The human eye is an excellent example of an organ that appears to be designed but upon close investigation shows a great deal of suboptimal design resulting from the constraints of its evolutionary legacy. This can be demonstrated not only by the obvious flaws in the design but also in a number of medical disorders. I will discuss the design flaws first and then go onto the medical conditions that result because of the flaws.
The most obvious design flaw of the retina is that the cellular layers are backwards. Light has to travel through multiple layers in order to get to the rods and cones that act as the photoreceptors. There is no functional reason for this arrangement. Even in a healthy and normally functioning eye, this arrangement causes problems. Because the nerve fibers coming from the rods and cones need to come together as the optic nerve, which then has to travel back to the brain, there needs to be a hole in the retina through which the optic nerve can travel. This hole creates a blind spot in each eye. Our brains compensate for this blind spot so that we normally do not perceive it—but it is there. If you search “blind spot” on Google there are many fascinating tests that show the effect of this.
Practically, this is a minor compromise to visual function, but it is completely unnecessary and can only be explained by evolutionary adaptation. If the rods and cones were simply turned around so that their cell bodies and axons were behind them (oriented to the direction of light), then there would be no need for a blind spot at all.
At this point I want to put something to bed. There are some creationist explanations that in effect say the layers need to be arranged this way. For example, Dr. George Marshall in an interview for answersingenesis.org says:
“The light-detecting structures within photoreceptor cells are located in the stack of discs. These discs are being continually replaced by the formation of new ones at the cell body end of the stack, thereby pushing older discs down the stack. Those discs at the other end of the stack are ‘swallowed’ by a single layer of retinal pigment epithelial (RPE) cells. RPE cells are highly active, and for this they need a very large blood supply—the choroid. Unlike the retina, which is virtually transparent, the choroid is virtually opaque, because of the vast numbers of red blood cells within it. For the retina to be wired the way that Professor Richard Dawkins suggested, would require the choroid to come between the photoreceptor cells and the light, for RPE cells must be kept in intimate contact with both the choroid and photoreceptor to perform their job. Anybody who has had the misfortune of a hemorrhage in front of the retina will testify as to how well red blood cells block out the light.” (read the full interview here http://www.answersingenesis.org/articles/cm/v18/n4/eye-for-creation).
But this answer makes no sense since there are examples where the photoreceptor layer has evolved to be at the top of the retinal, whereby providing a much better configuration. This much better “design” is found in the Cephalopods (squid, cuttlefish and Octopi). It would appear that although these creatures possess compound eyes that resemble those of vertebrates, they did in fact evolve independently (an example of convergent evolution). It is therefore demonstrably possible for this better configuration to work making Dr. Marshall’s statement above fallacious.
Intelligent design offers no explanation for the use of a superior design in one case and a suboptimal design in another. Marshall’s hypothesis assumes that the current design is necessary and cannot be changed, which is the case with a system that is contingent on the constraints of evolutionary legacy, but is not the case in a system designed from scratch. An intelligent designer would have designed a system where the photoreceptor discs are produced at the top (closest to the light) so that the older ones would migrate towards the bottom of the rod and cone cells where they would be absorbed. Below this absorption layer could be the blood vessels, and the axons from the rods and cones could also leave from the bottom of the rods and cones through this opaque absorption layer (the RPE).
It just so happens that the primitive vertebrates that first developed a primitive eye were small and transparent, and it is postulated that the arrangement of the retina therefore did not matter, but once the evolutionary path was started, it constrained all of future vertebrate eye evolution.
In addition to the retinal configuration design flaw is fact that the blood vessels that feed the retinal sit on top of the retina—between the light source and the receptive layer. If you and I were to design this we would connect the blood vessels that feed the retina from behind, so that they do not get in the way. In healthy eyes, these blood vessels do not cause any perceptible problem (but they are also partly responsible for the blind spot), but they do limit the total amount of light reaching the rods and cones, making our eyes much less efficient than they could be.
Another problem with this design is that these blood vessels they are vulnerable to diseases. For example a vast majority of diabetics will develop diabetic retinopathy In response to chronic ischemia (relative lack of oxygen), the retina will produce chemical signals that tell the blood vessels to proliferate to increase the blood supply. Because the blood vessels are above the retina, they increasingly get in the way, obscuring vision. At present, the primary treatment of diabetic retinopathy is to use a laser to burn some of the blood vessels and decrease their proliferation.
Having the blood vessels in front of the retina also means that even a small retinal bleed can significantly impair vision. And finally, any inflammation that occurs within the cell layers in front of the rods and cones will likewise impair vision. All of this could have been avoided or minimized were the rods and cones placed in the most superficial layer of the retinal, rather than buried at the bottom.