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I'm guessing - total guess here, not fronting - that super super early in the development of nervous systems, the smallest useful bilateral system would be 1 neuron on one side, and another neuron on the other side. Each respond to some kind of change in light, either dark to bright or bright to dark, and this caused a signal to float to the OPPOSITE side of the creature to activate that side's motor to move it toward the stimulus.
If this was a common strategy, then brains would begin to develop from this, developing more side-dependent sensors and triggers, and then the higher processing of the information would develop on that side and be relevant to the opposite side.
Once again, total guess, but if i was designing a primitive bilateral nervous system, i think i could get a huge bang for my buck with just 2 neurons. Which is damn near as simple as possible. Which if that's right, it's probably common to most complex life with nervous systems.
