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The Great American Exchange
Michael Ellis looks at one California critter among the very few that migrated north in the Great American Exchange.

By Michael Ellis

For millions of years, North and South America existed in geographic isolation -- there was no Central America. North America began as part of the supercontinent Laurasia -- that is basically all the land in the northern hemisphere. And South America began as part of another huge continent, which included Australia and Antarctica. In these separate regions plants and animals evolved in "splendid isolation" as Alfred Wallace first described it in the 1800s.

But beginning around three million years ago, volcanic activity created the Isthmus of Panama. The land that arose "suddenly" connected these two huge landmasses and caused one of the most remarkable events in the history of our planet. Abruptly there was a gigantic swap of fauna from one region to another, today known by biologists as the Great American Exchange. Camels, native to North America, headed south and evolved into llamas and vicunas. Deer, tapirs, cougars, skunks and foxes also went south into new territory and radiated out into a large number of species. Overall the exchange between north and south was quite uneven. The North American species basically out-competed, dominated and caused the extinction of many of South America's animals.

On the contrary, only a few species from South America succeeded in conquering the Northern Hemisphere. The modern descendants of these southern mammals are armadillos, porcupines and opossums. There are no armadillos here but porcupines are a native California critter. The opossums we see in the Bay Area were allegedly introduced into San Jose in 1910. Goodness knows why -- perhaps Granny was missing some of that possum stew she fondly remembered from back home in 'Kentuckee'. At any rate these marsupial mammals have landed fully in California and thriving in our suburbs.

So last week when I passed several flattened on the road, I was reminded of those Grateful Dead lyrics -- "what a long, strange trip it's been" at least for those possums.

This is Michael Ellis with a Perspective.

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