See 1491. This thread contains a reference to a very long article in the Atlantic (by Charles C. Mann) which is worth the read. It is a mixture of fact, hypothesis and speculation. Among other things it puts forth the proposition that the Amazon and the great plains of the U.S. are largely human (native american) artifacts! This article in the atlantic also was the basis for a recent good column by Jonah Goldberg (not sure if it was ever posted, though I would be surprised if it wasn't) entitled "No such thing as Natural"
When Columbus appeared in the Caribbean, the descendants of the world's two Neolithic civilizations collided, with overwhelming consequences for both. American Neolithic development occurred later than that of the Middle East, possibly because the Indians needed more time to build up the requisite population density. Without beasts of burden they could not capitalize on the wheel (for individual workers on uneven terrain skids are nearly as effective as carts for hauling), and they never developed steel. But in agriculture they handily outstripped the children of Sumeria. Every tomato in Italy, every potato in Ireland, and every hot pepper in Thailand came from this hemisphere. Worldwide, more than half the crops grown today were initially developed in the Americas.
Maize, as corn is called in the rest of the world, was a triumph with global implications. Indians developed an extraordinary number of maize varieties for different growing conditions, which meant that the crop could and did spread throughout the planet. Central and Southern Europeans became particularly dependent on it; maize was the staple of Serbia, Romania, and Moldavia by the nineteenth century. Indian crops dramatically reduced hunger, Crosby says, which led to an Old World population boom.