Corn is caught in a tug-of-war between ethanol plants and food, one of the first signs of a coming agricultural transformation and a global economic shift. Ever since our ancestors in the Fertile Crescent first figured out how to grow grains, crops have been used mainly to feed people and livestock. But now that's changing in response to the high price of oil, the cost in lives and dollars of ensuring a supply of petroleum imports, and limits on climate-warming emissions of fossil fuels.