The first evidence of the domesticated production of corn in the Americas can be found in Mexico and dates back to 7,000 BCE. It was developed from a wild grass called teosinte. After a few thousand years its usage and cultivation spread over South America and evidence has been found in areas even before the use of pottery.
     Until now the earliest evidence had only dated back as far as 3,000 BCE. Now a team of scientists from the Washington Natural History Museum (USA) have found evidence of processed (cooked) corn in Peru, dating from approximately 4,700 BCE.
     Dolores Piperno, a curator of New world archaeology at the Smithsonian National Museum of Natural History is quoted as saying "These new and unique races of corn may have developed quickly in South America, where there was no chance that they would continue to be pollinated by wild teosinte. Because there is so little data available from other places for this time period, the wealth of morphological information about the cobs and other corns remains at this early date is very important for understanding how corn became the crop we know today".  

Edited from EureAlert! (18 January 2012) BBC News, National Geographic News (19 January 2012)