Christopher Columbus first arrived in the Americas on October 12th, 1492. Columbus Day is celebrated in the United States today in memory of that event and how it undoubtedly changed the world forever. One of the biggest impacts his arrival had was due to the number of animals and crops introduced to the New World; this is known as the Columbian Exchange.
From a biological perspective, both North and South America were very different places before European settlers arrived. With the exception of alpacas and llamas there were virtually no large animals in the Americas suitable for domestication. The Europeans introduced cattle, sheep, goats and of course, horses; all animals considered vital to both the history and modern economies of the Americas.
Turkeys, llamas, alpacas and guinea pigs were all introduced to Europe and Asia, where they are now common.