The Evolution of the Eye

Somebody once told me at a party that the eye - the eye itself, the organ of sight - is proof that evolution didn't happen. The idea was that something so complex could never have evolved by degrees, because evolution requires that an adaptation have a direct survival benefit, and an eye that was only half-evolved would have none. I believe the tag-line to the argument was "of what use is half an eye?"

I pretty much disparaged that one right at the source, but then I ended up coming across the same argument again years later. Imagine my dismay to find that people in the wide world at large are looking at this argument as if it were something serious! I had assumed at the time that this person had just come up with it himself right then, off the top of his head - something that might sound good for maybe a second or two, until the butt-ass obvious implications jump out at you from all sides.

Sheesh. Why must it always be me who debunks these things?

OK, I'll do it! If you are one of these folks who subscribes to the "what good is half an eye?" argument, please gather round, and allow me to lay it out for you. Sit back, and I will unfurl for you my tale of wonder:

...

...It is the Dawn of Time...

You are a tiny one-celled dealy floating in a primordial ocean. The light flows right through you - you can sense it directly. Your cellular membrane, your protoplasm are no barrier to it. Sometimes you try to rise towards the light, so as to bask in its life-giving energy and/or to process nutrients.

And on...

You are a simple multi-celled organism. You can still sense light in a fairly direct way - it may not penetrate fully into your interior cells, but you can sense the direction of the light from the cells on your outer surface, and you can swim toward the stimulus. It is important for you to be able to sense light, to sense the direction from which it comes so you can move towards it or away from it. When hungry you may feed on the smaller organisms that thrive in the well-lit upper reaches. At other times you may retreat into the safer, darker depths.

And on...

You are a more complex organism. You are still quite small, but your cells are becoming more and more specialized. Most of your outer surface has grown thick to protect you - too thick for you to be able to directly sense light with it. It is still very important for you to be able to sense light, for all the same reasons outlined above - and so, small patches on your outer surface have retained their sensitivity to light, allowing you to sense and react appropriately to light's stimulus for your survival. A sudden flash of darkness may prompt you to bolt - it could be a predator!

And on...

You are a yet-more-complex organism. Your specialized light-sensitive tissues, situated near the clump of nervous-system tissues that serves as your rudimentary brain, have become more and more sensitive to variations in light. The earliest organisms could perhaps only have sensed light's presence, later organisms might sense its intensity or direction. You can sense vague shapes, patches of dark within a field of light or points of light within a vague blackness. This humble ability represents a huge survival advantage for you. Not only do you know when to bolt, you have a decent idea of which way to go.

And on...

You are complex. Your specialized light-sensing tissues are becoming more and more developed. The vague patches of light or dark you see are becoming better and better resolved into images. You can begin to distinguish inanimate from animate...even predator from prey. This is a huge survival benefit.

And on...

As the process continues down through the ages, differentiating, branching out down through all different types of life, those rudimentary specialized light-sensing tissues mature into disparate forms and organs, adapting within a multifariety of physiological limitations to meet the visual needs of different ecological niches.

And on...

You are a highly-complex organism. Your highly-specialized sight organs are situated adjacent to the large clumpy mass of cells that serves as your brain. You are balancing a drink and a small paper plate of dainties in one hand, while you use the other hand to point for emphasis with a piece of spinach-dipped cauliflower. You are explaining to me why an eye cannot possibly be the result of evolution, because a rudimentary eye would give no direct survival benefit.

I look at you. My own eyes brim with love and understanding for you, for the long, long journey that has led you here.

"Buddy," I begin, "you're a piece of work."