The Human Eye: A design review by Steven den Beste

Steven posted a link to his
superb essay, “The
Human Eye: A Design Review
“, in a comment to a Daily
Pundit discussion of the TNR
piece
on Conservatives and Evolution. Even if you are not interested
in evolution science, you will enjoy den Beste. Motivational excerpts (not
a subsitute for reading-the-whole-thing):

Occasionally I see creationists point to the human eye as a miracle of design, as if this somehow is evidence of divine origin for the human form. Unfortunately, from an engineering perspective, the human eye is seriously suboptimal. It simply isn’t that good a design.One of the things we studied was the vertebrate eye. And I was appalled by what I found. Frankly, not only is it not a wonderful design; on the contrary it’s one of the worst designed mechanisms in the body. (It contrasts, for instance, with the sheer elegance of the design of the kidney.)

So let’s do what we engineers refer to as a design review on the human eye, and discover what happens.

It may seem obvious, but we start with a formal declaration of the function the system is supposed to perform: to receive and process light of several frequencies in order to derive information about the environment, while using as little mass as possible to do so.

Because of this statement of function, it becomes clear that the critical component of the system is the retina.

The retina is a thin film of tissue a bit larger than a postage stamp which covers the inside surface of the eyeball away from the lens. It consists of light sensing cells embedded in a support matrix of epithelial cells, along with an array of blood capillaries which bring in nourishment and carry away wastes so that all these cells don’t die.

And right away, the flaws begin. The vertebrate retina is a terrible design. The optic nerve comes into the eyeball at a certain point, and the nerve fibers spread out across the surface of the retina. Each individual nerve fiber reaches its assigned point, burrows down into the retina through several layers of epithelial cells, and ends with the light receptor itself pointing away from the lens of the eye, which is the direction from which the light must come. As a result, incoming light strikes the surface of the retina and must penetrate through multiple layers of inactive cells and then through the body of the nerve itself before it reaches the active point where it might be detected. This both diffuses and attenuates the light, decreasing the efficiency of the retina in accomplishing its function.

It’s possible to do this better. We know this because the mollusc eye does it right. In the mollusc eye (typified by the octopus, squid and chambered nautilus, all of which have excellent vision) the optic nerve spreads out under the retina, and each nerve burrows up through the retina and ends with the light sensor on the surface of the retina, pointing towards the lens. This means that there is no attenuation of the light before it reaches the active components. (Just incidentally, this also means that molluscs have no blind spot. Vertebrates have a blind spot because there are no light receptors at the location where the nerve passes through the retina.)

The mollusc design is completely practical, but vertebrates don’t use it. Our design is second rate. This alone is sufficient to demonstrate the inelegance of our eyes, but the problems don’t stop there.

Some mammals have found a kludge which ameliorates this poor design to some extent. Beneath the layer where the light sensors are, there’s a reflective layer. If a given photon passes through the nerve and doesn’t set it off, it reflects and is given a second opportunity on the way out. That’s why the eyes of deer and cats seem to glow so brightly when you hit them with a flashlight at night; the light from your flashlight is reflecting off that layer. This substantially improves the sensitivity, though not to the level of the mollusc retina — but primates (including humans) don’t have this. So even among vertebrates, humans don’t have the best design available.

What we have makes perfect sense as the end product of a long sequence of incremental changes. However, it makes no sense at all as a unique design from scratch for this particular application.

If God designed the human eye from scratch for this application, then God is an incompetent engineer.

As I studied physiology, I found example after example of poorly designed mechanisms that didn’t make sense as original designs, but which made perfect sense as modifications of previous structures which were used for different functions. That is most of what convinced me that evolution is correct and that creationism is fantasy.

Some other examples:

Why does the birth canal run through the middle of the pelvis?
Why does the backbone run down one side of the trunk instead of through the middle where it would be more balanced?
Why does the ankle attach at one end of the foot instead of in the middle?
Why are there toes?
Why is it that nearly every part of the brain is as far as possible from the piece of the body with which it is associated?
Why is the motor control center for the right side of the body on the left side of the brain, and vice versa?
Why is the vision center at the rear of the brain, as far from the eyes as possible — and on the opposite sides?
Why is it that fully 90% of the genetic material we carry around is useless?
Why do we share a single canal through the neck through which we both breath and swallow?
Anyone who tries to claim that the human form is some sort of engineering marvel simply hasn’t looked closely enough.

And do not miss this related essay “Evolution,
Creation and Chocolate Cake”
, which offers an easily understood recipe-analogy
for evolutionary theory. Creationists won’t like this essay unless they can learn
from it.

One thought on “The Human Eye: A design review by Steven den Beste

  1. Your link is malformed: htttp://denbeste.nu/essays/humaneye.shtml has too many t’s in htttp. S/b http

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