Many animals flooded into the South American continent about 4 million years ago during the Great American Interchange in the Pliocene, when the string of volcanic islands linking with the North American continent, fused to form a land bridge (Isthmus of Panama). The animals quickly evolved and developed to fill the ecological niches they found. In most instances the animals that came south were more successful and began to replace the already indigenous animals that had dominated the continent for the previous 50 million years. The reason for this may be that the already indigenous animals had evolved few variations in that period of time. The continent itself had not shifted substantially and there had been little change in the environment and the climate except for the spread of the gra