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Parallel Evolution

November 3, 2012

I’ve read most of Steven Jay Gould’s books.  One of his stories about human evolution mentioned that in Miocene time there were five or six species of human living in Africa.  Another, about evolution of the horse, said that also in Miocene time there were five or six species of horse living in North America.  The Miocine was roughly ten million years ago.  This parallel between humans and horses struck me as a curious thing.

So, what is a species?  They have to be sufficiently different from other similar types of animals.  This means that they don’t interbreed.  Generally, they can interbreed but don’t do so in their initial stages of separation.  They can be separated geographically.  They can occupy different ecological niches in same location.  They even can be physically distinctive so they are not attractive to one another.

Humans continued to evolve after the Miocene, but the number of species declined.  There were several radiations from Africa to Europe and Asia.  All of these eventually became extinct except for one species, our own species.  Horses also continued to evolve.  The number of species also declined.  There were several radiations too, in this case from North America to Europe and Asia.  All of the earlier horse species also became extinct.  In fact, the modern horse even became extinct in North America but survived in Europe and Asia.  It was only reintroduced to North America a few hundred years ago.

Do humans and horses have something in common?  In terms of the number of species, both could be considered unsuccessful.  Both could easily have become extinct completely.  It’s only by chance that we are here, and that horses are here.

This curious parallel raises a question in my mind.  Was there something special about the Miocene that caused the proliferation of species?  I have no idea what the answer might be.

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