The migrants

Most people know of at least two reasons that organisms can evolve.  The first, of course, is natural selection; members of the same species with inheritable differences can have different survival rates or reproductive rates, leading to overall shifts in the genetic makeup of the population.  The second is catastrophe; a major external event, such as the eruption of the Siberian Traps or the collision of the Chicxulub Meteorite, can completely destabilize what had been a thriving ecosystem, and cause the selective pressures to go off in a completely different direction.  (The two I mentioned were the dominant factors in the Permian-Triassic and Cretaceous-Tertiary extinctions, respectively.)

Less well-known is the role that plate tectonics can play.  When two land masses split apart, the organisms then go their separate ways evolutionarily, especially once the two pieces drift far enough away from each other to experience significantly different climates.  This is what happened to Australia, which most recently was connected to Antarctica; once they diverged, Australia moved northward and Antarctica southward, resulting in just about everything in Antarctica becoming extinct as the temperatures dropped, and leaving Australia with its unique assemblage of species.

The opposite can happen when two continents run into each other.  This occurred when India separated from Africa and collided with Asia, about fifty million years ago, carrying with it species from the southern supercontinent (Gondwana) and introducing them to the northern one (Laurasia).  But an even more striking example occurred when North and South America got close enough that a bit of the seafloor was pushed above water, creating the Isthmus of Panama.

When this happened, on the order of three million years ago, it opened up an easy avenue of two-way migration between the two continents.  This reconnected land masses that had been separated since the breakup of Pangaea in the early Triassic Period, on the order of two hundred million years ago.  That's a long time for species assemblages to evolve in their own directions, and the result was two entirely different floras and faunas.  Those began to move back and forth across the gap as soon as the isthmus formed.

What is curious -- and still largely unexplained -- is why the survival rates of the northward and southward migrants were so drastically different.  Species went both directions; that much is clear from the fossil record.  But just looking at mammals, South America gained (and still has) various species of cats, wolves, foxes, peccaries, deer, skunks, bears, and mice that it gained from North America, to name only a few of the groups that moved in and thrived.  But going the other direction?

There were only three survivors.  The opossum, the armadillo, and the porcupine are the only mammalian South American imports we still have around today.  Others that attempted the northward trek, including ground sloths, glyptodonts, "terror birds," sparassodonts, notungulates, and litopterns, struggled along for a while but eventually became extinct.

[Image is in the Public Domain]

The surmise is that moving from wet forests where it's warm year-round into cooler, drier temperate deciduous forests or grasslands is harder than the reverse, just from the perspective of resources.  Whatever the reason, though, it altered the ecosystems of South America forever, as the North American species proved to be better competitors (and predators), driving entire families of South American mammals extinct.  Some groups continued to thrive and diversify, of course.  Hummingbirds come to mind; they're a distinctly South American group. increasing in diversity as you head south.  Where I live, there's a grand total of one species of hummingbird (the Ruby-throated Hummingbird).

The little country of Ecuador has 132.

The reason all this comes up is the discovery of the complete skeleton of an extinct species of porcupine in Florida, dating to 2.5 million years ago -- and therefore, one of those early migrants northward from its ancestral homeland.  It's related to the modern North American species, but definitely not the same; the extinct species, for example, had a prehensile tail, similar to modern South American species (and which our North American porcupines lack).  It's still unknown, however, if the Florida species is ancestral to our current North American porcupines, or if they're cousins; further study of the skeleton may help to resolve that question.

It's fascinating, though, to see the fingerprints of this mass migration that was to change so radically two different continents.  The process of plate movement continues; Australia will eventually collide with Asia, for example, with similar results, mixing together two sets of species that have been isolated for millions of years.  Change is inevitable in the natural world; it can happen quickly or slowly, and sometimes occurs in ways we're just beginning to understand.

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