I
have recently been reading essays by Loren Eiseley, a paleontologist of the
middle twentieth century. His books, such as The Firmament of Time and The
Immense Journey, were considered epics of popular science writing at the
time they were published. Many of his essays are very well-written, and I
recommend him to any of you who like classic nature writing.
One
of his essays, The Slit, is about a time in which he went down into a
cavern and dug out a mammal skull from shortly after the extinction of the dinosaurs.
Primates, the order to which humans and other apes belong, started to diversify
right after the dinosaurs became extinct. One of the directions that they
diversified was into arboreal forms, living in the trees. And that is where
most of them still are, except for baboons and people. Another direction was to
live by burrowing in the ground out in the newly-spreading grasslands. These
primates eventually became extinct. Why? It was probably because of another
order of mammals that was evolving and diversifying at the same time: rodents,
which includes prairie dogs and ground squirrels. [Here is a photo I took of a prairie dog in the Badlands of South Dakota in 2000.]
Very
early in the Cenozoic Era, it was not clear which direction mammalian evolution
might take. Out in open prairies, both primates and rodents diversified. But
the rodents were more successful, driving the burrowing primates to extinction.
Had things ended up just slightly differently, primates might have been
primarily burrowing animals, rather than leaping around in trees. And humans
might not have evolved.
Another
insight I got from reading Eiseley was a possible contributing factor to human
intelligence. Our big brains require a lot of oxygen, therefore a lot of blood
vessels. Why did our lineage, starting with the amphibians, have such
well-oxygenated brains? The answer might be that amphibians evolved not just in
shallow water, not just in shallow fresh water, but in shallow fresh anoxic
water, so that extra blood vessels were required to keep even their small
brains alive. It was this extra vascularization that made the evolution of
intelligent terrestrial vertebrates possible.
To
me, this was an excellent example of how the slightest change in the course of
events can make a huge difference in the evolutionary outcome.
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