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One of the trickiest tasks for proponents of the Theory of Evolution is to effectively trace the development of specialized structures such as the eye or the wing, and to explain how their intermediary structures served an evolutionary purpose. The problem is this: wings as they are found today are very useful for birds, but how much adaptive sense could they have made in their earlier stages, before they evolved into the perfectly-shaped, well-balanced structures that allow for flight? At some point they must not have worked well, and that would be a liability for the creature because not only would it have had these ungainly appendages that may not work very well, it would have had to spend valuable time and energy actually growing them. What good is growing these appendages if they don't have any significant purpose? What good is half a wing?

Even Darwin found this task to be daunting, remarking: "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." Charles Darwin, 1859

In order to have a viable theory, one must be able to trace an organ's development so that each stage is beneficial and evolutionally adaptive. Here is a possible evolution of the eye, taken from the book Eye and Brain by Richard Gregory, as presented to me in a Cognitive Science-related Visual Perception workshop:

First, let's start off with a water-based single-celled organism, for all intents and purposes the simplest form of life we can consider. Next we must come up with a simple way to register light, and probably the simplest way of doing so is with a light-sensitive molecule. Light-sensitive molecules, which change state when hit by a photon, are very common and it is reasonable to assume sometime during its development, the single-celled organism developed a molecule that just happened to be light-sensitive. Over time, this molecule could be built into an organelle which would probably be placed next to an edge for best light detection. (These organisms aren't that big, so basically anywhere would be near an edge)

 _____
|____o|
To improve directionality, we would place the organelle in a type of pit. This would allow it to restrict the direction of light that it detected, giving it more information about the light's source.
 _______
|    o__|
|____|
Next we could place a few more organelles along the pit, and if we keep track of which one was activated, we have even more idea about where the light emanated from.
 _______
|    o_o|
|___o|  \
         \
          \ -light from a particular direction
Finally, perhaps the organism would develop some sort of covering to the pit to protect it from grains of sand or other intruders. Over time, this covering could grow to deform and bend the light, and here we have the rudimentary basis for a lens, from which we can see how easily this can lead to an actual eye.
 _______
|    o_o|
|___o|__)
Whether or not this is actually the way eye developed, it is a fairly viable theory, and if nothing else, illustrates that complex structures can be built from scratch with every step of the way making good evolutionary sense.
Notes: Sorry for the horrible ASCII Art. By the way, cells most likely are roundish, and do not have 90 degree angles. Also, somewhere along the way we went from single-celled to multi-celled organisms, but the leap isn't that important. :) If anybody has a better way of drawing cells, /msg me!

Thanks to BluePlanet for pointing out that light-sensitive molecules respond to photons, not protons!

Creationism and its particular brand of intelligent design are dead.

Evolution is not only a superior theory in multiple ways, but has also been both historically and experimentally validated. The big remaining weapon seems to be the idea of irreducible complexity, even though that, too, has been largely accounted for, both with general and specific cases covered where and when necessary.

One of these cases is the so-called irreducible complexity of the eye. How does something like the eye come into being? Even Charles Darwin himself had trouble conceiving this fact, but he later came to the conclusion that, by the principles of evolution, if we could find a proper and logical chain of development which at every point, the "choice" could be justified by natural selection's ideas, then the eye could also have evolved into existence.

The evolutionary response, therefore, is the obvious one: like everything else, through generations upon generations of random, selected-for, genetic mutations. In particular, one suggested method—this is by far the most popular—for the mammalian eye to have become what it now is, is to go through the following intermediary stages.

A couple of notes about what is to follow. First off, each eye is facing towards the right, and the parts that look like line-noise represent the nerves. They will always be attached to the photosensitive cells or the retina, whichever is a more suitable name for the version present in that stage. Secondly, as much as the specifics of this phenomenon are speculative, a great deal of it has been confirmed in fossil records from the early Cambrian era and onwards, which is where scientists believe eyes began a rapid amount of development. That is to say, scientists are pretty sure the following is more or less how it actually happened.

== ONE ==========================
                              
                         |    
                          \   
                           |  
                           |  
                          /| 
                         / |
                        |, |  
                    ,."";+:|  
                  :"     \ |   
                          \|  
                           |  
                           |  
                           |  
                          /   
                         |    
                              
=================================

The eye begins its journey as a flat region of photosensitive cells attached to a nerve network. (The odd character garbage in the above diagram represents the nerves.) These "organs" can still be found on various species of protists as eyespots, located at the posterior end of the cell, near the flagellum. They are mainly used as very basic sensors for light, aiding in synchronizing circadian rhythms or seeking out light for photosynthesis. As light can hit the photosensitive surface from all directions, there is no real semblance of sight yet, just basic light detection.

