Robert Krulwich/NPR

Wondrous prehistory, thanks to Robert Krulwich:

This is the story of two continents doing battle, North America versus South America. It is also a biological mystery.

For a very long time, North America and South America were separate land masses. The Pacific Ocean slipped between them, flowing into the Caribbean. The Isthmus of Panama was there, but it was underwater. The two continents didn’t touch.

As a result animals on both continents, especially mammals, evolved independently. They didn’t, couldn’t, interbreed. And yet, both North and South America had mountains, plains, long lazy rivers, deltas and supported similar forms of mammalian life. In fact, when biologists look back at the fossils, they found almost mirror like populations.

North America had horses. They were a little thicker and hairier back then.

South America had a parallel version called “litopterns” with dangly Snuffleupagus-like noses.

North America had elephants and rhinos (gone now, but they once were natives).

South America had Astrapotheres and pyrotheres, who looked quite similar, being tusk-bearing, water-friendly mammals.

North America had a saber-toothed tiger.

So did South America.

Again, while they looked the same, on close inspection you’d find the northern one carried its fetuses in a uterus, while the southern tiger was marsupial; its fetuses grew in an outside pouch.

“This evolutionary convergence was the greatest on land that the world has ever seen,” writes biologist E.O. Wilson. The two continents had their own versions of shrews, weasels, cats and dogs. And then 2.5 million years ago, the two continents attached.

And Then There Was One …

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