Jul 03 2008

The Not-So-Intelligent-Design of the Human Eye

Intelligent design (ID) is the notion (I am deliberately not referring to it as a theory, because as yet no one has put forth a formulation of ID that qualifies as a testable scientific theory) that life on earth was designed from the top-down by an intelligent agent. The historical and cultural background of ID is clearly documented (as was demonstrated in the Dover trial) – it is the latest incarnation of fundamentalist Christian creationism – and so the “intelligent designer” is simply a stand-in for the Christian God.

Regardless of what you call the top-down designer, any reasonable interpretation of ID/creationism predicts that life should show signs of top-down design. ID/creationism fails to be a scientific theory because its proponents refuse to make such reasonable predictions. In essence they argue that the designer (God) could have created life to look like anything – even absent any evidence of top-down design.

Evolution is the scientific theory that the various species of life on earth arose through branching descent, change and speciation over time. This is a decidedly bottom-up process of design with populations adapting to their immediate needs using the raw material that happens to be available to them. This raw material includes their existing biology in all its facets, including the variation within the population. Evolution is therefore unguided, unplanned, messy, quirky, and historically contingent.

Therefore if we cast a scientific eye onto the details of biology we should be able to see whether life displays the signs of top-down design or bottom-up design. Stephen J. Gould pointed out that the difference will not be found in the complexity and perfection of anatomy and physiology – for both origins are compatible with biological forms that are well-adapted to their current functions.

However, evolutionary design is constrained by history and raw material, while top-down design is not. Therefore if life is the result of evolution we should see imperfections, sub-optimal design, quirky anatomy and other manifestations of contingent evolutionary history. We should see things that no self-respecting designer would have deliberately designed, at least not by any fathomable logic.

An examination of the human eye can serve as an example of such an analysis. I will take a decidedly medical perspective – I am a physician and physicians are familiar with all the ways in which biology can go wrong. Diseases and medical problems often arise from the quirkiness of bottom-up design, so I will examine various aspects of eye anatomy and discuss how they led to functional problems that could easily have been avoided by a more rational top-down design.

The Retina

The first figure above is the human eye, cut away to show the internal structures, including the retina, which lines this inside of the globe of the eye and is the tissue layer that reacts to light. The second figure below is a microscopic view of all the layers that make up the retina. In this figure light would enter from the bottom and travel up to the receptors (rods and cones) near the top.

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 photo-receptors. There is no functional reason for this arrangement – it is purely quirky and contingent.

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 don’t perceive it – but it’s there.

From a practical point of view this is a minor compromise to visual function, but it is completely unnecessary. 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.

Some creationists have tried to counter this argument by saying 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.

This answer is unsatisfactory, however, because it assumes that certain elements of the current retinal design are necessary and unchangeable – which is only the case in a contingent system, but is certainly not the case in a top-down designed system. For example, the rods and cones could have been designed so that the photoreceptor discs are produced at the top (meaning the layer closest to the direction of light), with older ones moving backward toward the bottom of the cells where they are 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).

The point is – a top-down designer could arrange the cells and the cell layers in any configuration that could logically work, and it is certainly possible to conceive of workable configurations that place the photoreceptors at the top, rather than the bottom. Evolution, or a bottom-up system, cannot do this. It is constrained by existing anatomy.

Also – the evolutionary line that led to modern squid evolved eyes independently from the vertebrate line leading to humans. The squid eye has the photoreceptor layer at the top of the retina, unlike the vertebrate configuration which has it at the bottom. It is therefore demonstrably possible for this better configuration to work. Evolutionary contingency reaches different results from different histories, but why would a top-down designer use a superior design in one case and a sub-optimal design in another?

Even worse than the backward arrangement of the rods and cones is the fact that the blood vessels that feed the retinal sit on top of the retina – between the light source and the receptive layer. A more logical arrangement would be to have the blood vessels feed the retina from behind, so that they do not get in the way. In healthy eyes the 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. The real problem is that they are vulnerable to various diseases.

About 80% of diabetics who have had diabetes for 10 years 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 hemorrhage can significantly impair vision.

And finally, any edema or 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.

Tomorrow I will continue with further examples of sub-optimal eye design.

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