Quizzing the Anonymous - Why do we have the blind spot?
Why do we have the blind spot?|
The "blind spot" (anatomical optic disk) is often given as the example of "nonintelligent" design. The contention is that having a region where the optic nerve and the blood vessels enter the retina creates a "hole" in the visual field that is only partially compensated by having two eyes and advanced neural processing. The greater claim is that having neural connections and blood vessels in front of the photoreceptor cells is detrimental and can only be explained by the haphazard character of evolution. Then something like that is added:
...Do these design problems exist because it is impossible to construct an eye that is wired properly, so that the light-sensitive cells face the incoming image? Not at all. Many organisms have eyes in which the neural wiring is neatly tucked away behind the photoreceptor layer. The squid and the octopus, for example, have a lens-and-retina eye quite similar to our own, but their eyes are wired right-side out, with no light-scattering nerve cells or blood vessels in front of the photoreceptors, and no blind spot. http://www.pbs.org/wgbh/evolution/change/grand/page05.html
see http://www.bio.davidson.edu/people/midorcas/animalphysiology/websites/2003/Muller/structure%20of%20the%20cephalopod%20eye.htm for the diagram of squid eye
...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. http://wiki.cotch.net/index.php/Vertebrate_eye#Vertebrate_eye
I've picked these two examples at random, by Googling, as these two dubious arguments are repeated in countless popular books and web sources, which does not make these any more convincing. In a more developed form, the argument can be read here http://www.pandasthumb.org/archives/2006/11/denton_vs_squid.html
I cannot trace the origin of this oft-repeated wisdom, but these are the classical arguments from suboptimal design. The problem with this sort of argumentation is that it is nearly always incorrect. The blind spot is no exception to this general rule. It has nothing to do with evolutionary biology vs. creationism, as the real problem is evolutionary reasoning itself. Vision is so central to animal way of life that the "suboptimal design," quite simply, would not do. Our eye has to be designed this way and the right approach is to seek the reason. In this case, the reason is that the "superior" cephalopods have different blood chemistry and oxygen metabolism. The idea that the eye started as a sparse set of photoreceptors that could've been innervated from either side is nonsense. The story of the blind spot is much more interesting and much more revealing about biological evolution than the pedestrian versions given above.
Our eyes have ciliary photoreceptors that are incapable of discerning the polarization of light (unlike the rhabdomeric photoreceptors of the molluscs, insects, and other arthropods). All animals, including both the vertebrates and the cephalopods, got their eyes from a common prebilaterian ancestor; the basic structure, the optical cascade that belies it, and the opsins themselves are monophyletic in origin, eg. http://www.pnas.org/content/105/40/15576.short
This line began to diverge about 600 Mya: we have c-opsin/Gt protein combo (typical for the ciliary photoreceptors) while the cephalopods have r-opsin/Gq combo (typical of the rhabdomeric photoreceptors), see e.g.
We do not know what was that first ancestor of all eye-equipped animals, but the eye proper has been developed only once, and the simplest animal that has it is a box jelly shown above. This most "primitive" eye already combines ALL of the specialized eyes seen in later animals: a lens eye, a pit eye, and a slit eye - alltogether. What it does not have is a brain attached to it, because the jellies do not have the nervous system; they only have neural nets. How this eye functions can be read in
Sensory physiology: Brainless eye, R Weher Nature 435 (2005) 157
The idea that different types of eyes have developed independently has been abandoned some time ago. See Gehring's 2005 review on this subject. Both the molecular design and the developmental genes for the eye are highly consereved among all bilaterians. The eye has started as a complex organ with dense photoreceptor area; what was before was not the eye proper, and it simply does not matter how that something was innervated, if it was innervated at all. There was no blind spot in these so-called "primitive" eyes (that were already nearly as complex as the eye can be). That was not due to the sparcity of the receptors. The reason is different: there was no brain to connect to. The eye had very minimal innervation and no blood vessels whatsoever, because the first animals (such as the jellies and flatworms) did not have blood; the oxygen freely diffused into their bodies, including the eyes.
