After doing some research on the evolution of the eye, I compiled this simple explanation of the key points. Anyone with greater knowledge please correct anything mistaken - and questions / comments welcome. Any writers that want to correct grammar/sentence structure issues would be great also.
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An eye, simply put, is photoreceptors + structure.
The structure can be something as simple as a cup-shaped depression or pinhole lens, or as elaborate as our camera eye, an insect’s compound eye, or the mirror eyes of a scallop. Each of these various structures from the very simple to the very complex are useful to its owner and ideally suited to their environment.
The simplest type of eye does nothing more than sense light and movement. More complex eyes, like our own, collect light from our surroundings, regulate its intensity through a diaphragm, focus it through an adjustable assembly of lenses to form an image, convert the image into a set of electrical signals, and transmit the signals to the brain.
The human eye is called a camera eye, since its mechanics resemble those of a film-loaded camera. Human eyes have 126 million photoreceptors on the surface of the retina. Birds also have camera eyes, but with 10 times the amount of photoreceptors their vision has a much higher resolution than ours.
Complex eyes evolved over a long period of time from a much simpler structure—one that could only sense the difference between light and dark. The intermediate stages along the way, from the simple to the complex, were completely functional and useful to each organism, and became progressively more useful with every improvement. Similar key intermediate stages can be observed in modern animals.
One of the simplest eyes today is found on the Euglena—a single cell organism. This tiny creature has an eyespot at the head end of the cell, which cannot form an image, but can distinguish between light and dark. The structure of this eyespot is simply a photoreceptive cell and a pigment cell. Some other transparent creatures do not have pigmentation, and can only sense if they are in light or in darkness. The pigment cell serves as a ‘backing board’, allowing the creature to have a sense of its bodies direction in relation to light. This simple eye structure helps the Euglena find an environment with optimum light conditions for photosynthesis, and establish day/night cycles and coordinate behaviour.
As cells replicate themselves, mutations randomly occur—which are the catalyst for evolution. A mutation that in some small way gives a creature an advantage and allows it to survive longer and reproduce, is natural selection at work.
Natural selection is the gradual process by which biological traits become either more or less common in a population as a function of the effect of inherited traits on the reproductive success of organisms interacting with their environment. In a competitive world, even the slightest advantage provides a greater chance of survival. Mutations which allow the creature to better escape predators, better catch prey, or better take advantage of its environment, gives it a higher chance to reach maturity and pass on, through it’s genes, the advantageous trait to it’s offspring.
An indented eyespot is just such a favourable trait.
A flat eyespot can detect light, but an indented eyespot can detect the direction of light. Light hits the photoreceptors on one side of the indent, and casts shadow on the other side, indicating the direction of the light source—this type of eye is called a cup eye. Even an extremely shallow indent is a marked improvement on a flat eyespot and an advantage to the organism.
The Planarian worm, which is commonly found in ponds and rivers, is a modern creature with just such eyes. A seemingly small advantage, but one which allows the Planarian to detect the shadows of predators passing overhead—a very handy ability.
Gradually, over many generations, the cup deepened—with each small improvement allowing greater precision. A creature that could react more quickly to predator or prey, and navigate more succinctly in response to light, had a clear advantage. Chance of survival to sexual maturity were higher, and reproduction passed the advantage on to its offspring. Natural selection drove the deepening of the cup eye forward in this manner. Organisms with deeper cup eyes flourished, and those without did not, or adapted and evolved in different ways.
The cup deepened until the rim began to constrict and formed a narrow opening. A light source shining through the opening hit an increasingly smaller and smaller area inside the eye, allowing greater perception as to its direction. Predator or prey momentarily blocking out such light indicated their relative position.
As the opening grew narrower it began to work as an aperture, forming a vague and blurry image against the retina—the narrower the opening, the clearer the image. The eye of a Nautilus is a perfect modern example of this. It has what is called a pinhole eye, which works using the same principle of a pinhole camera—light goes through the pinhole and projects an inverted and reversed image. Vision from such an eye is fairly dim because the small hole only allows a minimal amount of light to enter. It is also not as sharp as when using a lens, but for the Nautilus, a pinhole eye works perfectly for its environment.
