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Mass Spectrometry Unearths Mexico's Maize

Maize is a staple food crop of many prehistoric socieities and many scientists believe that the food originates from teosinite which is native to the Balsas River Valley of the Pacific Slopes in southern Mexico. Much of this region receives annual rainfall of 1500mm and annual mean temperatures of 25C and is covered by a tropical deciduous forest.

In 1966 a cave located in the valley of the Mexico highlands region of Oaxaco found several living floors believed to be have been occupied by early human inhabitants. Zone A, the uppermost level, contained a diverse array of domesticated plant life that was dated between 620-740 A.D. In addition to the long time occupants, shorter visits known as "ephermeral occupations" were thought to have been made to the caves pre 620 A.D. These visitors left behind four-small primitive looking maize cobs which occupy small areas of ash just below Zone A.

Accelerator mass spectrometry has been used to determine the age of maize cobs in Oaxaco. Radiocarbon dating placed the age of the maize cobs at over 6200 years old, or some 4200 years B.C., making them the oldest known agricultural crops in the Americas. This finding, in addition to the unequivocal domesticated nature of the crops, leaves open the question of when agricultural practices to cultivate maize and related crops began. These early farmers were likely mobile hunters and gatherers so that the handful of caves and rock shelters in the valley so far excavated do not necessarily represent those in which the practice began.

The full article appears in the February 13 issue of the Proceedings of the National Academy of Sciences USA.


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