A frequently raised criticism of evolution in any evolution-creation debate is that of the human eye. The creationist will say something like, "How can something as marvelous as the human eye have come about by chance alone? Surely there must have been a divine creation." These types of statements show two things. First, the creationist doesn't understand how evolution's 'chance' works. (i.e., They have yet to grasp the concepts behind cumulative natural selection.) Second, they haven't bothered to really examine the human eye to look for characteristics such as design flaws. (Note that the quote from Ernst Mayr under the above "creationist will say" link has been taken completely out of context. Not only does Mayr's entire book provide evidence after evidence of "improved" random mutations, but the same paragraph as quoted also states that "the objectors to random mutations have so far been unable to advance any alternative explanation that was supported by substantial evidence." This page looks at the substantial evidence against the 'intelligent' designer of creationism.)
As Frank Zindler (former professor of biology and geology) stated,
"As an organ developed via the opportunistic twists and turns of evolutionary processes, the human eye is explainable. As an organ designed and created by an infinitely wise deity, the human eye is inexcusable. For unlike the invertebrate eyes ..., the human eye is constructed upon the foundation of an almost incredible error: The retina has been put together backwards! Unlike the retinas of octopuses and squids, in which the light-gathering cells are aimed forward, toward the source of incoming light, the photoreceptor cells (the so called rods and cones) of the human retina are aimed backward, away from the light source. Worse yet, the nerve fibers which must carry signals from the retina to the brain must pass in front of the receptor cells, partially impeding the penetration of light to the receptors. Only a blasphemer would attribute such a situation to divine design!Biologist George Williams wrote an entire book on the subject of design and purpose in nature. Near the beginning of The Pony Fish's Glow, Williams responds to Paley's watchmaker argument using various body parts as examples of why Paley's argument may look good on the surface, but it lacks credibility when closely examined using modern technology and biology. Here he discusses the human eye:
Although the human eye would be a scandal if it were the result of divine deliberation, a plausible evolutionary explanation of its absurd construction can be obtained quite easily--even though we can make little use of paleontology (because eyes, like all soft tissues, rarely fossilize)."
"not all features of the human eye make functional sense. Some are arbitrary. To begin at the grossest level, is there a good functional reason for having two eyes? Why not one or three or some other number? Yes, there is a reason: two is better than one because they permit stereoscopic vision and the gathering of three-dimensional information about the environment. But three would be better still. We could have our stereoscopic view of what lies ahead plus another eye to warn us of what might be sneaking up behind. (I have more suggestions for improving human vision in chapter 7.) When we examine each eye from behind, we find that there are six tiny muscles that move it so that it can point in different directions. Why six? Properly spaced and coordinated, three would suffice, just as three is an adequate number of legs for a photographer's tripod. The paucity of eyes and excess of their muscles seem to have no functional explanation.He continues on the eye later as follows:
And some eye features are not merely arbitrary but clearly dysfunctional. The nerve fibers from the retinal rods and cones extend not inward toward the brain but outward toward the chamber of the eye and source of light. They have to gather into a bundle, the optic nerve, inside the eye, and exit via a hole in the retina. Even though the obstructing layer is microscopically thin, some light is lost from having to pass through the layer of nerve fibers and ganglia and especially the blood vessels that serve them. The eye is blind where the optic nerve exits through its hole. The loose application of the retina to the underlying sclera makes the eye vulnerable to the serious medical problem of detached retina. It would not be if the nerve fibers passed through the sclera and formed the optic nerve behind the eye. This functionally sensible arrangement is in fact what is found in the eye of a squid and other mollusks (as shown in the figure below), but our eyes, and those of all other vertebrates, have the functionally stupid upside-down orientation of the retina.
Paley did not really confront this problem. Little was known about mollusks' eyes at the time, and Paley merely treated the blind spots as one of the problems the eye must solve. He correctly noted that the medial position of the optic nerve exits avoids having both eyes blind to the same part of the visual field. Everything in the field is seen by at least one eye. It might also be claimed that the obstructing tissues of the retina are made as thin and transparent as possible, so as to minimize the shading of the light-sensitive layer. Unfortunately there is no way to make red blood cells transparent, and the blood vessels cast demonstrable shadows.
