It resulted in the joining of the Neotropic (roughly South America) and Nearctic (roughly North America) definitively to form the Americas. The interchange is visible from observation of both stratigraphy and nature (neontology). Its most dramatic effect is on the zoogeography of mammals but it also gave an opportunity for non-flying birds, arthropods, reptiles, amphibians and even freshwater fish to migrate.
After the late Mesozoic breakup of Gondwana, South America spent most of the Cenozoic era as an island continent whose "splendid isolation" allowed its fauna to evolve into forms found nowhere else on earth. Its endemic mammals initially consisted of marsupials, xenarthrans (i.e., armadillos, anteaters and sloths, like the giant ground sloth Megatherium), and a diverse group of native ungulates: notoungulates (the "southern ungulates"), litopterns, astrapotheres (e.g. Trigonostylops, Astrapotherium), and pyrotheres (e.g. Pyrotherium).
Marsupials may have traveled (via Gondwanan land connections) from South America through Antarctica to Australia and/or vice versa in the late Cretaceous or early Tertiary. One living S. American marsupial, the Monito del Monte, is believed to be more closely related to Australian marsupials than to other South American marsupials. A 61-Ma-old platypus-like monotreme fossil from Patagonia may represent another Australian immigrant. It appears that ratites (relatives of S. American tinamous) migrated by this route around the same time, more likely in the direction from S. America towards Australia/New Zealand. Other taxa that may have dispersed by the same route (if not by flying or floating across the ocean) are parrots, chelid turtles and (extinct) meiolaniid turtles.
The marsupials present in South America included didelphimorphs (opossums) and other small forms, but many larger predatory forms also existed, like the borhyaenids and the sabertooth Thylacosmilus. The marsupials shared the ecological niches for large predators with fearsome flightless "terror birds" (phorusrhacids), whose closest extant relatives are the seriemas. (Terrestrial ziphodont sebecid crocodilians were also present at least through the middle Miocene.)
The notoungulates and litopterns had many strange forms, like Macrauchenia, a camel-like litoptern with a small proboscis. They also produced a number of familiar-looking body types that represent examples of parallel or convergent evolution: one-toed Thoatherium had legs like those of a horse, Pachyrukhos resembled a rabbit, Homalodotherium was a semi-bipedal clawed browser like a chalicothere, and horned Trigodon looked like a rhino. Both groups started evolving in the Lower Paleocene, possibly from condylarth stock, diversified, dwindled before the great interchange, and went extinct in the Pleistocene. The pyrotheres and astrapotheres were also strange but were less diverse and disappeared earlier, well before the interchange.
The invasions of South America started at least 31.5 Ma ago (late Eocene/early Oligocene), when cavimorph rodents arrived. Their subsequent vigorous diversification displaced some of S. America's small marsupials and gave rise to – among others – capybaras, chinchillas, viscachas, and New World porcupines. (The independent development of spines by New and Old World porcupines is another example of parallel evolution.) This invasion most likely came from Africa. The crossing from West Africa to the northeast corner of Brazil was much shorter then due to continental drift, and may have been aided by island-hopping (e.g. via St. Paul's Rocks, if they were an inhabitable island at the time) and westward oceanic currents. Crossings of the ocean were accomplished when at least one fertilised female (more commonly a group of animals) accidentally floated over on driftwood or mangrove rafts. (Island-hopping cavimorphs would subsequently colonize the West Indies as far as the Bahamas).
A little later (at least 25 Ma ago) primates followed, probably from Africa in a fashion similar to that of the rodents. Primates capable of migrating had to be small. With little effective competition they also diversified widely, giving rise to the New World monkeys. (Not long after arriving, monkeys apparently most closely related to titis island-hopped to Cuba, Hispaniola and Jamaica.) The South American cavimorph rodents and monkeys are both believed to be clades (i.e., monophyletic).
