[blam]

"If people were going back and forth between the Americas and other continents, why weren't any food plants being taken between the continents? Native American plants include maize, beans, pumpkins, squash, tomatoes, peppers, tobacco, potatoes."

It may be because they would have had to travel for an extended length of time through some seriously cold regions.

[SunkenCiv]

:') PreColumbian Central America (including Mexico) shows population peaks three or four times (or more) estimated to exceed those of the postcolumbian era until the 20th century -- followed by collapse. Could be due to the climate cycle (which wouldn't be that surprising), but it could also be an indirect consequence of that same cycle -- IOW, introduction of disease germs from the Mediterranean basin.