MadSci Network: Evolution

Re: How do evolutionists explain how the eyeball evolved?

Date: Wed Sep 8 14:29:34 1999
Posted By: Dean Jacobson, Faculty Biology, Whitworth College
Area of science: Evolution
ID: 934919750.Ev

	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 

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.

Dean Jacobson

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