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31 May 2010
Geneticists track origins of maize

10,000 years ago a revolution occured. Humans began to domesticate animals and plants and move from hunter-gatherer societies to permanent settlements. Nine of the major food crops were developed in the Americas, including corn, or maize (Zea mays). Corn is the source of 21 percent of human nutrition.
     Maize is not found in a wild form, so its origins have been a mystery. Botanists have never found other living plants with similar charactertistics. It was long assumed that modern corn originiated from an extinct or yet undiscovered ancestor. But, in the early 1930's, George W. Beadle, a graduate student at Cornell University, USA, discovered similarities in chromosomes between modern maize and teosinte, a Mexican grass. He was able to cultivate fertile hybrids of corn and teosinte with characteristics of both plants, and to get kernals of teosinte to pop. He concluded that they were members of the same species with teosinte being the ancestral form. Beadle continued to make discoveries in genetics and shared the Nobel Prize in 1958.
     For 30 years, Beadle's hypothesis was not widely accepted. Teosinte is a thin, finger-like ear, blue-grey in color with hard kernals attached to to a central stalk. It was first classified as a relative of modern rices. Many scientists felt that the differences between the plants were too great to have allowed modern corn to evolve in the short interval since humans had begun to grow crops. Beadle continued his research after he retired. He cross-bred corn with teosinte until he was able to demonstrate that four or five genes produced the major structural differences between the plants. This meant that they shared most of their DNA, and were very closely related.
     Beadle's genetic studies did not provide information about the geographic evolution of modern corn. This required DNA typing of the sort used in paternity studies. Botanists from the University of Wisconsin, USA, lead by John Doebly, collected 60 samples of teosinte from sites in North and Central America. DNA profiles were created for the samples, and it was discovered that all modern maize is related to teosinte from the Central Balas River Valley in southern Mexico. By examining variations in genes, they were able to estimate that maize appeared approximately 9,000 years ago.
     Archaeologists then began excavating in the Balas Valley, looking for evidence of agriculture and the use of corn. Anthony Ranere from Temple University, USA, and Dolores Piperno from the Smithsonian National Museum of Natural History, USA, examined rock shelters and caves. The Xihuatoxla shelter yielded stone milling tools that still retained residues of maize on them. The tools were dated to 8,700 years BP, consistent with the 9,000 year estimate for the origins of maize.
     It is remarkable that humans who had just begun to learn about agriculture wer able to domesticate a minimally useful plant like teosinte into a crop with high nutrition and good yields that is easy to harvest. It probably took hundreds and perhaps thousands of years to refine the characteristics of the plants and remove unwanted features such as the hard cases around the kernals. Perhaps plants with a softer casing an evolutionary advantage. Or, the early farmers may have selectively bred individuals plants that, through random genetic variation, had more useful characteristics.
         
Source: New York Times (24 May 2010)

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