I read this article this evening since it was recommended in the course of a conversation about the plagues that swept the Americas after European contact occured. Most people are aware that disease killed a lot of native peoples. Most people are not aware that disease probably killed somewhere between 90% to 98% of the native peoples. The "new world" was far from unpopulated when the ancestors of most of us showed up. How big that population was is open to a bit of debate. Not exactly the empty wilderness of childhood schools though.
"Before it became the New World, the Western Hemisphere was vastly more populous and sophisticated than has been thought, an altogether more salubrious place to live at the time than, say, Europe. New evidence of both the extent of the population and its agricultural advancement leads to a remarkable conjecture: the Amazon rain forest may be largely a human artifact"
Another interesting point, not mentioned in this article, is that it could all very easily happen again, especially given the interconnection in our global society.
You can ping this entry by using http://www.khephra.org/mt-tb.cgi/367 .