Charles Darwin eloquently expressed his own struggle with imagining how the vertebrate eye could have evolved. If you spend any time reading the writings of creationists or proponents of intelligent design (not recommended unless you have an extraordinary tolerance for pretzel logic), you'll find a quote from The Origin of Species:
To suppose that the eye with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest degree.
This quote causeth much crowing and fist-bumping amongst the holy, lo unto this very day, usually followed by something like "Even Darwin admitted that evolution by natural selection doesn't work."
It's wryly amusing, given the degree to which anti-evolutionists cherry-pick the scientific evidence they accept and the (much larger amount of) evidence they ignore completely, that this quote is itself cherry-picked, as you'd find out if you went on to read the next two sentences of the book:
When it was first said that the sun stood still and the world turned round, the common sense of mankind declared the doctrine false; but the old saying of Vox populi, vox Dei, as every philosopher knows, cannot be trusted in science. Reason tells me, that if numerous gradations from a simple and imperfect eye to one complex and perfect can be shown to exist, each grade being useful to its possessor, as is certainly the case; if further, the eye ever varies and the variations be inherited, as is likewise certainly the case; and if such variations should be useful to any animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, should not be considered as subversive of the theory.
So the argument -- if I can dignify it by that name -- of the anti-evolutionists boils down to our old friend Argument from Incredulity: "I can't imagine how it could have happened, therefore it must be God."
The truth is, we understand the evolution of the eye pretty well. Lots of animals (for example, flatworms) have light-sensitive spots; and as Richard Dawkins brilliantly explains in his tour-de-force defense of evolution The Blind Watchmaker, once you have any kind of light-sensing ability at all, incremental improvements can result in some amazingly complex structures. The eye isn't "irreducibly complex" -- the intelligent design cadre's favorite phrase -- at all; simple photosensitive spots led to "cup eyes" which led to eyes like a pinhole camera, and so on. In fact, the whole process has been repeated more than once. Complex eyes have evolved independently at least three times, possibly more.
The vertebrate eye is a particularly interesting case. The transparent proteins in the lens, appropriately named crystallins, were found in 1988 by molecular biologist Joram Piatigorsky to come from the same genes that produce heat-shock proteins, enzymes that protect other proteins against damage from fluctuating temperature. Take heat-shock proteins and assemble them in layers, you get a lens. This is an example of exaptation (also called preaptation or preadaptation), where a gene, protein, or structure that evolved in one context develops a function giving it an entirely different use, and that use kind of moves in and takes over.
It's another example of exaptation in the eye that is why the whole topic comes up; in fact, it's not only exaptation, it's exaptation of a gene that was borrowed from another organism entirely. A paper this week in Proceedings of the National Academy of Sciences looked at a protein in all vertebrate eyes called IRBP (interphotoreceptor retinoid-binding protein), without which our sense of sight wouldn't work. When light strikes your eye, protein-bound complexes containing retinol (a derivative of vitamin A) absorb the energy, causing them to kink. This triggers a neuron to fire, sending a signal to your brain. However, something needs to unkink the complex, thus resetting the switch so it can respond to the next photon to come along.
That's what IRBP does. Without it, your retinal cells would be able to respond exactly once, then they'd shut down permanently.
This week's paper found something astonishing. The gene that codes for IRBP doesn't exist in our nearest invertebrate relatives, nor in any other group studied, with one exception -- certain species of bacteria. What apparently happened is that the common ancestor of all vertebrates swiped a gene from bacteria that coded for a pepsidase -- an enzyme that breaks down and recycles proteins. This kind of gene-stealing isn't uncommon. (I did a post a few years ago about a pair of viral genes that seem to be critical for our forming memories, if you want another good example of this phenomenon.) But like the heat-shock proteins becoming crystallins, the pepsidase made by the gene our ancestors grabbed was useful for something else -- unkinking the protein complexes in our rapidly-evolving eyes.
So our eyes work not only because of proteins gaining additional functions, but because we stole a gene from bacteria.
"Horizontal gene transfer can help to endow organisms with new functions," said Julie Dunning Hotopp, of the University of Maryland School of Medicine’s Institute for Genome Sciences. "Once these genes take root in a new species, evolution can tinker with them to produce totally new abilities or enhance existing ones. It is the biological equivalent of upcycling that happens in my Buy Nothing Group."****************************************
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