Posted by Ian Musgrave on November 14, 2006 06:24 AM
In a recent article in Touchstone Magazine, Jonathan Witt, fellow for the Discovery Institute’s Center for the renewal of science and culture, has written a review of Francis Collins’ book “ The Language of God: A Scientist Presents Evidence for Belief”. Amongst other things in this review he claims that Michael Denton has demonstrated that the “backwards wiring” of the mammalian retina improves oxygen flow and is good design.
Denton of course, has done no such thing. Since I am on a role with things visual, I am reposting an updated version of an earlier article on this topic.
Just to recap, vertebrates (like ourselves), and the invertebrates Squid and Octopi have “camera eyes“. They differ in how the photoreceptors in the retina, the part of the eye that receives the image, is wired up to the brain. The vertebrate wiring system is often cited as an example of “bad”, or at least quirky, design that is explainable by evolution.
The vertebrate retina is wired “backwards”. That is the photoreceptors point to back of the retina, away from incoming light, and the nerves and blood vessels are on the side of the incoming light, this means that any image formed on the vertebrate retina has to pass though layers of blood vessels and ganglion cells, absorbing and distorting the image.
To get decent visual acuity, vertebrates must focus light on a small patch of retina where the blood vessels and nerves have been pushed aside, the fovea. This patch must be small because of the nutrient requirements of the retina. Also, the construction of the vertebrate retina means that blood vessels and nerves must pass through the retina, creating a “blind spot”, where no image is formed. Finally, the “backwards” retina means that vertebrates have a high risk of retinal detachment. Altogether this shows that having the nerves and blood vessels in front of the photoreceptors is less than optimal design.
Imagine taking a pane of glass, then smearing it thickly with vaseline, then wiping a tiny hole in the vaseline. That is what the vertebrate retina is like.
Now consider the eye of squids, cuttlefish and octopi. Their retinas are “rightway round”, that is the photoreceptors face the light, and the wiring and the blood vessels facing the back (1). Squid and octopi have no blind spot; they can also have high visual acuity. The octopus also has a fovea-equivalent structure, which it makes by packing more (or longer) photoreceptors into a given area (1). Because it doesn’t have to create a hole in the supporting tissue it can have arbitrarily large “fovea”, and greater visual acuity. Cuttlefish have better visual acuity than cats (2) and because of their “rightway round” retinas; this level of acuity covers nearly the entire retina (1,2) unlike vertebrates where it is confined to the small spot of the fovea.
The vertebrate retina is a prime example of historically quirky “design”. The vertebrate retina is backwards because the development of the retina was first elaborated in rather small chordates, where issues of acuity and blind spots were non-existent; all subsequent vertebrates got stuck with this “design”. Vertebrates do very well with the limitations of the design of the eye, but it is clear that this is no system a competent designer would make. Naturally, this annoys the proponents of an Intelligent Designer, and they have been looking for ways to put a better spin on the kludged design of the vertebrate eye.
ID advocates have a hard time dealing with the quirky design of the eye, both Witt and Behe have used the “better blood flow” argument in order to show the backwards retina really is good design.
This invokes an argument that has been doing the rounds of creationists for a while. The True.Origins site (which is a rip-off of Talk.Origins) has a page that claims that the “backwards” retina improves the blood supply. It is probably the canonical page where these claims come from. Denton’s argument is slightly different, but follows on from the canonical creationist argument, so I will deal with the creationist argument first.
In vertebrates, underneath the photoreceptors is a layer of pigment and pigment cells called the choroid (the squid, cuttlefish and octopus have similar arrangements - more on this later), this layer of pigment absorbs stray light that is not caught by the photoreceptors, which might reflect back and fuzz up the image.
In terrestrial vertebrates, the amount of light landing on the retina produces a significant amount of heat, enough to damage the retina itself (3,4). The True.Origins page gives the impression that it is light focused on the retina that produces the heat. The article implies that by having the most thermally sensitive bit of the photoreceptor bang up against a heat sink (the blood vessels of the choroid, whose rapid blood flow removes the heat, see below), vertebrates can tolerate light intensities that “right way round” retinas could not.