The evolution of the camera eye however, branched off at a sooner point than this, when the cup eye still had a wider opening. Around this time, a jelly-like mucus secreted by cells began to form inside the eye to help hold the shape of the cup, to protect the light sensitive cells from chemical damage and to keep mud and other debris out of the eye. This adaptation is quite simple for most creatures, requiring perhaps only one or two mutations.
Anything translucent and refractive, through which light can pass and bend, works as a lens. The transparent jelly that formed in the early cup eye better focussed light on the retina, and formed an initially vague image. Mutations with slight increases in the refractive index produced greater visual acuity. Slugs and sea snails have these kind of primitive lens eyes consisting of translucent jelly.
When the jelly substance hardens, it forms a hard lens found in creatures such as the Octopus, whose eyes are very similar to our own. The octopus eye is called an aquatic camera eye and has 25 million photoreceptive cells to our 126 million.
For the complex focussing mechanism, some species move the lens back and forth, some stretch the lens flatter. Another mechanism regulates by controlling growth of the eye and maintaining focal length.
Complex eyes evolved perhaps 50—100 different times, using similar techniques, some arriving at a very similar outcome, others taking different evolutionary paths. The human and the octopus eye have an interesting distinction that highlights the different evolutionary paths they both took.
Octopus and all cephalopod eyes, have photoreceptive cells assembled in a similar fashion—pointing towards the opening of the eye, with the nerve fibres coming out of the back of each cell and trailing off to connect to the brain. This seems to be a logical arrangement.
Vertebrate’s eyes however, have the photoreceptors pointing towards the brain, with the nerve fibres coming out on the inside of the retina. Thus, to connect to the brain, the nerve fibres must run along the surface of the retina and route together through a small hole in the optic disc. This conglomerate of tiny nerve fibres forms what we call the optic nerve, which connects to the brain.
Where the optic nerve passes through the optic disc, there are no photoreceptive cells to detect light, and a part of the field of vision is not perceived. Normally however, we do not notice the blind spot, because the brain interpolates it based on surrounding detail and information from the other eye.
To demonstrate the blind spot in each eye, close one eye and focus the other on the appropriate letter (R for the right, or L for the left). Keep your eye a distance from the page approximately 3 times the distance between the R and the L. Move your eye towards or away from the page until you see the other letter disappear.
Besides this small blind spot, the backwards wiring has little effect on our vision. The nerve fibres that travel along the inside of the retina are so thin that a projected image is not impaired by them. The photoreceptors also work just as well in their backwards facing positions.
The reason for this seemingly counter-intuitive outcome is that much earlier in our evolutionary history, when the eye was still in its simple stages, the photoreceptor cell developed facing inwards. Which made no difference at this early stage—only later, as the eye grew more complex did the optic nerve develop to deal with the problem. All invertebrates on our evolutionary branch, have optic nerves. Cephalopods however, who took a different evolutionary branch, do not.
Evolution does not go backwards to correct mistakes. It does not move towards perfection or any kind of ultimate goal, but simply uses the tools it has to work with to better equip an organism to adapt to it’s environment.
All the variation in the animal kingdom is a product of environment.
The mantis shrimp, which mostly lives near brightly coloured coral reefs, has eyes with far more colour receptors than we have. As a result, they can see 16 pigments compared to the 3 pigments human eyes can detect.
Birds, which also have an optic nerve and thus the same blind spot as us, have significantly better vision than ours. The human eye has a cluster of photoreceptor cells in the centre of our retina—surrounding the optic nerve. However, along the walls of the retina moving towards the lens, the photoreceptors become less dense. This causes the centre of our vision to be clear, and our periphery vision blurred. Bird’s eyes, have photoreceptor cells spread out more evenly towards the edges of the retina meaning their peripheral vision is crystal clear and completely focussed.
As diverse and complex as eyes are, they all evolved from simple beginnings, using the very same biochemical tool kit:
Photoreceptors + structure.