The dilemma of the blind spot arises only when the oxygen is delivered using blood. That is where the crucial difference between the cephalopods and us emerges. The cephalopds belong to the clade of animals having blue blood: instead of haemaglobin in which O2 is carried by Fe in a porphyrin ring, they use haemocyanin that has two Cu complexed by two sets of triple histidine ligands. These are two entirely different O2-carrier designs showing independent origin of these two blood systems. The difference is crucial, because heamocyanins of the cephalopods do not bind O2 in a cooperative fashion, as done by the haemoglobin complex in other animals, so it is only 25% as efficient as an O2 carrier. To compensate, the metabolic rates have to be increased. Furthermore, the cephalopods do not have blood cells; their oxygen carrier is extracellular, freely floating in blood and through the tissues. Their main competitor, fish, has the superior, cellular blood design, and so the cephalopds have to increase both the uptake and the delivery rate of O2:
...the capacity of hemocyanin for carrying oxygen is limited. This is due to the unfavorable increase in colloidal osmotic pressure and blood viscosity at high pigment concentrations. At an oxygen-binding capacity of only 3 mM (as opposed to 10 mM in fish), cephalopods rely on fully oxygenating their pigment at the gills and on releasing the majority of bound oxygen during each passage through the tissue capillary beds. Under resting conditions, about 80% of bound oxygen are being released in the tissues in the cuttlefish S. officinalis. http://icb.oxfordjournals.org/cgi/content/full/47/4/645
The cephalopods pass huge amount of water through their gills, extract lots of oxygen and immediately deliver it to their organs in a single pass. Now, it happens that our retina is one of the highest O2-consuming tissues of the body. It is, for example, consuming more O2 per gram than brain. It combines frenetic pigment synthesis with expensive neural processing which requires a lot of oxygen and nutrients to sustain. Like the muscle tissue that has its own O2 carrier, myoglobin, the retina has its own O2 carrier, neuroglobin, but O2 is delivered to the retinal receptors directly by haemoglobins in the blood vessels. These vessels are absolutely essential there, next to the cones, because oxygen has no means of diffusing through its thickness at such consumption rates, see http://www.jbc.org/cgi/reprint/M209909200v1.pdf
The true reason why cephalopds do not have the optic disk blocking their visual fields is that they can get away with it. They deliver oxygen to their retinas using extracellular haemocyanins, so they do not need the blood vessels to go right next to their rhabdomeres. Faster O2 metabolism and lack of cooperativity make this mode of oxygenation possible. The animals that deliver oxygen using haemoglobin containing specialized blood cells cannot allow themselves such a luxury. Since the arterial blood has to enter the retina anyway, Nature chose to use this entrance for the optic nerve, which makes good practical sense: the nerve grows around the artery, so the nerves get their oxygen, too. Better still, our sharp central vision is by fovea, which is cleared of blood vessels and innervated and oxygenated from behind, through the choroid. That is the reason why the fovea (which is only 1% of the retina) operates under hypoxic stress under bright light and why we avert our eyes from it. The marine animals do not encounter this problem because they do not deal with bright light, so the oxygen demand of their retinas is lower. That is another reason they can get away with letting the oxygen only through the posterior arteries.
The blind spot is not about nerves; it is about oxygen and blood. The design of our eye is optimal for us and the design of cephalopod eyes is optimal for them. It would be ridiculous to redo the blood chemistry for solving a minor problem with the oxygen supply to the retina, so a different solution was found. I do not think it is possible to have cellular delivery of oxygen and cameral eyes in any other way. If that were possible, it would've been tried over the 500 Myr that the two exist. It is ridiculous to lay a claim of suboptimality for the design of such ubiquity and antiquity.
In short, if you want to have clear vision, have blue blood. On this issue, Mother Nature sides with the Victorian gentry.
Why do we have the blind spot?
Tags: evolution, whys
very interesting, thank you!