However, when one reads the paper they reference (3), a completely different picture emerges.
It is the choroid itself that generates the heat that threatens the retina! As noted above, the pigments in the choroid absorb light that is missed by the photoreceptors. This light is re-radiated as heat. 25-30% of the light falling on the retina ends up being absorbed by the choroid and re-radiated as heat (3,4). So we have the most thermally sensitive part of the photoreceptors bang up against the bit that generates the most heat. Good design? I think not.
To cool down the choroid, very fast blood flow through the tissues below and in the pigment layer is needed (3,4). But let’s be clear about this, the Creationists have it back to front. The “backwards” arrangement of the vertebrate retina does not make possible fast blood flow, it requires fast blood flow to cool the tissue down. This is yet another area where vertebrate design is flawed, with the fragile photoreceptors hard up against the source of the damaging heat.
Of course, the question of why fish, which have more species than all terrestrial vertebrates combined, must suffer with a backwards retina so that terrestrial vertebrates can have high blood flows to an area that wouldn’t need them if the system was designed correctly in the first place, is never addressed. The other question is why terrestrial gastropods which have camera eyes have a “right way round” retina if invert retinas are important for terrestrial vision? Their camera eyes are relatively small compared to terrestrial vertebrates, and so should loose heat readily. However, arthropod eyes of this size are subject to light-induced retinal damage. See the references in this paper.
In squid, octopi, cuttlefish and terrestrial gastropods, the pigment layer is below the photoreceptors, in an area of dense blood vessels (1). This arrangement blocks stray light and provides sufficient blood flow to cool the tissue and provide nutrients without the added layers of ganglion cells over the top of the photoreceptors that distort and absorb the image. Even better, squid, octopi and cuttlefish do not have the most thermally sensitive part of the retina next to the source of waste heat, as it is in vertebrate eyes, needing an outrageous amount of blood flow to cool the system.
The vertebrate eye does very well indeed, but it is a kludge. The fovea is a cute trick to squeeze greater acuity out of a flawed design, but octopi and squid do it better. The cooling blood flow to the choroid is needed as the pigments of the choroid generate waste heat, but this is irrelevant to whether the photoreceptors are forward or reverse facing. The arrangement of the vertebrate eye does not improve the blood supply, and it looks like the vertebrate eye has to kludge up a high blood flow to the choroid because the vertebrate inverted retina is poorly designed to get blood to where it is needed.
This brings us to Denton’s argument. This is that the blood flow through the choroid needs to be high for the metabolic requirements of the retina. This is a variant of the “cooling bath” concept, and has exactly the same problems. The retina is an energy hungry system, but it doesn’t need to be inverted to get a high blood flow. In fact, the way the vertebrates do it is just plain silly. Molecules used for providing the energy to run light detection are formed in the mitochondria in the cell body from blood born nutrients, then passed along to the photoreceptors in the modified cilia projecting from the cell body (see diagrams in links above). As the retina is invert, the cell bodies are further away from the choroid, with the light harvesting disks between them and the choroid. Consequently, all blood born nutrients delivered by the choroid in vertebrates must diffuse from the choroid, through the pigmented epithelium, then past all the photoreceptor disks to the mitochondria in the cell body to be used (and all waste diffused in the reverse direction). Delivery from the cell body end would result in a shorter diffusion distance through less restricted space; ie, more efficient delivery. This point is born out by the fact that choroid oxygen tension drops by only 3% from artery to vein. In consequence, the retinal artery, though it only carries 5% of the blood supplied to the retina, carries 40% of the oxygen used by the retina.
