About three weeks ago I posted something by Carl Zimmer on the evolution of the eye here, but then forgot to post the second part. Here it is:
In my last post, I went back in time, from the well-adapted eyes we are born with, to the ancient photoreceptors used by microbes billions of years ago. Now I’m going to reverse direction, moving forward through time, from animals that had fully functioning eyes to their descendants, which today can’t see a thing.
This may seem like a ridiculous mismatch to my previous post. We start out with the rise of eyes, a complex story with all sorts of twists and turns, with gene stealing, gene borrowing, gene copying; and then we turn to a simple tale of loss, of degeneration, of a few genes mutating the wrong way and–poof!–billions of years of evolution undone.
In fact, loss is never such a simple matter. I can illustrate this fact with two disparate beasts: fleas and cavefish.