A few years ago I read a wonderful book by Marc W Kirschner and John C Gerhart called The Plausibility of Life. One point the authors make is about the Cambrian Explosion, when all the animal phyla evolved. The reason no new animal phyla have come about since then, is that animals in an incipient new phylum would just get eaten by much more specialized animals and other organisms that are already around.
In Stephen J Gould’s Wonderful Life, Gould makes the point that rather than being shaped like trees, the evolving animal phyla stop branching after a while and start to taper, as more successful species and genera thrive and other, less successful ones go extinct.
Now I’ve come across an amazing quotation from Charles Darwin about the evolution of organic molecules, though he knew nothing of DNA:
“It is often said that all the conditions for the first production of a living organism are now present, which could ever have been present. But if we could conceive in some warm little pond… that a …compound was chemically formed ready to undergo still more complex changes, at the present…such matter would be instantly devoured…which would not have been the case before living creatures had formed.”
So there was Darwin, at the threshold of figuring out the mechanism of biological evolution, already figuring out that once evolution has reached a certain point, it can’t go back again. I am gradually realizing what genius it took to come up with Darwin’s understanding of evolution.