Blood absorbs light strongly, …. From this we can immediately discount one possible way of supplying the photoreceptors in a non-inverted retina where the photoreceptor would form the inner layer–pointing directly towards the light, i.e., by placing a choriocapillaris-type system of blood vessels in front of the photoreceptor cells, i.e., between the photoreceptors and the light. While such an arrangement might well deliver sufficient quantities of oxygen to the photoreceptors, the sensitivity and acuity of any such hypothetical “eye” would be greatly diminished by the highly absorbent complex of blood vessels positioned between the light and the photoreceptor layer
This is pretty silly, with the current arrangement, the photoreceptors have a range of ganglion cells, supporting cells, nerve cells and blood vessels already piled thickly on top of it (when you look into the eye with and ophthalmoscope, you can see the superficial blood vessels on top of the retina, there are also capillaries that dive deep into the cell layer as well. The retina already has a mass of blood, and lots of other things, getting in its way. Of course there is a better way to do it, the way cephalopods do it.
In cephalopods the blood vessels are right next to the terminal parts of the photoreceptor process, the photoreceptor cell bodies and the pigment cells where it is needed. You can see the blood vessels and pigments in this paper on the octopus retina. It is far more efficient than the vertebrate system for both cooling and nutrient delivery. No wonder cephalopods require a much smaller blood supply to the eye.
Both the “cooling bath” and the “nutrient/oxygen delivery” arguments actually reveal that the vertebrate eye is a kludge. The high flow rates are required because the quirky design means more efficient methods can’t be used.
Denton brings in other arguments for the “superiority” of the vertebrate “back-to-front” retina, but they are irrelevant. Fore example, vertebrate photoreceptors can detect a single photon as he claims, great, but so can cephalopod photoreceptors, and they are not covered with gunk that absorbs or scatters the incoming photons. Cephalopods occupy many niches, from shallow water tidal zones with high light intensities to the abyssal depths where every photon counts, some are ambush predators, and some are active hunting predators. Some see in black and white, some see in colour, some see polarized light (which vertebrates can’t). Many have visual acuity equivalent to many vertebrates; cuttlefish have equivalent visual acuity to cats as befits their status as active hunters. All this without an invert retina. When Denton says
that in redesigning from first principles an eye capable of the highest possible resolution (within the constraints imposed by the wavelength of light16) and of the highest possible sensitivity (capable of detecting an individual photon of light) we would end up recreating the vertebrate eye
he is just plain wrong.
The pre-adaptation concept Denton prattles on about is nonsense. We are to expect that an intelligent designer will give the marine vertebrates, which are significantly more numerous in species and population than the terrestrial vertebrates, a poorly designed retina so that a very few percent of all terrestrial vertebrates can have supposedly superior vision? This is a definition of “good design” of which I was not previously aware. And again, cephalopods do it better.
Let’s be clear, the vertebrate eye works, and works rather well given its limitations (one merely has to contemplate the visual acuity of the eagle to see that the “design” works well). But it is a suboptimal Heath Robinson “design” where the limitations of the original invert retina setup (which were irrelevant to amphioxus and the small chordates in which the vertebrate eye evolved) are worked around by kludges. It is like claiming that the misground Hubble mirror with its correcting lenses is the “best possible design” because it gives clear pictures.
Once again, the vertebrate eye fails as Intelligent Design. ID proponents loudly proclaim they are not creationists and one is left to wonder why they have appropriated a bad Creationist argument.
(1) Matsui S et al., Adaptation of a deep-sea cephalopod to the photic environment. Evidence for three visual pigments. J Gen Physiol. 1988 Jul;92(1):55-66
(2) Schaeffel F, Murphy CJ, Howland HC Accommodation in the cuttlefish (Sepia officinalis). J Exp Biol. 1999 Nov;202 Pt 22:3127-34.
(3) 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.
(4) Parver LM. Temperature modulating action of choroidal blood flow. Eye. 1991;5 ( Pt2):181-5.
(5) Denton, M The Inverted Retina: Maladaptation or Pre-adaptation? Origins & Design 19:2