salutations, space cadets. What follows is the transcript and images from my mini fold up comic on the evolution of the eye at the end i'll include the printable files so you can make your own, free of charge no less! enjoy.
The eye is an intricate, amazing and absurdly useful organ; to imagine it as a product of evolution boggles the mind. To be sure, the evolution of the eye is complex but certainly not irreducibly so. The steps in its evolution are ultimately easy to follow and, indeed, demonstrably true. For one to understand the progression of this evolution one needs some imagination, as well as cold, hard facts and living examples of the several in-between phases of the eye as we know it today.
The first stop on our journey is the mild-mannered, unicellular Euglena. Near its whip like flagella, a small sheet of light-sensitive proteins has-evolved through the random nature of genetic mutation. These “eyespot’s” only real ability is to sense the difference between light and dark. Although this may not seem like much of an advantage, it is certainly better than blindly bumbling around. Because of its advantages the ‘eyespot’ was favored, passed on to, and improved on by other organisms like the flat worm. If one pictures the original light sensitive proteins as a flat sheet, then the next logically advantageous step would be for this sheet to become concave, like a bowl. Why is this concave shape preferable? Think about it. If you hold a bowl up to a light one very important fact can be ascertained: the direction of the light. This is exactly the kind of eye flat worms have. It enables them to recognize, say, the safety of a cave, but not enough to distinguish much else from its surroundings.
Because of its advantages the ‘eyespot’ was favored, passed on to, and improved on by other organisms like the flat worm. If one pictures the original light sensitive proteins as a flat sheet, then the next logically advantageous step would be for this sheet to become concave, like a bowl. Why is this concave shape preferable? Think about it. If you hold a bowl up to a light one very important fact can be ascertained: the direction of the light. This is exactly the kind of eye flat worms have. It enables them to recognize, say, the safety of a cave, but not enough to distinguish much else from its surroundings.
The next stop on our journey (our unofficial mid-point) would be the precursor to much of the tentacle’d life in the ocean today--the Nautilus. As our rudimentary eye becomes more and more concave, the edges of the ‘bowl’ come close to touching, creating a small pinhole for light to filter through, not unlike a pinhole camera. Although the images, if one could even call them that, are blurry and largely indistinguishable, the nautilus is now able to differentiate objects from their surroundings. The advantages of this eye are great. It simultaneously allows its user to easily flee from predators and catch more prey.
The next step is somewhat harder to imagine: the development of the lens. Modern lenses are indeed complex, but their ancestor need not have been. The first step would be the development of the aqueous humour, or the gunk inside ones eyes. To start with, this gunk need not be anything special. Any clear, viscous liquid will do. It allows the light to be focused as it travels through the eye to the light receptors in the back. The Sea Snail is one of the animals who have done just that. Around the creatures ‘eye holes’ a pocket has formed, capturing somewhat thick plasma-like liquid, enabling it to see much better than any of its antecedents.
If one imagines a bowl of pudding sitting in a refrigerator, eventually forming a thicker skin on top, the next step in our process is easy to understand. As this ‘skin’ formed it created the different layers of the eye as we know them today, the lens, the sclera, the cornea, etc. it’s easy to see (no pun intended) how advantageous this evolutionary step was and is. The more useful something is, the probability of it being passed on increases greatly. As the advantage, whatever it is, is passed from generation to generation, from organism to organism, the chance of it beneficially mutating increases and increases. Ultimately, after millions of years we end up with the multitude of creatures, the vast majority on earth, with the ability to see and to see in amazing ways. It is said the Tools of evolution are time and death, understanding this makes the evolution of the eye not only comprehensible but a virtual inevitably on a world such as ours.
The vertebrate and the invertebrate eyes evolved separately from one another. As remarkable as the similarities are, the invertebrate eye it actually arguably better. In the vertebrate eye, the retinal wiring is ran through a hole in the image capturing retina, creating a blind spot and more matter for light to travel through before it’s registered. In the invertebrate eye, the wiring is plugged into the back of the retina, creating no blind spot and more directly capturing light. Why do squids get the good eye!?
Thanks to: Richard Dawkins and Dan-Eric Nilsson