Christopher Columbus, 1451

Of course we know without a doubt that Columbus did not in fact discover America.  The Vikings were here first and foremost, but they apparently did not like it enough to stay put.  We also know without any doubt that China had the wherewithal to sail to North America almost a century before the Vikings did so, but after a brief foray around to Africa, went home and did not pursue either trade or colonization on another continent. 
So it was left to the shipping powers of the 15th century to find and colonize North and South America.  These folks were hungry for power and money and they found both in large abundance.  I think reading Charles Mann's companion books '1491' and '1493' are an easy way to think about what exactly happened when the Spanish rapidly colonized and conquered the Aztec and Incan civilizations and brought all that silver (and gold, but it was silver that changed world commerce) home with them.  It is hard to know what would have happened if our continent had been colonized earlier or later, but good to remember the time that changed the world irrevocably.