Published online by Cambridge University Press: 23 April 2018
A large collection of maize macro-specimens has been gathered from archaeological sites across the American continent, but only a few have been directly dated by accelerator mass spectrometry (AMS). We recently conducted two new excavations in several rock shelters of Tehuacán valley (San Marcos, Coxcatlán, and Purrón) and uncovered 132 non-manipulated macro-specimens of maize suitable for morphological and paleogenomic analysis, including many complete cobs, stalks, internodes, and leaves. Direct AMS dates for 43 samples found in San Marcos or Coxcatlán confirm the previously reported chronologies for these sites. By contrast, a cob found in Purrón was dated to 3060±30 before present (3360–3180 cal BP) (2σ), demonstrating that maize was present at that site at least 1500 calendar years earlier than previously expected, and suggesting that other specimens of similar age are still likely to be found in the southeastern region of the Tehuacán valley. A global comparison of macro-specimen chronology across the continent shows that the current archaebotanical record does not yet reflect the chronology of dispersal from central Mexico to northern or southern regions, opening the possibility for finding the missing links in subsequent expeditions within Mexico and Central America.