At this stage of the evolution of intelligence a lot of minor advances would be made: multiplication of reflexes, the addition of sensors sensitive to the absence of a stimulus, the combining of more sensors and effectors so, perhaps, a purple cell with white dots and smell B and a long flagellum will be pursued if the temperature of the water is over eighty degrees but not if it is under.
By this time, something of central importance had to have happened, or be ready to happen: the evolution of sensors sensitive to pain and pleasure. For that to happen, “endo-sensors” (sensors sensitive to external stimuli) would have to have broken free of the cell membrane to become potential “intra-sensors.” And somehow become sensitive to a chemical due to damage to the cell mem- brane–probably excessive water (a biochemist would know). Or maybe the infra-cell might become sensitive to pieces of the membrane which it would never have contact with unless the membrane were damaged. If the intra-sensor were attached to an away-from effector, natural selection would select it because of its value in helping its cell get away from whatever had damaged is membrane.
Eventually similar intra-sensors connected to toward effectors would become sensitive to some by-product, say, of a successful hunt–something eaten but not digested, that would cause the cell to pursue whatever it had gotten a good taste of. I’m now going to name all such components of a cell that carry out functions like those of the sensors and effector “infra-cells” to make discussion easier. Let me add the clarification that the connections between sensors and effectors may begin as physical channels but will soon almost surely come to be made by precursors of neuro-transmitters: i.e., a sensor with “connect” to its effector by a distinctive chemical that only the effector recognizes and is activated by. The cell’s cytoplasm will act as a primitive synapse.
Various other “neurophysiological” improvements should soon also occur. One would be an intra-sensor’s gaining the ability to activate a toward effector when it senses pleasure but activate an away-from effector when it senses pain. The accident resulting in such an infra-cell would not be too unlikely, it seems to me: simply the fusion of two cells, one sensitive to pain and connected to an away-from effector, the other sensitive to pleasure and connected to a toward effector. Obviously an evolutionary improvement.
It also seems likely to me that intra-sensors would evolve sensitive to the activation of effectors. They would connect to other infra- cells carrying out reactions to, say, a successful capture of prey: a toward effector becomes active due to signals from a sensor sensitive to a certain kind of prey, in which case the outcome should be dinner, so a sensor sensitive to the effector’s activation which is connected to some infra-cell responsible for emitting digestive juices or the like, would be an advantage.
Certain other infra-cells should evolve to allow the step up to memory, but right now I can’t figure out what they might be, so will stop here, for now.