|MadSci Network: Evolution|
Jen: The eye is still a controversial subject among creationists and others who do not believe evolution, but it is not a hard structure to explain. The central idea is that if all creatures were originally blind, 1% of an eye is better than nothing and will be preferentially selected. The best idea of this apparent evolutionary change (a flat area of skin or epithelium with light sensors, then folding inward to form first an open cavity and later a closed cavity, first without a lens and then with a lens) is illustrated in many evolution textbooks and popular books; checking the index of library books on evolution might work. A technical paper (hard to read, but with a good illustration) on this very subject appeared only 5 years ago: A pessimistic estimat of the time required for aneye to evolve, by Nilsson adn Pelger (1994), in Proceedings of the Royal Society of London, series B, p. 256. (This is found only in the libraries of the largest colleges or universities) What is really neat is that we can find invertebrate animals like scallops, worms, etc. that have eyes with each of these "intermediate" conditions (i.e., open cavity with no lens, etc.). It is also neat that while our eye and the octopus eye look very similar, our retina is inside out (the rods and cones on the outside of the retina, *below* the blood vessels and the neuronal wiring) while the octopus has a retina with the rods and cones on the inside of the retina. (This is why we can see moving blood cells when an ophthamologist shines a bright light into your eye.) It is clear that these different eyes evolved separately. A clue to the origin of eyes is also found in the protein structure of our lens and those of other animals. Since these proteins, called crystallins, are simply globular proteins that don't stick to each other (the lens is not a crystal, but merely a fluid of nonattracting hard spheres, the proteins themselves) it turns out that many types of proteins, originally used for different metabolic functions in ordinary cells, were co-opted for use in the eye. Crocodiles and some birds use a lactate dehydrogenase enzyme in their lens (this enzyme removes a hydrogen from lactic acid, which you eat in yoghurt and saurkraut), most birds and reptiles use argininosuccinate lyase, and we and many other vertebrates use a heat shock protein, a helper molecule called a chaperone that repairs other proteins if they get bent out of shape (inproperly folded) after being exposed to excess heat. It is amazing that so many types of metabolic enzymes have been used in the eye in this structural role of focusing light, rather than a unique protein being created just for this purpose. By the way, I have no trouble believing in a supernatural creator who used evolution. Cheers, Dean Jacobson
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