The earliest nervous systems served to coordinate muscle activity in multi-cellular creatures. Swimming around in primordial seas, our little proto-fishies needed to make sure that one set of muscles all relaxed whenever the opposing set of muscles contracted. Perhaps such early systems were exclusively peripheral in nature, lacking any need for a central nervous system to coordinate body-wide behavior. However, it surely didn't take long (in phylogenetic time) before the need for central coordination of muscle activity led to the formation of a central nervous system -- the first brains. Simultaneously, the value of linking sensation to behavior led to the basic structure of all subsequent nervous systems. Consider, for example, little Peter Primordial Phishie. He doesn't have an eye, but he has managed to come up with a light-sensitive neuron mounted on the upper side of his body. This handy device tells him when some large object swims overhead. Presumably anything larger than Peter is a predator. The perception of this darkness can then be profitably employed to predicate whatever precautionary behavior is most pertinent to Peter's predicament.
Of course, Peter's progeny will have improved upon his primitive system. By elaborating on the photosensitive neuron, they would have developed a primitive eye, with some capability for perceiving simple images. This capability would enable these higher-tech fishies to distinguish predator (bigger than me) from prey (smaller than me). We might diagram the basic neuron layout as something like this:
This is, of course, not an anatomical diagram; it's a stylized representation that does for nervous system what a stick figure man does for portraiture. Each circle represents something like a neuron; if the situation described inside the circle arises, then that pseudo-neuron fires. The upper row represents output from the eye; "lower left activity" means that there is something happening in the lower left quadrant of the visual field. The triangles represent the nature of the output to the lower row. A sharp point touching the lower circle represents an excitatory input, while a blunt base touching the circle represents an inhibitory input, while the size of the triangle indicates the strength of the input. Again, I emphasize that the real system is much messier than this; the diagram deliberately suppresses some important truths in order to clearly communicate the essential truth that I seek to present: the pattern-recognizing nature of this system. The system's behavior is simple: if something is moving around in the visual field, and it's big enough to trigger activity in all four quadrants, the the "Run for your life!" neuron is activated. If it does not trigger activity in all four quadrants, then the "Dinner is served!" neuron is activated.
This system, then , can distinguish between two very simple visual patterns. The basic design is extensible: more complex patterns can be recognized by simply adding more neurons in more complicated connection arrangements.
Extend it they did, as animals built ever more complex brains capable of recognizing all sorts of subtle visual patterns. They even integrated multiple sensory inputs to further refine the pattern recognition, as in the old joke "looks like moose shit... feels like moose shit... smells like moose shit... tastes like moose shit...". And so animals just got better and better, and the Darwin-Jones index reached record highs. This takes us up to about the early Triassic.
Our next exciting episode: Sequential Thinking Appears!