But this visit, together with some questions on optics and vision asked by my AP Physics students last week, reminded me of just how incredibly intricate our vision is, from the way the placement of our eyeballs gives us stereoscopic vision, to the manner in which our brain interprets and reverses the images so that we see right side up.
An “Inverted” Retina
But in contrast to our intuitive recognition of its wonders, the human eye is often cited as an example of something that is sub-optimally designed, or so flawed that it could not have possibly been designed by an intelligent designer. Now, I know that my own eyes are certainly not optimal—just take a look at how thick my glasses are—but the argument is that even the most perfectly seeing eye is flawed, due to a critical “mistake” in the design of the human eye that can only be explained by an unintelligent evolutionary process rather than an intelligent designer.
A sampling of this argument:
But the eye betrays its evolutionary origin with a tell-tale flaw: The retina is inside out. The nerve fibers that carry signals from the retina’s light-sensing cells lie on top of those cells and have to plunge through a large hole in the retina to get to the brain, creating the eye’s blind spot. Any intelligent designer would be offended by such a clumsy arrangement. The human eye was not designed; it was inherited as the result of long-term evolutionary development. (Art Hobson, “Unintelligent Design”)
But only an idiot designer would have the wiring from the image-capture system (i.e. screen sensitive to light) intrude into the darkened cavity through which the light has to pass from lens to image-capture screen. The most blindingly obvious solution, which even a quite severely dumb designer can reasonably be expected to come up with, is to have the image capture aparatus [sic] receive light from one side (the cavity) and have its wiring go out to the image-processing system from its other side. (“Intelligent Design is not a Theory”)
An intelligent designer, working with the components of this wiring, would choose the orientation that produces the highest degree of visual quality. No one, for example, would suggest that the neural connections should be placed in front of the photoreceptor cells -- thus blocking the light from reaching them -- rather than behind the retina. Incredibly, this is exactly how the human retina is constructed. Visual quality is degraded because light scatters as it passes through several layers of cellular wiring before reaching the retina. Granted, this scattering has been minimized because the nerve cells are nearly transparent, but it cannot be eliminated because of the basic design flaw. Moreover, the effects are compounded because a network of vessels, which is needed to supply the nerve cells with a rich supply of blood, also sits directly in front of the light-sensitive layer, another feature that no engineer would propose. (Kenneth Miller, “Life’s Grand Design”)
The above illustration shows the forest of tissues and cells
that light has to travel through to reach the photoreceptor cells.
From http://www.dericbownds.net/bom99/Ch08/Ch08.html
Here is Richard Dawkin’s take:
Any engineer would naturally assume that the photocells would point towards the light, with their wires leading backwards towards the brain. He would laugh at any suggestion that the photocells might point away from the light, with their wires departing on the side nearest the light. Yet this is exactly what happens in all vertebrate retinas. Each photocell is, in effect, wired in backwards, with its wire sticking out on the side nearest the light. The wire has to travel over the surface of the retina, to a point where it dives through a hole in the retina (the so-called ‘blind spot’) to join the optic nerve. This means that the light, instead of being granted an unrestricted passage to the photocells, has to pass through a forest of connecting wires, presumably suffering at least some attenuation and distortion (actually probably not much but, still, it is the principle of the thing that would offend any tidy-minded engineer!). (The Blind Watchmaker, pg. 93)
The squid and the octopus are seen as examples of the way that an eye should be designed, or the “Obviously Correct Solution” as the writer of one of the above quotes put it. The image at the right compares the cephalopods verted retina with the inverted human retina (from “The Evolution of the Human Eye” by Sean D. Pitman).
A Designer Faux Pas?
So what’s up with this? Did Someone make a mistake in the design of the human eye? Or is this really an example of a perfectly natural evolutionary error? The answer is: Neither! To call the inverted retina a “design flaw” or an “error” is to turn a blind eye to the intricately coordinated series of systems and structures designed to bring clarity of vision to humans.
When an example of “sub-optimal” design is singled out, critics often focus narrowly on one apparently compromised aspect of a system, and miss seeing the big picture of how the entire system works as a cohesive and finely tuned whole. See my previous post on “Design Flaws” in the Human Breathing System” for another example. Though at first the inverted retina appears to be a backward design, a deeper understanding of the eye reveals just how purposefully the parts were arranged, such that optical engineers today are gaining new ideas on how to dramatically improve sensing equipment by applying the techniques used in the eye.
The Greedy Photoreceptors
Those who have studied the vertebrate eye in depth have concluded that, rather than being a “flawed design,” the inverted retina appears to be an ideal and necessary solution to the specific demands of the eye’s photoreceptors.
Biochemist Michael Denton writes, “[C]onsideration of the very high energy demands of the photoreceptor cells in the vertebrate retina suggests that rather than being a challenge to teleology the curious inverted design of the vertebrate retina may in fact represent a unique solution to the problem of providing the highly active photoreceptor cells of higher vertebrates with copious quantities of oxygen and nutrients” (“The Inverted Retina: Maladaptation or Pre-adaptation?” Origins and Design 19:2). He describes the incredible capability of the mammalian photoreceptor to detect even a single photon of light. This extreme sensitivity comes at a price, a “greedy” need for both nutrients and oxygen. These are provided for by a layer of capillaries (seen at the bottom of the illustration at left) connected to the photoreceptors by the retinal pigment epithelium (RPE). Without this continuous maintenance, our photoreceptor cells would quickly overload and stop functioning. This layer of capillaries and the RPE would completely block light from the photoreceptors if they were facing the inner surface of the retina, hence the need for the “inverted” retina.