== TWO ==========================
                              
                         |    
                          \   
                           |  
               _______     |  
              /__-----"____/  
             / /              
            / |               
            | |               
            |/|               
         ,."";+\_______  __   
       :"    '._______,'/  |  
                           |  
                           |  
                          /   
                         |    
                              
=================================

A depression or folding occurs at the site of the photosensitive cells, pushing them inwards with respect to the surface of the organism. This allows for limited specificity and sensitivity when it comes to direction. An example of this in organisms today would be the planarian, who is also one of the organisms that exhibits the ability to regenerate lost body parts to the extent that you could split it in half and come out with two new planarians.

== THREE ========================
                              
                         |    
                          \   
               ________    |  
              / ,.----.\   |  
             / /      "'\  \  
            / |          \  | 
            | |           \_| 
            | |            _  
            |/|           / | 
         ,."";+\        _/  | 
       :"    \  \_____,'/  /  
              \________/   |  
                           |  
                          /   
                         |    
                              
=================================

Now the eye's opening has shrunken to a pinhole-size. This gives the eye a lot more directional sensitivity, to the potential extent of being able to discern images as a human might. Some argue that the new 'chamber' is filled with water or some other fluid, as a precursor to the vitreous humour, while others say it is just plain. An interesting problem here is that small objects may potentially get stuck inside the cavity if they are forced in somehow, and that leads into the next evolutionary step.

== FOUR =========================
                              
                        ||    
                         \\   
               ________   ||  
              / ,.----.\  ||  
             / /      "'\ \\  
            / |          \ || 
            | |           '|| 
            | |            || 
            |/|           ,|| 
         ,."";+\        _/ || 
       :"    \  \_____,'/ //  
              \________/  ||  
                          ||  
                         //   
                        ||    
                              
=================================

The chamber is now a little more rounded out, and has been enclosed by the precursor to a cornea. The chamber itself has filled with the vitreous humour, and, if it could not yet have been called a retina, the photosensitive cells at the back now fit that qualification nicely. This is a relatively obvious step to select for*, as now the image, as soon as it enters the eye, is regulated and could not be disturbed without destroying the eye itself, whereas in the previous stages, there was potential room for intrusion via small objects.

== FIVE =========================
                              
                       | |    
                        \ \   
               ________  | |  
              / ,-----.\ | |  
             / /       " \ \  
            / |         A | | 
            | |        / \| | 
            | |       |   | | 
            |/|        \ /| | 
         ,."";|         V | | 
       :"    \+\_____,.' / /  
              \______/   | |  
                         | |  
                        / /   
                       | |    
                              
=================================

The cornea is now more properly developed, and a lens has formed. The lens allows the eye to focus in on particlar depths of vision to the retina, as well as just generally gives finer control over the picture transmitted to the brain and subsequently 'seen' by the animal**. This already looks a whole lot like our mammalian camera eye, which we also share with cephalopods (though cephalopod eyes are formed from slightly different origins, and can be considered a case of convergent evolution).

== SIX ==========================
                        .     
                         .    
              _________   .   
            _/ _______ \  .   
           /  / ,-----' \     
          /  / /       \ \    
         /  / |         \ |\  
         |  | |         /\| | 
         |  | |        |  | | 
        _|  |/|         \/| | 
         ,."";|         / |/  
      ,:"_\  \ \____   / /    
           \_ \_____:.' / .   
             \_________/  .   
                         .    
                        .     
                              
=================================

Finally, we arrive at the contemporary ocular apparatus. The cornea has become a direct part of the eyeball, and now houses an aqueous humour. (In the diagram, the bulge in the front of the eye is the aqueous humour, and it is enclosed by the cornea. The little bean behind it is the lens.) The eye itself is now no longer fused into the surrounding surface, as well. The lens has become smaller and better—humans change the focus level by changing the shape of the lens, while cephalopods actually move the lens about in their eye, so 'better' here means 'more efficient at doing whichever it is supposed to be doing in that particular species'. Irreducible complexity believers now arise as start citing this form as something that could only be created by a god.

Now, I'm not a proponent of intelligent design, but there truly is something divine about the elegance of this process...

=================================

* Yes, I'm aware that natural selection can't really technically make choices, but by considering the process in the more abstract sense of "the better version wins", this sort of language isn't too much of a stretch.

** It is much more likely than not that the organism of stages 1 and 2 has, by stage 5, evolved into something we can refer to as an animal.

I'd like to thank Wikipedia for being so helpful in this w/u. I'd also like to thank my high school bio classes, in which I learned about what the hell protists and planarians were, and Darwin for his excellent read on evolution, On the Origin of Species. Also, apologies for the quality of the ASCII art. For anyone equipped with an SVG viewer and the desire to view these diagrams in a more palatable form, a proper image can be found here.

[IN12#10]

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