in small steps
The Big Question ... to me
The biggest question in evolution to me is the development of multi-cellular animals. This took hundreds of million of years. To begin with, each cell fought it out with all the others. At some point, they stuck together. First as colonies ... then as individual multi-cellular animals. The difference is often arguable. If an individual cell can be culled from the "mass" and it survives on its own, I guess that's a colony. If it is too specialized to survive, it's part of a single animal.
I believe that a sponge and a jellyfish are considered colonies even though the cells are "stuck" together to make a single thing. But controversially, an ant colony is sometimes called a single animal even though its individual members can't survive (for long) on their own. I refer you to Sir David Attenborough.
At any rate, notice that in multi-cellular animals, you don't find an organ made from just one cell. Nature uses a large amount of redundancy in making organs. You wouldn't risk any important aspect of an animals life on a single cell. They die off to quickly.
The Single Cell
Single cell animals move around. That's one of the first things that nature endowed them with ... an ability to move around in the environment. Those that don't move are generally called "food". If you move ... you encounter more food than if you just sit there and wait for it to come to you.
And because you are moving around, you develop a front and a back. An amoeba doesn't have a front or back because it's that primitive ... it just moves, period (just the first step in evolution).
Why have a front and a back? Because you want to streamline the cell to go through the water with greater ease. Less energy for swimming around requires less food and more territory is covered in the search for it. A spindle then has a front and a back end. But which is which? What determines the definition of front and back?
In addition to moving around, it would be advantageous to acquire information about the environment through which you are now moving with greater ease. If an organism can sense heat, chemicals, vibrations, light, etc. it can move around with a directed purpose. Just moving randomly is less energy efficient than moving directly to the source of energy.
Now, the development of a sensory organ requires energy too. You want just the number that gives you an advantage ... not so many that you waste energy producing unusable redundancy. So, for our first, hypothetical "eye" on a multi-cellular animal ... not a huge animal but something the size of a hydra for instance ... let's put some sensory cells on the front end. That's the end that encounters the environment first so that's where sensory organs are initially placed. (And that's why your face is so crowded with such sensory organs ... smell, taste, sound, light, some touch, balance?)
Check out the rollover image below.
Here, we put some light sensing cells on the front end. They provide light direction information only. They can do so because they are situated on the front with the main body blocking light stimulation from the rear. This is a key point. If the body did not block light from the rear, the cells could be stimulated from any direction and would be rendered useless.
Now, you can understand why the initial setup tends progressively to the concave. The more some cells are shielded from the rear, the more effective they are at pinpointing the direction of the light source. Now, if we continue this natural progression, we end up with a ball with a small opening at the front. This then maximizes the acquisition of directional information. We can do no better than this.
Also, at this point we will have a covering over the opening develop to bar stuff from getting into the ball (clutter that would render the eye useless useless). If it was a mouth, there would be no cover since the purpose would be to admit visitors who are most welcome. Nature likes to make cavities (for mouths and other things) and extrusions (for limbs). They are extremely practical.
One of the unforeseen advantages of the above development is that the small opening at the front of the ball is that it forms a camera obscura (a pinhole camera). What this does is put a shadow of any object passing between the light source and the detector on the cells at the rear of the ball. That's the next great leap forward in the development of the mammalian eye.
If you now develop a larger brain system, that shadow information can be processed into a pursue or flee strategy depending on whether you are predator or prey.
Something here about the camera obscura ... if the opening is too big you get a fuzzier image ... if too small, you get a better defined image but less illuminated since less light gets in. There is a compromise to be made ... and ... another organ develops.
A sphincter muscle closes your mouth and anus. You have something similar in your eye to lower or increase the amount of light being admitted ... an iris. This is the next development in the eye. It is neither more nor less improbable a development than the door at your rear end. It adjusts the amount of light that gets in and thus the animal can "choose" the compromise setting that suits its present needs.
It would certainly be advantageous to be able to discern not only a blurry motion but also the shape coming at you. Is it a female or a predator? Perhaps just the shape would be helpful.
Color and Shading
This is a terribly obvious development. Clearly, if you could not only detect a shape but also gradations in the amount of light ... reflected off of ... an object, you'd have a real advantage because your probability of acting correctly on this better information would increase.
The same would, of course, be true for adding the luxury of color receptor cells. What's required here is more brain power to utilize the information.
Remember our covering over the opening? It hasn't been idle in its development either. If it gets thicker or thinner, it affects the sharpness of the image projected onto the back of the ball when the opening is made bigger or smaller.
Nature seems to like to make layers over layers (as in skin) so there may be multiple layers of coverings. We can then insert between those covers a collection of fluid. More fluid makes for a thicker lens. It's got to be of a different refractive index than the fluid in the ball though ... and also it must be clear.
And ... since it is a different material than the internal fluid, we might as well let it harden a bit into a lens. If it's somewhat rubbery, we can then mush it from the sides to make it change shape as well with another sphincter gizmo. Then, it will be able to alter itself at the will of the animal which yields ... what? All together now ... Depth Perception by way of depth of field as in an SLR camera. What an advantage an ability to focus is !!
While we're at it ...
Let's make it steerable. If you can move your eye around you don't have to move your whole body to look to the side. That's an advantage.
And while we're at it ... I'll take two of 'em (up front of course). That gives depth perception by way of trigonometry.