<P> The Great American Interchange was an important late Cenozoic paleozoogeographic event in which land and freshwater fauna migrated from North America via Central America to South America and vice versa, as the volcanic Isthmus of Panama rose up from the sea floor and bridged the formerly separated continents . The migration peaked dramatically around three million years (Ma) ago during the Piacenzian age . It resulted in the joining of the Neotropic (roughly South America) and Nearctic (roughly North America) ecozones definitively to form the Americas . The interchange is visible from observation of both biostratigraphy and nature (neontology). Its most dramatic effect is on the zoogeography of mammals but it also gave an opportunity for reptiles, amphibians, arthropods, weak - flying or flightless birds, and even freshwater fish to migrate . </P> <P> The occurrence of the interchange was first discussed in 1876 by the "father of biogeography", Alfred Russel Wallace . Wallace had spent 1848--1852 exploring and collecting specimens in the Amazon basin . Others who made significant contributions to understanding the event in the century that followed include Florentino Ameghino, W.D. Matthew, W.B. Scott, Bryan Patterson, George Gaylord Simpson and S. David Webb . The Pliocene timing of the formation of the connection between North and South America was discussed in 1910 by Henry Fairfield Osborn . </P>

When did south america and north america join