Friday, July 13, 2012

Our Eyes: Suboptimal Design?

Rabbi Klinghoffer responds to Richard Dawkins about how an engineer would reject the eye because of its suboptimal design. I like the Rabbi's argument that the eye is still much better than anything engineers have been able to design, so it's a little premature to reject it. But then the question still remains, is the vertebrate eye suboptimal design? Could it have been designed better? According to the blogger "shkrobius," no: Why do We Have the Blind Spot?:

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.

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

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.

I don't know if shkrobius is right, but it's interesting.

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