What might Paley's reaction have been to the claim, which I will elaborate in the next chapter, that mundane processes taking place throughout living nature can produce contrivances without contrivers, and that these processes produce not only functionally elegant features but also, as a kind of cumulative historical burden, the arbitrary and dysfunctional features of organisms?" (page 9-10)
"What would Paley's reaction have been to the suggestion that the creator's wisdom is as finite as ours, and that the engineering perfection of such instruments as the eye...depends...on much trial-and-error tinkering that supplemented the creator's limited understanding? And what about the suggestion that the creator had no understanding at all, but accomplished sophisticated engineering entirely on the basis of trial and error?" (page 11-12)Williams concludes his section on trial-and-error and the eye argument with the following:
"This is no doubt true of all the implements we use: cameras, cars, computers, and even the watch that Paley reasoned must have had an intelligent designer. How far is it possible to go with trial and error alone? All the way to the human eye and hand and immune system and all the other well-engineered machinery by which we, and all other organisms, solve the problems of life...Another creature to consider is the mole rat. Which theory holds water when the eye of the mole rat is considered? The ancestor of the mole rat presumably used its eyes as it lived above ground and needed them for survival. However, the mole rat has adapted to living underground in complete darkness. Its eyes have become useless--indeed, they have been buried beneath skin and fur and couldn't be used even if the mole rat came into the light. The neurons that were used for sight have been put to better use in the mole rat's brain for other sensory functions. Evolution by natural selection perfectly explains the eyes of a mole rat. A creationist must resort to faith and/or a poor designer. (See Lucy's Legacy p. 25 and Jared Diamond's "Competition for brain space" in Nature 382: 756-757.)
Darwin was challenged repeatedly on this matter. Critics would point to the precision and design features of the eye and claim that an organ of this perfection could not possibly have been produced by an accumulation of small changes, each of which made the eye work slightly better. A grossly imperfect eye, which could be improved by this process, would supposedly never evolve in the first place. Slight improvements in one part, such as the retina, would be useless without an exactly matching improvement in another, such as an increased precision of the lens. This is an utterly fallacious kind of reasoning. An improved retina may be useless without an improved lens, but both retinas and lenses are subject to individual variation. Some of the better retinas would be found in individuals who also had better lenses, so that the improvements, on average, could be favored.
The criticisms were also factually erroneous, and their proponents were ignorant of biology. As Darwin pointed out, familiarity with the animal kingdom shows the existence today of just about every stage in a plausible sequence from primitive light-sensitive cells on the surfaces of tiny wormlike animals, through the rudimentary camera eyes of scallops, to the advanced optical instrumentation of squids and vertebrates. Every stage in this sequence is subject to variation, and every stage is clearly useful to its possessor." (page 13-14)
Those interested in this subject should also see chapters four and five of Richard Dawkins' Climbing Mount Improbable, section 13.3 in Mark Ridley's Evolution, pages 110 to 114 in Cells, Embryos, and Evolution, the faulty deductive reasoning of Paley--especially as it relates to intelligent design inference for human eyes--from p. 140-3 of Science As a Way of Knowing: The Foundations of Modern Biology, Ted Gaten's research interests, the section entitled "Eyes and Evolution" on pages 161 - 165 of Songs, Roars, and Rituals, and Evolution of the Eye and Visual System by J. R. Cronly-Dillon and R. L. Gregory. On a related topic, see the inefficiencies created by natural selection (and lack of design) as illustrated on this page. Also see the November 2006 issue of National Geographic.
In summary, the eye not only lacks evidence of divine creation, it exemplifies the problems that natural evolution can create (along with the virtues) in organisms. Rather than being a chief argument for creationism, the human eye should be a topic that 'special creation' and/or 'intelligent design hypothesis' apologists avoid.