Tens of millions of years ago, a force of nature set two giant masses on an unavoidable collision course that would change the face of the Earth and spell life or death for thousands of species. The force of nature was plate tectonics, and the bodies were North and South America. And even though they were hurdling towards each other at an underwhelming 2.
5 cm per year, their collision actually did have massive biological reprocussions by causing one of the greatest episodes of biological migration in Earth's history: The Great American Biotic Interchange. Our story begins 65 million years ago, the beginning of the age of mammals, when what is now North and South America were continents separated by a marine connection between the Pacific and Atlantic Oceans. During this time, South America was the home of fauna that included armored glyptodonts as large as compact cars, giant ground sloths weighing more than a ton, opossums, monkeys, and carnivorous terror birds.
North America had its own species, such as horses, bears, and saber-toothed cats. Over 20 million years, the shifting of the Farallon and Caribbean Plates produced the Central America Volcanic Arc, a peninsula connected to North America, with only a very narrow seaway separating it from South America. As these plates continued to surf the Earth's magma layer far beneath the Pacific Ocean floor, the Caribbean Plate migrated eastward, and about 15 million years ago, South America finally collided with this Central American Arc.
This gradually closed the water connection between the Pacific and the Caribbean, creating a land bridge, which connected North America to South America. Terrestrial organisms could now cross between the two continents, and from the fossil records, it's evident that different waves of their dispersals took place. Even though plants don't physically move, they are easily dispersed by wind and waves, so they migrated first, along with a few species of birds.
They were followed by some freshwater fishes and amphibians, and finally, various mammals began to traverse the bridge. From South America, mammals like ground sloths and glyptodonts were widly distributed in North America. Moreover, many South American tropical mammals, like monkeys and bats, colonized the forests of Central America, and are very abundant today.
South American predator marsupials went extinct 3 million years ago, at which point North American predators, such as cats, bears and foxes, migrated south and occupied the ecological space left behind. Horses, llamas, tapirs, cougars, saber-toothed cats, gomphotheres, and later humans also headed south across the land bridge. But what happened on land is only half the story.
What had been one giant ocean was now two, creating differences in temperature and salinity for the two bodies of water. The isthmus also became a barrier for many marine organisms, like mollusks, crustaceans, foraminifera, bryozoans, and fish, and separated the populations of many marine species. It also allowed the establishment of the thermohaline circulation, a global water conveyor belt, which transports warm water across the Atlantic, and influences the climate of the East Coast of North America, the West Coast of Europe, and many other areas.
It's a challenge to track all of the ways the collision of the Americas changed the world, but it's safe to say that the ripples of the Great American Biotic Interchange have propagated through the history of life on the planet, and that of mankind. What if these species hadn't gone extinct, or if there were no monkeys in Central America, or jaguars in South America? What if the thermohaline circulation wasn't flowing?
Would the East Coast of North America be much colder? It all goes to show some of the most impactful transformations of our planet aren't the explosive ones that happen in an instant, but the ones that crawl towards irreversible change. We are the product of history.
