For a cellular creature to evolve there must be some incentive for it to do so. In the natural world incentive is optimized survival; simply because things that want to survive and over-achieve so do and those that do not quickly become extinct. The things that help an organism survive and over-achieve are:

Better food/energy gathering capabilities
Better survival
Better reproductive success.
Better survival of offspring.
Better metabolic economy.

This list could be expanded or condensed depending on your definition of the catagories. For instance you could expand 1. could be subdivided into energy gathering and food gathering. Energy gathering could then be subdivided into Light gathering and heat gathering and chemical gathering. Possibly 3. and 4. could be combined into a single category. Here isn't the proper place to get particular, I'm sure you get the idea. However as you proceed into this investigation you will find that you do need to get progressively more particular in your details. Its the same for any field of study.

Now you have to ask: 'What could happen to (my) cell that would increase its performance in one of these catagories?'

To take an example from nature. The ancester of Blue-Green Algea developed a resistance to oxygen poisoning. In the early Earth the presence of life lead to (over a longggggg period of time) Carbon being buried and Oxygen polluting the atmosphere. Oxygen was a poison back then and bacteria had to develop the ability to go down or die in the poisonous heights. BG algae however evolved an Oxygen tolerance was then able to exploit the higher waters that the other bacteria had abandoned. This increased its achievement and reproductive success as it had un-competed resources to exploit.

Unfortunately in the higher waters there was also Light (another poison). So BG developed a photopigment to protect itself from the radiation. Again opening up to it regions that other creatures avoided. BG then developed a means of trapping the energy in Light for its own metablism. A BIG step forward and the BG population explodes.

So now BG wants to stay up near the Light. So some become sticky and cling to the rocks up where the Light is. (Increase in energy gathering) But being sticky also increases your chances at getting buried. So BG develops sticky free tails that wave in the currents. Today a BG is a linear chain of cells.

Now in developing such cause and effect evolutionary chains you have to remember that you can only work with what you have. You can not suddenly decide to add an eye onto your creature without having some precursory organ that could conceibably give rise to such a function.

Considering BG and the developement of the eye. We start out with a photosynthetic chemical and a linear collection of cells. The cells higher on the chain of course get more light and are more industrious while those lower down get shaded and have a miserable existence. It would do the collective creature good if those higher-up shared their wealth with those lower-down as both serve a vital function to the creatures survival and reproductive success. So what if the actual cell structure of BG were to become a more expanded pointy star-shape (3D of course). Those up near the surface would suffer when the tide went out as the increased surface would hasten dessication. Those under the tidal zone would benefit from the increase in surface area as it would promote better resource collection. This conflicting preference would favour hybreds in the near tidal zone and those BG that had the hydred of smooth sides but pointy ends would find that the increased surface area between cells would have better sharing of photo-products simply because there is more area for interactions between the chain of cells. This pointiness could then develop into a long-tail like trait so surface area interaction is further increased, even by-passing one cell in the chain to get down the one below, and on and on and on. In the daylight hours then the cells of the root and stalk would have reason to respond to the influx of photo-production (stimulous) perhaps by lengthening their cells and becoming a more effective stalk. From here I hope you can see the beginnings of a nervous system, a pumping system, a respritory system, and a primetive eye.

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