Following their discovery by Europeans in 1492, North America and South America have been collectively known as the Americas or the New World, two immense landmasses that had been close geographical neighbors for time immemorial. However, the geological histories of North and South America are very different, and for huge expanses of time, there has been no physical link between them whatsoever. All of the landmasses on earth were once assembled in a superlandmass, Pangea. Over millions of years, Pangea fragmented, and all of the continents in the modern Southern Hemisphere were grouped as a southern supercontinent, Gondwana-land, while the continents of the Northern Hemisphere formed the northern supercontinent, Laurasia. Over millions of years, these supercontinents were wrenched apart by the colossal forces of plate tectonics into the landmasses we are familiar with today, and they were rafted over the viscous rock of Great American Interchange—The emergence of a land the earth's mantle to more or less their bridge between North and South America allowed ani-current positions. Although South mals to migrate between these two landmasses. Several America faced North America across types of North American mammal moved into South the equator, there was no physical con- America, but relatively few of the South American mam-nection between the two landmasses. mals made it to the north and thrived. (Phil Miller)
North America retained a connection to the other landmasses by way of the intermittent land bridges that formed between its northwestern corner and the eastern tip of Asia. South America, on the other hand, has been completely isolated during its history for immense stretches of time.
The animal inhabitants of South America evolved in isolation to form a fauna that was amazing and unique. The mammals were particularly interesting, and many groups were known only from South America. Although South America was isolated from the other landmasses, some animals managed to set up home there by inadvertently rafting across the then narrow Atlantic Ocean from Africa on floating mats of vegetation. This is how rodents and monkeys are thought to have reached South America between 25 and 31 million years ago. Much later, at around 7 million years ago, some representatives of the group of mammals that includes raccoons and coatis managed to reach South America from North America using stepping stones of islands that were appearing between the two landmasses. These islands were the highest reaches of modern-day Central America, which was being uplifted from below the waves.
The isolation of South America and the uniqueness of its fauna was upset completely about 3 million years ago when the gradual geological upheaval forced the Isthmus of Panama out of the ocean completely, directly connecting the two landmasses. This was the beginning of the Great American Interchange and over the next few thousand years, animals and plants used the corridor of dry land to move between North America and South America. Many species of mammal we associate with South America actually originated in North America, for example, the llamas and tapirs. Other migrants from the north included horses; cats such as the cougar and jaguar; dogs; bears; and several types of rodent, to name but a few animals. Some South American mammals managed to cross the land bridge into North America, but many of these are now extinct, including the glyptodonts and giant ground sloths. The only surviving North American mammals to have their origins in South America are the Virginia opossum, the nine-banded armadillo, and the North American porcupine.
For reasons that are not completely understood, the South American species did not fare well when it came to invading the north, while the North American species thrived in the South American lands. The only ancient South American animals to make any lasting impression in North America were the ones with some sort of protection. The extinct glyptodonts, like the armadillos, were protected with a tough carapace, while the ground sloths had powerful claws, thick skin, and great size on their side. Apart from mammals, one other group of South American animals, the terror birds, managed to survive in North America for a while, but it is possible that they crossed by island hopping before the two landmasses became connected by a corridor of land.
The animals that moved into South America from the north thrived, and most of them are still around today, even though this continent has been massively altered by humans. All of the South American cats, bears, and dogs have their origins in North America, but they all adapted to the varied habitats offered by this continent and may have even played a role in driving some of the South American native mammals to extinction. The giant, native animals that were unique to this continent are all extinct, and all that we have as reminders of their existence are dry bones and a few pieces of parched hide. Although the original South American giants are all gone, their smaller relatives live on. Today, more than 80 species of marsupial survive in South America, but they are mostly tree-dwelling animals with a liking for insects and fruit. The relatives of the giant ground sloths live on in the trees as the five species of forest sloth, famous for their sluggish behavior. The anteaters, strikingly different to all other mammals, are not unique to South America, but it is here they reach their greatest size in the shape of the giant anteater. Superficially similar to the glyptodonts, the armadillos live on as 20 living species, but they are distantly related to the armored giants of the Pleistocene, which grew to the size of a small car.
Many hundreds of thousands of years after the Great American Interchange reached its peak, humans moved into the Americas via the Bering land bridge, although there is increasing evidence that early seafarers may have reached these lands a long time before people walked across. Regardless of how humans got to North America, they also moved south into South America. Early crossings may have been made using boats, but the land bridge used by the animals of the New World for millennia was certainly used by humans as well.
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