Eye

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The right eye of a domestic cat (Felis silvestris catus). Note the slit-shaped iris.
The right eye of a domestic cat (Felis silvestris catus). Note the slit-shaped iris.

The eye is a sensory organ that detects light. There are a number of different classes of eyes in use throughout the animal kingdom, ranging from the extremely rudimentary (i.e., light-sensitive patches of skin) to the exactingly precise (i.e., lens-and-retina). The human eye in particular is firmly in the 'exactingly precise' part of this spectrum. As a result, some Creationists assert the human eye to be so complex and advanced that it must have arisen in one single Act of Creation, rather than being the current state of a continuing process of descent-with-modification. However, given the fact that there are a number of different classes of eyes which, collectively, exhibit a wide range of degrees of complexity, it is difficult to understand what would absolutely prevent a lineage from acquiring a series of successive increases in the complexity of its eyes, eventually ending up with something akin to the human organ.

Contents

Vertebrate Eye

Suboptimal design > Vertebrate eyes

Vertebrates (mammals and closely related classes) have evolved a suboptimal eye. In the eyes of vertebrates the photo-sensitive cells lie behind the optic nerve endings and a layer of blood vessels. These must emerge through the back of the retina, leaving a hole in the organ with no light-sensitive cells, forming a blind spot. Other animals, such as cephalopods (e.g. squids and octopuses) and insects, which have evolved sight independently, have much more efficient eyes because they do not have a blind spot, or nerves between the photo-sensitive cells and the light source. In cephalopods, the photocells in the retina each point towards the light, with their nerves out behind.

There are three failings of this system.

Moreover, it is not just that a blind spot in an eye (think about it!) is a poor design. This 'design' also has consequences. Firstly, we do not normally notice the blind spot, because our brains fill in the missing bit for us. Which means that extra brain processing is required to give us an unimpeded picture of the world -- unimpeded, that is, by the design flaw. More design to correct the initial design is surely not intelligent design!

The second problem is a minor loss of visual acuity due to having the extra material between the retina and the light. The photons of light that hit the retina can be scattered slightly before they hit the photoreceptors, causing a very slight blur. This is, however, so minor it can be ignored.

The worst result of the design is the relative ease with which the vertebrate retina can become detached from layer beneath it - the pigment epithelium. Each beat of the heart stretches and then relaxes the retina, pulling it away from this layer. Any retinal damage, usually a tear caused by direct trauma to the eye (e.g. a blow to the head) to the retina, is thus exaggerated by each and every beat of the heart. This would not occur if the blood vessels were on the 'right' side of the retina. In addition, trauma can cause fluid to build up between the retina and the pigment epithelium, forcing the retina to detach. Detached retina is one of the most frequent causes of visual impairment around the world and is entirely a result of this moronically bad design. The analogous cephalopod eye, with its 'right way round' retina, cannot suffer from this because the retina is anchored into the blood-supplying tissue by the nerve wiring itself.

The vertebrate 'design' can easily be explained by the theory of evolution - a proto-eye evolved which was simply an area of light-sensitive skin. By chance, nerve endings happened to be between the light sensitive area and the light sensitive cells - it was 50/50 how the cells were developed. Since the light sensitive cells gave the individuals which possessed them a slight selection advantage they stuck, and developed into an eye.

Humans' eyes are even worse design than just having a back-to-front retina. Many animals have bi-focal lenses and are able to magnify objects much further away than humans are capable of seeing.

Creationists ask "what use is half an eye?" The answer is "a lot". After all, we've got one!

Creationist responses

Creationists have, as yet, not produced an effective explanation for the design of the vertebrate eye, but they have produced many failed hypotheses.

By and large, creationists focus only on the first two problems of eye design, the blind spot and the minor loss of visual acuity. Since both of these problems are relatively minor, they can brush them off with a shrug, usually while smugly accusing scientists of trying to make mountains from molehills. However, it is not remotely realistic to brush off retinal detachment, the single most common cause of blindness in the world.

Another common creationist claim is that the eye, for various reasons, couldn't be designed any other way. This crashes hard into the fact that other animals, such as gastropods and cephalopods, DO have eyes designed another way, with the blood supply and nerves attached round the back. Creationists who are aware of this try to explain it by pointing out cephalopods live in the ocean and try to salvage the situation from the different environments they and humans experience. However, fish are also found in the oceans and yet have the vertebrate-style eye, whereas terrestrial gastropods such as snails live in a similar environment to humans and yet have their retina arranged similar to those of cephalopods. If, for the sake of argument, we accept that the vertebrate eye was well designed for land and the cephalopod eye well designed for the water, then fish and land snails both have badly designed eyes! (See also: Twin-nested hierarchy.)

