I was thinking about this last night.
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.
As soon as the Europeans showed up after Columbus, they started taking all of these plants all over the world.
But before that, no.
Also, no animals. After Columbus, the following animals were introduced to the Americas: chickens, horses, cows, pigs, goats, sheep. None existed in the Americas before Columbus.