Covered that later. Without the agricultural developments of the American indians this world would probably be unable to to support more than 500 million people instead of its current 7 billion.
I think that the world ate just fine before the discovery of the "new world" I don't think that the American Indian had the concept of planting a seed to produce a crop. They, of course ate what the earth provided, but to assume that a race of people who had no concept of the wheel could have established an agricultural program is nonsense.
Immediately to their south were civilizations creating cities, pyramids, stone buildings, metal tools and weapons,and a civilization that compared with the rest of the world. I have american indian blood in many of my relatives, but the American Indian population were VERY basic human beings.