains from domesticated maize (Zea mays, not teosinte), even those recovered from the Early Archaic/Paleoindian layers. Phytoliths from domestic squash (Cucurbita spp.) were recovered from both sediments and stone tools in every layer, again including the Early Archaic and Paleoindian layers. This is the earliest date for domesticated maize discovered to date.
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Piperno, Dolores R., et al. 2009 Starch grain and phytolith evidence for early ninth millennium B.P. maize from the Central Balsas River Valley, Mexico. Proceedings of the National Academy of Sciences Early Edition.
Ranere, Anthony J., et al. 2009 The cultural and chronological context of early Holocene maize and squash domestication in the Central Balsas River Valley, Mexico. Proceedings of the National Academy of Sciences Early Edition.