Tortoises also arrived in South America in the Oligocene. It was long thought that they had come from N. America, but a recent comparative genetic analysis concludes that S. American members of Geochelone are actually most closely related to African hingeback tortoises. Tortoises are aided in oceanic dispersal by their ability to float with their heads up, and to survive up to six months without food or water. S. American tortoises then went on to colonize the West Indies and Galápagos Islands. Skinks of genus Mabuya apparently floated across the Atlantic from Africa during the last 9 Ma.
The earliest mammalian arrivals from North America were carnivorous procyonids that island-hopped from Central America prior to the formation of a land bridge, around 7 Ma ago. Some South American procyonids then diversified into forms now extinct (e.g. the "dog-coati" Cyonasua, which evolved into the bear-like Chapalmalania). However, all extant procyonid genera appear to have originated in North America. It has been suggested that the first S. American procyonids may have contributed to the extinction of sebecid crocodilians by eating their eggs, but this view has not been universally viewed as plausible. The procyonids were followed to S. America by island-hopping sigmodontine rodents, peccaries and hog-nosed skunks.
Similarly, megalonychid and mylodontid ground sloths island-hopped to North America by 9 Ma ago. Megalonychids had colonized the Antilles previously, by the early Miocene, and may have reached N. America by that route, since the oldest N. American fossils are from Florida.
The formation of the Isthmus of Panama led to the last and most conspicuous wave, the great interchange, around 3 Ma ago. This included the immigration of North American ungulates (including llamas, tapirs, deer and horses), proboscids (gomphotheres), carnivorans (including felids like cougars and saber-toothed cats, canids, mustelids, procyonids and bears) and a number of types of rodents into South America.
In general, the initial net migration was symmetrical. Later on, however, the Neotropic species proved far less successful than the Nearctic. This misfortune happened both ways. Northwardly migrating animals often were not able to compete for resources as well as the North American species already occupying the same ecological niches; even when they became established, they usually did not diversify much. Southwardly migrating Nearctic species diversified more, and are thought to have caused the extinction of a large proportion of the South American fauna. (There were no extinctions in N. America obviously attributable to S. American immigrants.) Although terror birds were initially able to invade N. America, this success was temporary; all of the large Neotropic avian and marsupial predators ultimately disappeared. South America's native ungulates also fared very poorly, with only several of the largest forms, Macrauchenia and a few toxodontids, withstanding the northern onslaught. (Among the notoungulates, the mesotheriids and hegetotheriids did manage to survive into the Pleistocene.) Its small marsupials fared better, while the primitive-looking xenarthrans proved to be surprisingly competitive. The African immigrants, the cavimorph rodents and platyrrhine monkeys, generally held their own during the interchange, although the largest rodents (e.g. the dinomyids) seem to have disappeared. With the exception of the North American porcupine and several extinct porcupines and capybaras, however, they did not migrate past Central America.
The presence of armadillos, opossums, and porcupines in the United States today is explained by the Great American Interchange. Opossums and porcupines were among most successful northward migrants, reaching as far as Canada. While only one example each of xenarthrans, marsupials and cavimorph rodents currently lives in North America, 32 species from these taxa are present in tropical Central America. Prior to the end-Pleistocene extinctions, most major groups of xenarthrans were established in Central or North America (as a result of at least seven successful invasions of the latter, and at least six more of only the former). Among the megafauna, ground sloths were notably successful invaders of North America; Megalonyx spread as far north as the Yukon.
Generally speaking, however, the dispersal and subsequent explosive adaptive radiation of sigmodontine rodents throughout South America was much more successful (both spatially and by number of species) than any northward migration of S. American mammals. Other examples of N. American mammal groups that diversified conspicuously in S. America include canids and cervids, both of which currently have Great American Interchange #Canid and cervid species/genera listings by continent. Although Canis currently ranges only as far south as Panama, S. America still has more extant canid genera than any other continent.
