Great American Biotic Interchange
Once the Isthmus of Panama joins the continents, land mammals cross both ways. North American predators and grazers spread south, and many South American species die out.
The story
For about 60 million years South America was an island continent, and its mammals evolved in isolation: giant ground sloths, armored glyptodonts, odd hoofed grazers and marsupial predators, along with huge flightless 'terror birds'. When the Isthmus of Panama joined the continents, the barrier fell. The main wave of crossing is dated to about 2.7 million years ago, though some animals had made it across earlier by sea or island hopping. Ground sloths, armadillos, opossums, and porcupines moved north, while cats, dogs, bears, horses, deer, camels, and mastodon-like elephants moved south.
The exchange was uneven. Many native South American hoofed mammals and marsupial predators declined or vanished after the northern animals arrived, and northern groups such as deer, cats and canids spread widely in the south. Biologists debate why: competition, predation, and climate change all played a part, and some extinctions happened more slowly than the old story suggests. Even so, the exchange reshaped the animals of two continents and left a mixed fauna, from jaguars to llamas, whose ancestors were camels that had crossed south. Later, people in the Andes domesticated llamas and alpacas from those southward-moving camelids.
Why it mattered
- The animals of the Americas were reshuffled, producing the mixed wildlife of both continents we know today.
- Many unique South American mammals declined or died out, an early large-scale case of invasion and extinction.
- Camel ancestors that crossed south became the llamas and alpacas that Andean peoples later domesticated.
- The exchange became a textbook example of how the joining of two land masses can change ecosystems.
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