The Worst Animal Invasion You’ve Never Heard Of

By: Greg Schmalzel

Twelve thousand years ago, South America stood on the brink of collapse. Megafauna like the giant ground sloth and toxodon were vanishing under the invasion of Native American hunters. But millions of years earlier, an equally cataclysmic upheaval played out. And humans weren’t to blame for this one. When North and South America collided 2.8 million years ago, a tsunami of foxes, big cats, and even bigger bears  stormed across a fresh corridor.

Imagine a land bridge rising from the sea. Two isolated worlds colliding in a single, earth‑shaking instant. Two evolutionary crucibles, each brimming with creatures honed in isolation, suddenly bled together. The beasts of North America meet the beasts of South America. A fight for survival ensues. Ecosystems buckle, species vanish, and only the fittest endure.

This prehistoric clash is known as the Great American Biotic Interchange, and it completely reshaped animal kingdoms. It also set the stage for the first human pioneers, who stepped into a world of giants.

This is the hidden story of one of our planet's most deadly continental collisions.

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The Collision

K. Cantner, AGI.

For over 60 million years, North and South America were isolated worlds—two evolutionary laboratories divided by ocean. In the south, giant ground sloths, armored glyptodonts, and flightless terror birds ruled. In the north, horses, saber-toothed cats, and bears evolved in parallel.

But deep beneath the waves, tectonic forces were slowly setting the stage for collision. As the Caribbean and Farallon plates shifted, they forged a chain of volcanic islands—the Central American Arc. Around 15 million years ago, this arc slammed into South America, closing the seaway and forming what would become the Isthmus of Panama.

By 2.8 million years ago, the continents were connected by land. Ocean currents were rerouted, global climates shifted, and a new corridor for life opened up. Plants and birds came first, then amphibians and freshwater fish. Finally, mammals crossed in both directions: camels, horses, and wolves surged south, while sloths, opossums, and armadillos marched north.

This wasn’t a peaceful migration—it was a biological upheaval. Entire ecosystems were reshaped, and countless species vanished in the aftermath. This event, known as the Great American Biotic Interchange, rewrote the evolutionary story of both continents—and set the stage for the world early humans would one day enter.

The Great American Biotic Interchange

Guillermo Torres Carreño

Life surged across the newborn land bridge in waves. First came wind-blown seeds, spores, and pioneering trees, followed by birds claiming new forest canopies. Freshwater fish and amphibians splashed into new rivers. Then the giants moved.

South America sent armored glyptodonts north—massive, dome-shelled “living tanks” with clubbed tails. Giant ground sloths, elephant-sized with enormous claws, browsed high branches and may have rumbled low-frequency calls like elephants. Anteaters followed, snouts and sticky tongues raiding termite mounds, while opossums slipped into forests with their prehensile tails and pouches. Even towering terror birds, with hooked beaks and lethal kicks, briefly terrorized the north before vanishing.

But the north struck back even harder. Horses and camels thundered south, grazing fresh grasslands. Tapirs used flexible snouts to feed in dense forests. Massive gomphotheres, elephant relatives with four tusks, tramped through swamps and savannas.

Predators sealed the fate of many natives. Dire wolves surged south in coordinated packs, bone-crushing jaws targeting megafauna. Saber-toothed cats ambushed prey with deadly precision, their huge fangs slicing throats. Cougars prowled new territories from mountains to jungles. Foxes quietly filled edge habitats, outcompeting native small predators. And the short-faced bear, Arctodus, stomped south like a shaggy juggernaut, using its massive frame and claws to dominate the Pleistocene landscape.

The Great American Biotic Interchange wasn’t a gentle migration—it was a brutal reshuffling of life that forever changed the evolutionary story of two continents.

So What Happened?

At first, migrations between North and South America went both ways evenly. But over time, northern animals proved far more successful. Southern species that moved north often struggled to compete, rarely diversified, and many vanished from the fossil record. Meanwhile, North American migrants flooded south, adapted quickly, and drove mass extinctions of native species.

Why the imbalance? Geography played a big role. Northern animals evolved in larger, more competitive ecosystems with links to Eurasia and Africa, honing traits like grazing teeth, pack-hunting strategies, and strong immune systems. Climate mattered too: many South American species couldn’t handle the cooler, drier habitats north of the Isthmus. And crucially, South America’s own predators were already in decline before the interchange, leaving open niches for northern carnivores like cougars and bears.

Fossil evidence backs this up. Studies show similar numbers of early migrants, but much higher extinction rates among South American natives. Species like Sparassodonta, a line of carnivorous marsupials, went extinct before the interchange even peaked. Others, like Chapalmalania, a giant raccoon relative, vanished shortly after northern predators arrived.

By the time Paleoindians arrived, the Americas were already hybrid ecosystems shaped by millions of years of invasions and extinctions. Humans entered a world where saber-toothed cats prowled alongside glyptodonts and sloths grazed beside imported horses. This wasn’t untouched wilderness, but a landscape layered with disruptions—just as human hunters would soon add their own.

Sources: 

[1] Marshal, Ll et al. 1982. “Mammalian Evolution and the Great American Interchange.” Science 215,1351-1357.

[2] Webb, D. 1991. “Ecogeography and the Great American Interchange.” Paleobiology 17(3):266-280.

[3] ​​Domingo, L., et al. 2020. “The Great American Biotic Interchange revisited: a new perspective from the stable isotope record of Argentine Pampas fossil mammals.” Sci Rep 10, 1608.

[4] Webb, D. 2006. “THE GREAT AMERICAN BIOTIC INTERCHANGE: PATTERNS AND PROCESSES,” Annals of the Missouri Botanical Garden 93(2):245-257

[5] Webb, D. 1976. “Mammalian faunal dynamics of the great American interchange.” Paleobiology 2(3):220-234.

[6] Carrillo, J., et al. 2020. “Disproportionate extinction of South American mammals drove the asymmetry of the Great American Biotic Interchange.” Proc. Natl. Acad. Sci. U.S.A. 117(42):26281-26287.

[7] Palmer, D. (1999). The Marshall Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals. London: Marshall Editions.