The eventual triumph of the Nearctic migrants was ultimately based on geography, which played into the hands of the northern invaders in two crucial respects. The first was a matter of climate. Obviously, any species that reached Panama from either direction had to be able to tolerate moist tropical conditions. Those migrating southward would then be able to occupy much of South America without encountering climates that were markedly different. However, northward migrants would begin to encounter drier and/or cooler conditions as soon as they reached the doorstep of North America. The challenge this climatic asymmetry (see map) presented was particularly acute for Neotropic species specialized for tropical rainforest environments, who had little prospect of penetrating beyond Central America.
The second and more important advantage geography gave to the northerners is related to the land area available for their ancestors to evolve in. During the Cenozoic, North America was periodically connected to Eurasia via Beringia, allowing multiple migrations back and forth to unite the faunas of the two continents. Eurasia was connected in turn to Africa, which contributed further to the species that made their way to North America. South America, on the other hand, was connected to Antarctica and Australia, two much smaller continents, only in the earliest part of the Cenozoic, and this land connection does not seem to have carried much traffic (apparently no mammals other than marsupials and perhaps a few monotremes ever migrated by this route). Effectively, this means that northern hemisphere species arose over a land area roughly six times larger than was available to S. American species. This calculation may not be entirely fair, in that migrations between continents would have been more difficult and less frequent than migrations within S. America. Nevertheless, it is clear that N. American species were products of a larger and more competitive arena, where evolution would have proceeded more rapidly. They tended to be more efficient and brainier, generally able to outrun and outwit their S. American counterparts. These advantages can be clearly seen in the cases of ungulates and their predators, where S. American forms were replaced wholesale by the invaders.
Against this backdrop, the ability of S. America's xenarthrans to compete effectively against the northerners represents a special case. The explanation for the xenarthrans' success lies in their idiosyncratic approach to defending against predation, based on possession of body armor and/or formidable claws. The xenarthrans did not need to be fleet-footed or quick-witted to survive. Such a strategy may have been forced on them by their low metabolic rate (the lowest among the therians). Their low metabolic rate may in turn have been advantageous in allowing them to specialize on less abundant and/or less nutritious food sources.
At the end of the Pleistocene epoch, about 12,000 years ago, three dramatic developments occurred in the Americas at roughly the same time (geologically speaking). Paleoindians invaded and occupied the New World, the last glacial period came to an end, and a large fraction of the megafauna of both North and South America went extinct. This wave of extinctions swept off the face of the Earth many of the successful participants of the Great American Interchange, as well as other species that had not migrated. All the pampatheres, glyptodonts, ground sloths, equids, proboscids, dire wolves, lions and Smilodon species of both continents disappeared. The last of the South and Central American notoungulates and litopterns died out, as well as North America's giant beavers, dholes, native cheetahs, scimitar cats, and many of its antilocaprid, tayassuid, cervid and bovid ungulates. Some groups survived in their adopted homes but disappeared over most or all of their original range, e.g. South American tapirs, camelids and tremarctine bears (cougars and jaguars may have been temporarily reduced to S. American ranges also). Others, such as capybaras, survived in their original range but died out in areas they had migrated to.
The near-simultaneity of the megafaunal extinctions with the glacial retreat and the peopling of the Americas has led to proposals that both climate change and human hunting played a role. However, a number of considerations suggest that human activities were pivotal. The extinctions did not occur selectively in the climatic zones that would have been most affected by the warming trend, and there is no plausible general climate-based megafauna-killing mechanism that could explain the continent-wide extinctions. The climate change took place worldwide, but had little effect on the megafauna in areas like Africa and South Asia, where megafaunal species had coevolved with humans. Numerous very similar glacial retreats had occurred previously within the ice age of the last several Ma without ever producing
Extant or extinct (†) Central American taxa whose ancestors migrated out of South America during the last 10 Ma:
Extant or extinct (†) South American taxa whose ancestors migrated out of North or Central America during the last 10 Ma: