A bridge between worlds
For millions of years, North and South America were separate continents, allowing unique animal populations to evolve in isolation. South America was a land of strange marsupial predators, giant ground sloths, and car-sized armadillo relatives called glyptodonts. North America had the ancestors of modern bears, cats, dogs, camels, and horses. This separation ended when the Isthmus of Panama fully formed around 2.8 million years ago.
The formation was a slow, geological process. Beginning around 15 million years ago, volcanic activity between the Pacific and Caribbean tectonic plates created a series of islands. Over millennia, sediments washed from the continents and filled the gaps between these islands, creating a continuous strip of land, the Darién isthmus being the final link. This new land bridge connected two continents; it severed the connection between the Atlantic and Pacific oceans. This event rerouted ocean currents, leading to the formation of the Gulf Stream and altering global climate patterns.
The great migration begins
With the land bridge in place, animals began to move in both directions in an event called the Great American Biotic Interchange. From the north came waves of invaders. Gomphotheres (four-tusked elephant relatives), saber-toothed cats (Smilodon), deer, camels, horses, and bears all pushed south. These newcomers were highly successful. Today, South American animals like llamas, guanacos, and jaguars are all descendants of these northern migrants.
The southern migration was less successful but included some spectacular animals. Giant ground sloths like Eremotherium, some the size of elephants, lumbered north. Armadillos, porcupines, and opossums also migrated, and their descendants still exist in North America today. Perhaps the most formidable southern migrant was Titanis walleri, a flightless "terror bird" that stood over 2 meters tall and became a predator in what is now Texas and Florida.
The interchange was lopsided. The North American mammals proved to be better competitors and colonizers. Many of South America's unique native species, including most of its marsupial predators and strange hoofed mammals (notoungulates), went extinct. Today, roughly half of South America's mammal genera are descended from North American ancestors, while only about 10% of North American mammals came from the south.