If anyone else is surprised by that claim, turns out it's not quite true (in a blurry way):
[The variety between eye types] has led to the dogma that eyes have evolved in all animal phyla 40 to 60 times independently (Salvini-Plawen and Mayr 1961). However, recent genetic experiments cast serious doubts on this notion and argue strongly in favor of a monophyletic origin of the various eye types followed by divergent, parallel, and convergent evolution.
Even at a wider structural level, there is enough common ancestry to put a serious damper on the idea that it evolved independently 20 times. It's more like a single origin for major photosensitive proteins, and one origin per major eye type.
That's not to say it never happens, notably, there's a behavioral trait in ants (seed dispersal) that appears to have evolved independently at least 100 times. Sometimes everything is set up to favor convergent evolution with dozens or more independent occurrences, but that probably wasn't the case with the eye.
Just realized that the eye is a counterexample to their assumption. Took three billion years to evolve but is easy to do so as we've seen it evolve independently many times.
On the other hand, some physical features have evolved multiple times independently. The eye is one key example. IMO this is a strong indication that creatures that evolved in places other than Earth probably also have something we'd recognize as eyes.
There was this notion that the human eye, in its astounding complexity, couldn't have evolved gradually, but had to have been created all at once, called "irreducible complexity".
Eyes (complex, image forming ones) are also believed to have evolved independently. Nature got it more right with the squids -- their eyes got wired so they lack a blind spot! Actually there are many cases: http://en.wikipedia.org/wiki/Convergent_evolution
Reminds me how they eye was seen as counter argument against evolution because its complexity couldn't have developed naturally. But today we pretty much know how it evolved from a photosensitive spot to the complex mechanism we see (huh!) today.
Speaking of the evolution of the eye, recently I began reading the book The Vital Question (Nick Lane), and incidentally, last night I closed my reading on a discussion on that exact topic.
Allow me to quote [topic: 'The missing steps to complexity', page-45]:
"In The Origin of Species Darwin made the point that that natural selection actually predicts that intermediates should be lost. In that context, it is not terribly surprising that there are no surviving intermediates between bacteria and eukaryotes [e.g. plants, humans]. What is more surprising, though, is that the same traits do not keep on
arising, time and time again -- like eyes.
We do not see the historical steps in the evolution of eyes, but we do see an ecological spectrum. From a rudimentary light-sensitive spot on some early worm-like creature, eyes have arisen independently on scores of occasions. That is exactly what natural selection predicts. Each small step offers a small advantage in one particular environment, with
the precise advantage depending on the precise environment.
Morphologically distinct types of eye evolve in different environments, as divergent as the compound eyes of flies and mirror eyes of scallops,or as convergent as the camera eyes that are so similar in humans and octopuses. Every conceivable intermediate from pinholes to accommodating
lenses, is found in one species or another. We even see miniature eyes, replete with a 'lens' and a 'retina', in some single-celled protists [e.g. amoeba]."
Amazing! This will also refute some creationists and intelligent design arguments that point to the eyeball's complexity as something evolutionists could not well fit into their theories of explain. Now it seems the first step could be made towards his the eyeball may have evolved.
Eyes evolved independently in six different phyla. Compound, lensed, eyes are even found in brainless box-jellyfish, which helps them negotiate the mangrove swamps where they live.
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