After the continent of South America split from Africa it was isolated from all other life on the earth except for a few migrations over the oceans from Africa. When South America finally collided with North America by 3 million years ago the animals from North and South America began to migrate north and south. In the end the animals from the north were the dominant species. The only real South America successes were the armadillos, opossums and the porcupines. There are several reasons that are viewed as the reasons for this victory by the northern animals. One was that It was easier for the North American animals to travel from the colder harsher North American climes to the more tropical South American climes. Another factor was that since the South American animals were isolated they evolved on their own without any major competition while the North American animals had competition from Asia across the Bering land bridge so they were more hardened by competition.

(The Great Courses – A New History of Life)