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The domestication of maize and its spread throughout the Americas has long generated interest. Four symposia on maize at the March-April 2004 meeting of the Society of American Archaeologists in Montreal1 helped to push research (and debates) forward. While there is at present widespread acceptance that one of the teosintes (likely Zea mays subsp.parviglumus) is the ancestor of domesticated maize (Zea mays subsp. mays), many questions remain about where, when, and how domestication took place, and about the subsequent spread of cultivated maize throughout the Americas (Society for Ameri can Archaeology 2004a, b; Smalley and Blake, 2003). Botanical evidence for the presence of maize at different sites and times is key to the debate. This evidence consists of maize macrofossils (usually cobs, kernels, or frag ments) and maize microfossils (pollen grains and phytoliths) for which ages can be deter mined using radiocarbon analysis. Until recendy, almost all radiocarbon ages for maize macrofossils or microfossils were based on standard radiocarbon assays of associated charcoal or other organic remains in the soils or sediments from which the maize fossils were recovered. The development of accelerator mass spectrometry (AMS) radiocarbon daring, which requires a much smaller mass of sample, has made it possible to date indi vidual maize kernels or cob fragments, and has led to die application of more exacting standards to botanical evidence of the history of maize and other New W;orld crops (Smith, 1994?95; Fritz, 1994). Under these new standards, die primary class of botanical evidence consists of seeds or other plants parts that show morphologies indicative of deliberate planting and harvesting2 and that are direcdy dated by AMS methods (Smith, 1994-95). Cultigen pollen and phytoliths that are not direcdy dated constitute less desir able, secondary classes of evidence, as do macrofossils that are not directly dated. Rein terpretation of the history of maize based only on primary botanical evidence, including direct AMS dates on maize cobs from rockshelters in the Tehuacan Valley of Mexico that had previously been dated only indirecdy (Long era/., 1989), led Fritz (1994) to question the standard 5000 BC date for maize domestication. The earliest primary botanical evi dence she found was for a direcdy dated cob from the San Marcos (Tehuacan) cave that yielded an AMS date of 4700 ? 110 14C years BP, a radiocarbon age that corresponds to a calendar age of between 3700 and 3100 BC when corrected for known variations in atmospheric 14C (see Table 1 for calibration mediods and for.calibrated age ranges for all dates for maize subsequendy presented here). More recendy, Piperno and Flannery (2001: 2102) dated two "primitive-looking maize cobs" from Guild Naquitz cave in Oaxaca, Mexico, to 5420 ? 60 14C years BP and 5410 ? 40 14C years BP. Based on an analysis by Beadle and Ford, the dated cobs are thought to represent either "maize-teosinte hybrids

Pollen evidence of the earliest maize agriculture in Costa Rica