Westmont College biology professor George Ayoub, goes in-depth on the importance of the RPE in his article, “On the Design of the Vertebrate Retina,” Origins and Design 17:1. From the abstract:
It has been commonly claimed that the vertebrate eye is functionally suboptimal, because photoreceptors in the retina are oriented away from incoming light. However, there are excellent functional reasons for vertebrate photoreceptors to be oriented as they are. Photoreceptor structure and function is maintained by a critical tissue, the retinal pigment epithelium (RPE), which recycles photopigments, removes spent outer segments of the photoreceptors, provides an opaque layer to absorb excess light, and performs additional functions. These aspects of the structure and function of the vertebrate eye have been ignored in evolutionary arguments about suboptimality, yet they are essential for understanding how the eye works.
He also addresses the issue of the “blind spot” and engages in a thought experiment that shows that visual acuity would actually degrade significantly if the human eye was wired any other way. So rather than being “wired wrong,” it seems that this is exactly the most optimal solution for our vision.
Living Optical Fibers
So is the degradation in image quality as light passes through these nerves and blood vessels an adverse but necessary compromise for our eyes? I would suggest that the more appropriate question here is, “Who says that image quality in our eye is compromised?” The statements given by evolutionists like Dawkins, Miller, and others imply that the inverted retina is obviously sub-optimal, and that visual quality is of course degraded by the fact that light has to travel through all those layers of cells and tissue. But is any evidence provided that image quality is actually compromised? The truth is that vertebrates generally have the sharpest visual acuity of all animal species, but how is this possible given such a backwards arrangement of the vertebrate retina?
A fantastic study conducted in 2007 explored this very question, and what they discovered was a new function of a certain type of cell in the vertebrate eye that in essence serves as biological “fiber optics” to channel light through the retinal layers to the photoreceptors. The study was titled “Müller cells are living optical fibers in the vertebrate retina” (2007) and was published in the Proceedings of the National Academy of Sciences (PNAS)104 (20): 8287-8292. The below graphic from the study illustrates how these Müller glial cells collect light from the inner surface of the retina and transmit it virtually intact through the layers of nerves and tissue to the photoreceptors.
From the abstract:
“…these cells provide a low-scattering passage for light from the retinal surface to the photoreceptor cells. Using a modified dual-beam laser trap we could also demonstrate that individual Müller cells act as optical fibers. Furthermore, their parallel array in the retina is reminiscent of fiberoptic plates used for low-distortion image transfer. Thus, Müller cells seem to mediate the image transfer through the vertebrate retina with minimal distortion and low loss.”
An interesting summary of the study and their technique can be found here, and includes the below graphic comparing an artificial fiber optic panel with Müller cells, the “living optical fibers.”
From http://www.vision-research.eu/index.php?id=160
Some additional fascinating details emerge from this article. Müller cells span the entire thickness of the retina, and widen at the inner surface into funnels that cover the entire retinal surface, thus making it possible, according to the researchers, to capture every photon that enters the eye. As the researchers described, “[T]he funnel-shape of Müller cells and their refractive index gradient provide an optimal optical coupling of the retina to the vitreous. Thus, Müller cells not only allow low-loss light transfer through the scattering inner retinal layers to the photoreceptor cells but also optimize the coupling of the retina to the transparent media of the eye.” No talk of sub-optimality here—quite the reverse, in fact.
Lead researcher Andreas Reichenbach commented, “Nature is so clever. This means there is enough room in the eye for all the neurons and synapses and so on, but still the Müller cells can capture and transmit as much light as possible” (quoted in “Living optical fibres found in the eye” by Lucy Sherriff). The design of these cells is so efficient that it could very well provide the innovation for the next generation of fiber optic technology. The ability of these Müller cells to transmit light from a wide surface area through such a small tube can lead to new “intelligent” sensors that incorporate computer circuitry attached to more compact fiber optic bundles.
To say that a design is “sub-optimal” is to imply that there is a better way to do something. So the next time you hear someone comment about the “flawed design” of the human eye, ask them to elaborate on what aspect of the eye they consider to be flawed. Is there any better way to wire the photoreceptor cells that would still connect them with the replenishing blood flow necessary to maintain sustained vision? And what reason is there to talk of “degraded visual quality” when the eye features living optical fibers that transmit images with greater efficiency than the best optical plate that engineers have devised?
The “flaw,” it seems, is not in the actual design of the eye itself, but rather in that person’s understanding of the degree of integration and coordination exhibited by all the parts making up the purposeful and optimal design of the eye.
Additional Resources and References
“Living Fibre Optics Light Up Our Eyes” – An excellent illustrated description of the 2007 study on Müller cells, with great pictures! Also describes how engineers hope to adapt the design of these cells into fiber optic technology.
Andreas Reichenbach, et al., “Müller cells are living optical fibers in the vertebrate retina”
George Ayoub, “On the Design of the Vertebrate Retina,” Origins and Design 17:1
Jerry Bergman, “Inverted Human Eye a Poor Design?,” Perspectives on Science and Christian Faith 52 (March 2000): 18-30
Michael Denton, “The Inverted Retina: Maladaptation or Pre-adaptation?” Origins and Design 19:2
Rich Deem, “Bad Designs in Biology? Why the "Best" Examples Are Bad” – A discussion of the common examples of “bad design” in biology are not so bad after all.
Sean D. Pitman, “The Evolution of the Human Eye”
Image Credits
Optomap retina image courtesy of Optos.com
All other images linked to their original sources.