North and South America have been known collectively as the Americas or the New World since their discovery by Europeans in 1492, despite the fact that they had been close geographical neighbors for millennia. However, the geological histories of North and South America are very different, and there has been no physical link between them for vast stretches of time. Pangea was a super landmass that once included all of the landmasses on the planet. Pangea fragmented over millions of years, and all of the continents in the modern Southern Hemisphere formed the southern supercontinent, Gondwanaland, while the continents of the Northern Hemisphere formed the northern supercontinent, Laurasia. These supercontinents were ripped apart over millions of years by the colossal forces of plate tectonics into the landmasses we know today, and they were rafted over the viscous rock of the earth's mantle to roughly their current positions. Despite the fact that South America faced North America across the equator, there was no physical connection between the two continents.
The Great American Exchange— The formation of a land bridge connecting North and South America allowed animals to migrate between these two continents. Several North American mammal species moved into South America, but only a few South American mammals survived and thrived in the north.
North America remained connected to the rest of the world via the intermittent land bridges that formed between its northwestern corner and Asia's eastern tip. South America, on the other hand, has spent vast periods of its history completely isolated.
South America's animal inhabitants evolved in isolation to form an amazing and unique fauna. The mammals were especially fascinating, and many groups were only known from South America. Although South America was isolated from the other landmasses, some animals found their way there by inadvertently rafting across the then-narrow Atlantic Ocean on
floating mats of vegetation from Africa. It is thought that rodents and monkeys arrived in South America between 25 and 31 million years ago. Many millions of years later, around 7 million years ago, some members of the group of mammals that includes raccoons and coatis were able to reach South America from North America by using stepping stones of islands that were forming between the two landmasses. These islands were the highest points of modern-day Central America, which was rising from beneath the waves.
South America's isolation and the uniqueness of its fauna were completely disrupted about 3 million years ago when gradual geological upheaval forced the Isthmus of Panama completely out of the ocean, directly connecting the two landmasses. This was the start of the Great American Interchange, and for the next few thousand years, animals and plants used the dry land corridor to travel between North and South America. Many of the mammals we associate with South America, such as llamas and tapirs, originated in North America. Other northern migrants included horses, cats such as the cougar and jaguar, dogs, bears, and several types of rodents, to name a few. Some South American mammals made it across the land bridge into North America, but many of them, including glyptodonts and giant ground sloths, are now extinct. The Virginia opossum, the nine-banded armadillo, and the North American porcupine are the only surviving North American mammals that originated in South America.
For unknown reasons, South American species did not fare well when it came to invading the north, whereas North American species thrived in South American lands. The only ancient South American animals that left a lasting impression in North America were those that were protected. Extinct glyptodonts, like armadillos, had a tough carapace, whereas ground sloths had powerful claws, thick skin, and a large size on their side. Aside from mammals, one other group of South American animals, terror birds, survived in North America for a time, but it is possible that they crossed by island hopping before the two landmasses became connected by a land corridor.
The animals that migrated to South America from the north thrived, and the majority of them are still alive today, despite the fact that this continent has been drastically altered by humans. All of the South American cats, bears, and dogs originated in North America, but they all adapted to the diverse habitats offered by this continent and may have even contributed to the extinction of some of the South American native mammals. The giant, native animals that were unique to this continent are all extinct, and we only have dry bones and a few pieces of parched hide to remind us of their existence. Although the original South American giants have all died out, their smaller relatives continue to exist. More than 80 marsupial species survive in South America today, but they are mostly tree-dwelling animals that eat insects and fruit. The five species of forest sloths, famous for their sluggish behavior, are relatives of the giant ground sloths. Anteaters, which are strikingly different from all other mammals, are not unique to South America, but it is here that they reach their greatest size in the form of the giant anteater. The armadillos are closely related to the glyptodonts and have 20 living species, but they are not related to the armoured giants of the Pleistocene, which grew to the size of a small car.
Humans moved into the Americas via the Bering land bridge many hundreds of thousands of years after the Great American Interchange peaked, though there is growing evidence that early seafarers may have reached these lands long before people walked across. Regardless of how humans arrived in North America, they also made their way south into South America. Early crossings may have used boats, but the land bridge used by New World animals for millennia was almost certainly used by humans as well.
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