Another common argument suggests that the arrangement was necessary for heat management reasons[1]. It is often proposed that the choroid is responsible for acting as a heat sink, by taking away the heat that could threaten the retina[2]. However, the choroid is a substantial producer of heat too (light that passes through the photoreceptors is absorbed by the choroid and the retinal pigment epithelium as heat). The photoreceptors in vertebrate eyes are located very close to these layers. This can be advantageous in low-temperature conditions, but also threatens the integrity of the retina if they overheat (ocular hyperthermia). The cells of the retinal epithelium produce the 11-cis retinal molecule required for sight, so it could be reasoned that these pigmented cells must be located near to the photoreceptors, and to counter this the choroid is required to remove the heat. However, in cephalopods, the photoreceptors produce their own 11-cis retinal [3].

Other creationists, such as Jonathan Sarfati have claimed that the cephalopods have very poor vision[1], but their visual problems aren't due to the retinal design, but certain problems with the lens they haven't evolved a solution for yet. Besides, their visual acuity isn't in general worse than that of fish, which have the vertebrate eye. Besides, pointing out another aspect of bad design would only make the "Intelligent Designer" seem less competent!

Additionally, Dr Gurney cites the evidence that the xanthophyll pigmentation of cells covering the retina contain, while clear to allow visible light through, so absorb blue and ultraviolet light and therefore protect the retina from photic damage[1]. But given how little UV light is blocked by these thin layers of pigment (20-40%)[4], this is a bit like praising the bullet-stopping power of tissue. Other animals have UV protection methods that work far more effectively, such as polarized lenses.

A slightly less common argument is that the orientation of the vertebrate eye helps to prevent light from being reflected off the the choroid onto the photoreceptors, which would cause blurry vision.[5]. While this would be the case if light was reflected, it is not clear from the creationist arguments exactly how the vertebrate eye provides any protection against reflection. It is clear that human eyes do reflect light, as evident by the 'red eye' effect in flash photographs, and even more so in other vertebrates (bats, cats, dogs, crocodiles, horses etc) due to their reflective layer behind the retina known as the tapetum lucidum.

Perhaps the most common creationist response to the problems of the vertebrate eye is, "Well, you couldn't build a better one!" Well, maybe not actually build it, but I can certainly envision one. Of course, I can't build a car either, but I can certainly identify flaws in the design of cars, or am I not supposed to be able to tell that a Rolls Royce™ is a higher quality car than a Chevy Lumina™? If creationists really believe this, you should never ask them for shopping advice!

Nautilus eye

Suboptimal design > Nautilus eye
A Chambered Nautilus (Nautilus pompilius)
A Chambered Nautilus (Nautilus pompilius)

As photographers will tell you, a 'pinhole camera' is an effective way to form an image out of light; the small aperture at the front forms an image on the screen behind it. The smaller the hole, the sharper the image. The down side is that the smaller the aperture, the less overall light gets in. To solve the problem, camera designers add a lens. A lens is an obvious improvement, since it allows both a sharp and bright image. We might therefore expect the Intelligent Designer of organisms to use lenses in eyes.

For the most part, the 'designer' did. Octopus and squid eyes have lenses... vertebrate eyes have lenses. There is however one cephalopod mollusc, the nautilus (family Nautilidae), which is considered more primitive than octopuses and squid, since it has an external shell. And it lives at considerable depths, where light is at a premium. The nautilus has a very good pinhole camera eye; considerable thought presumably went into its design. But the designer apparently saw fit not to give that eye a lens. Its eyes are therefore far less efficient than it easily could have been. Did the designer just forget?

References

  1. ↑ 1.0 1.1 1.2 Gurney, P., Is our inverted retina really bad design? Journal of Creation 13(1):37–44, 1999; [1]
  2. ↑ Parver LM. Auker CR. Carpenter DO. "The stabilizing effect of the choroidal circulation on the temperature environment of the macula." Retina. 1982, 2(2):117-20
  3. ↑ Takimoto N., Kusakabe T., Tsuda M. "Origin of the vertebrate visual cycle" Photochemistry and Photobiology 83(2): 242-247 Mar-Apr 2007 PMID: 16930093 [2]
  4. ↑ Landrum, J. T. and Bone ,R. A., "Lutein, Zeaxanthin, and the Macular Pigment" Archives of Biochemistry and Biophysics 385(1), 1 January 2001, p28-40 [3]
  5. ↑ " Vestigial Organ" at CreationWiki. Retrieved August 28, 2007 from http://creationwiki.org/Vestigial_organ

External Links

See also

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