h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Though it has been covered better elsewhare, I'll try and present a short piece about the evolution of the eye.

It is a commonly used agument that half an eye is worse than useless, but the many intermediary stages between complete blindness and a fully lensed and iris eye can provide a huge evolutionary advantage for the posessing organism. In the land of the blind, the one-eyed truly can be king.

An animal posessing a simple flat patch of nerve cells near the suface of the skin with a small response to light can detect a shadow passing across it, caused by a large predator swimming between it and the light above. Alternatively, given sensory cells arrayed around the animal's body, it could detect where 'up' is, or be able to navigate into, or out of, crevices under rocks.

If the patch of sensory cells becomes depressed into a pit, directional information can be obtained, enabling the animal to better gauge which way to swim away from danger, or towards a desired destination. Changes in the shape of the pit can improve matters.

Increased transparency (at any time) of the cells coverng the sensory cells can increase sensitivity while providing mechanical protection, and in an animal with a pit of cells, if these cells form a crude focussing cornea/lens, the efficiency is improved, and further modification to the lens shape will provide better information.

Mutations (at any time) that improve the sensitivity of the optically detecting molecules are a clear selective advantage.

Given a sufficiently sensitive eye, some method of closing off part of the light (iris or external covering cells) can help the eye to adapt to varying light levels, as well as improving the focussing of the eye (the smaller the aperture, the better the depth-of-field).

From a design point of view, the human eye is not perfect - the retina is back to front, with the cells linking the light sensing cells to the brain being in front of the sensory cells, which results in the blind spot where the bundle of neural wiring passes through the sensory layer, and lower sensory efficiency. Any half-decent designer would not have created it the way it is.

Doesn't it also say a lot the way he talks about "The eye". Like there is only one kind. There are of course several different models of them.

Another point - eyes not wildly dissimilar to humans have evolved independently several times. Example - the eye of the octopus. Most major features in common with the human eye (retina, lens, pupil, humours) but evolved completely independently from the eyes which led to ours.

H.

Yep, I have to say that the 'back-to-front' aspect of the eye has always baffled me... no doubt the examiners will ask me a question to catch me out on that...

It may be a developmental feature - in fact, I strongly suspect that it is. Many things which seem totally convoluted in the human body are actually down to what happens in the womb, and how certain structures develop before others, or indeed, render themselves redundant after birth.

Take the case of the recurrent laregeal artery. It recurs around something called the ductus arteriosum - a small, redundant structure which used to be a blood vessel that connected the pulmonary arteries to the aorta (please don't ask me to explain that, or I'll cry).

This nerve goes down from the head, enters the chest, then does a u-turn, and goes back up to supply motor fibres to the muscles of the vocal chords. Simply becase when it was first developing, it was pulled down by this vessel - which is now redundant.

Stay ,
WD

From a practical point of view, the actual effect of the blind spot isn't too significant.
I guess in the case of animals with a reflective tapetum, the bounced-back light will only have passed through the neural layer once, rather than twice, so some efficiency lost from the inverted design (assuming it's significant in the first place) may be countered. Of course, that depend on the sesory and communication layers having to be next to each other. The best design would be sensory layer -> reflective layer -> communication layer.

Does anyone know if non-vertebrate eyes have similar (or different)design issues?

Actually, the human eye is perfect for where we live. Some try to say that the octopus eye is the best eye and that the human eye needs to be like the octopus eye because there are no nerve cells blocking the retina. The eye is perfect for the octopus because of its environment. Since there is less light in the ocean, the octopus needs the least amount of visual obstruction it can get, that way it can see shadows. For humans, we have the best eye we can have for our environment. Anyone who says humans need eyes like an octopus is off their rocker. If humans had eyes like an octopus, the minute we would leav the womb, the light would blind us. The reason why there are nerve cells and blood vessels blocking the retina is to protect it from the sunlight which would burn the retina in the blink of an eye.
Therefore, the human eye is perfect.

Hmm that doesn't sound quite right. It is only in one spot the nerve cells go in front of the eye. And the rest of the eye is not burned to bits anyway.

"the human eye is perfect"

I'll remember that the next time I can't find my glasses...

H.

Well, perhaps the "human eye is perfect barring mutations like Hoovooloo's", then

Hurray! I'm a mutant! Does that mean I get to join the X-Men?

H.

Unfortunately, I believe the position of 'mutant with dodgy eyes' is already taken. The Mystery Men might have an opening for you tho'...

I'm unsure what part of the eye would burn up if the retina *were* the other way around. I assume either a smaller pupil (with attendant increased depth-of-field (good) and diffraction (bad) would be one solution. I wouldn't have thought that it was the photosensitive pigments that were the problem, as they already provide a pretty effective autoexposure system through their reversible bleaching properties, and I can't see it would be desperatel;y hard to evolve less sensitive pigments either.

In my wish-list for the 'perfect' eye, I'd have a greater depth of field, greater focal range to allow for short/long sight to be corrected internally, and much faster adaptation from bright light to full night vision. Especially for someone like me, 15 minutes simply isn't good enough.

Better than colour vision could be useful as well, though I suppose tetrachromactity or better with added UV capacity could make TV sets and photography more complex. "What, you only use *three* different phosphors and have the nerve to call this a *colour* set?"