Mar 17 2010

Eye Evolution and Irreducible Complexity

A creationist commenter on a post of mine discussing lame creationist arguments first admitted that he did not actually read my post, and then began to repeat the same tired creationists lies and logical fallacies we hear over and over again.

I had asserted a well-established biological fact – the eye is not irreducibly complex. There are examples in nature of simpler eyes that represent probable stages through which the vertebrate eye evolved. I made this as part of a broader point, that many structures and systems claimed to be irreducible have known simpler antecedents, and I even provided a link to a page on Talk Origins that linked in turn to many articles with the evidence for this claim.

Creationists, however, are apparently not interested in making sound arguments or what science actually has to say about any particular question – only obfuscating the truth with misdirection and debating tactics. The commenter claimed I had not bothered to provide evidence to back up my claim, and inferred that I therefore could not.

He wrote:

That link has absolutely nothing to do with scientifically showing that these two biological systems, specifically vision or blood clotting could have evolved from simpler systems that were functional but served a different purpose from their current one. It is no different than your approach – simply declare that the argument is flawed without specific scientific proof to the contrary but broad statements of the sort …”that is wrong and this is how it works,” without any proof.

He then challenged me to provide evidence for simpler antecedents in eyes. Well, challenge accepted. Of course, what he is really saying is that he is too intellectually lazy and/or dishonest to find the information himself. It is not hidden away in dusty university libraries – it’s just a few Google clicks away.

Without much trouble I was able to find links documenting what I had read from many sources – there are multiple examples of living creatures with simpler eyes – highly functional, often adapted specifically for their function, and representing a plausible path of the evolution of vertebrate eyes from nothing but a patch of light sensitive cells.

I will anticipate the likely creationist misdirection – these examples do not represent an actual evolutionary sequence. Of course not – they are examples from extant (living) species – all of whom share a common ancestor but represent current examples of many different lineages that split off at various times in the past. They are not a sequence.

The point of these living examples is to refute the claim that the eye is irreducibly complex – that if it were any simpler it could not function, or more specifically could not confer an evolutionary advantage to the host.

Eyes are soft tissue – they do not generally fossilize. So we have scant fossil evidence of eye evolution. We do have some, from species with preserved skull bones showing muscle insertions and the structure of the tissue around the eye (for example), but not the soft parts of the eye itself.

Here are some simple eye structures that work just fine for the organisms that have them:

Eye spot – patch of tissue or cells that are photosensitive. Organism can move toward light, or synchronize circadium rythm, but not see shapes.

Example: Euglena

Once you have an eye patch, the next step is for the patch of cells to become progressively depressed. This increases progressively the ability to distinguish direction, each step providing a slight advantage, until you have an Eye cup. Eye cups are able to tell direction of light better than eye patch – the more cupped the eye, the finer the angle of discrimination.

Example: Planarian

When an eye cup continues to deepen, eventually the outer rim of the cup with close in on itself, forming a Pinhole eye. The pinhole eye works like a simple camera, and is able not only to distinguish direction quite well but also make out basic shapes.

Example: Nautilius

A pinhole is still open to the outside world, however, so the formation of a transparent cell layer over the hole has an obvious advantage. Once this happens, that opens the door for layering and specialization, including forming a pocket of cells to act as a lens. A primitive lens would focus light into the eye, increasing the amount of light falling on the retina. Before this was sufficiently refined for the lens to focus an image sharply on the retina, a weak primitive lens would still amplify light. This would allow sea creature to see at greater and greater depths, until they had a strong lens that could focus in on the retina.

Example:  Box Jellyfish – have primitive lenses that do not focus, adapted for low light environments. Jellys in brighter environments have simpler eyes without lenses.

And now you have all the basic elements of a vertebrate eye, requiring only progressive refinement.

Again – this line of evidence does not prove that or how the vertebrate eye evolved. It simply demonstrates that simpler eyes, all the way down to a patch of light sensitive cells, could work and provide an adaptive function – therefore the eye is not irreducibly complex.

There are other lines of evidence, however, from genetics, fossil evidence, and the suboptimal design of the vertebrate eye, that point directly at evolution.

Of course this evidence will not stop trolling creationists from leaving comments claiming the eye could not have evolved because it is just too complex. Creationists have a flat learning curve when it comes to evolution.

Addendum: I also found this article: Evolution of the vertebrate eye: opsins, photoreceptors, retina and eye cup.

This is a nice article published in Nature that reviews the evolution of the vertebrate eye giving examples of the various simpler stages with vertebrate relatives. In other words, this is as close to an evolutionary sequence as we can get with extant species. This goes beyond plausibility, and beyond demolishing irreducible complexity, to documenting the evolution of the vertebrate eye specifically.

It is ironic that creationists continue to use the eye as the example of the complex structure that defies evolutionary explanation – when in reality the various eyes that have evolved in nature represent one of the best lines of morphological evidence for evolution.

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