Simple Eye Make Up-What stages of evolution are there to have the simplest eye?

by Serena.

Question by @al: What stages of evolution are there to have the simplest eye?
For an unicellular spicy having advantage of an eye, it needed at least nerve fiber or some kind of connection to brain, an understanding of the information it receives and also needed intelligence for reacting upon receiving that different data, all this together makes a functional simple eye and an advantage in survival. Without one of them the light sensitive spot should have been gone before it evolved to become a functional eye because of economical adoption.
For example: as I know the simple organisms mutated to have light sensitive cells on the body and they could only understood the existence of the light or differences of day and night, so they could move or stay near to water surface closer to sunlight, from that point evolution of human eye looks understandable but I have trouble understanding the evolution of a functional light sensitive cells.

Best answer:

Answer by novangelis
Many neurotransmitters are also hormones. A substance secreted by a cell which induces a chemical change in a neighboring cell, which in turn releases another chemical which impacts a third cell has done the same thing as a neuron, only not as quickly, and there are many intercellular reactions like this. The electrochemical action potential of a neuron accelerates the process. Skin and nerves derive from the same tissue layer. A brain is overkill; all you need is a intermediate cell (such as a nerve) that arcs to any primitive response (e.g. a muscle that contracts in response to such). Brains (local concentrations of neurons) and complex network produce create more potential modulations of responses, but any cell that links a photochemical response to the trigger is a very primitive visual system.

Being able to switch from a slow process (e.g. floating to the surface in the morning and sinking at night) to a rapid process (reacting to the shadow of a potential predator) is a matter of the intermediate cell specializing for speed.

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