How did Columbus begin a Global Exchange?

Answer 1

Columbus's arrival in the New World, as an agent for a country with the power, resources, and will to exploit it, marks the beginning of regular contacts between the New and Old Worlds, or more accurately, the Americas and Eurasian supercontinent. Flora and fauna, as well as microbes, that had been evolving separately for thousands of years were suddenly thrust into easy contact with each other. This was particularly significant for the Americas where the main spread of land runs north-south and would otherwise serve as a barrier to such contact (changing climate bands tend to do this).
The Europeans were able to use new foodstuffs suitable to marginal lands (particularly the potato) and power an astonishingly big population explosion. Meanwhile, Native Americans, protected from cohabitation with many nasty diseases because they didn't live with large amounts of domesticated livestock, were suddenly made vulnerable to Eurasian diseases and, just as suddenly, decimated.