Those textbook diagrams showing the supposed evolution of vision reveal a real blind spot, for there are big problems with this evolutionary narrative. For instance, the biochemistry, even in primitive eyes is numbingly complex. The notion that it evolved is nowhere motivated by the scientific evidence.
Second, the sensing of light signals, alone, does nothing for it needs to be interpreted in the brain. And third, speaking of signals, the signal processing that goes on between the initial signal transduction and the brain is profound. The signal transduction, as phenomenally complex as that is, is only the beginning.
The incoming light is converted into an electrical signal (action potential) and then undergoes massive processing before making its impact on the brain. And new research is revealing new levels of complexity in this processing. If you stare at a horizontal line first, then a circle appears stretched out, like an ellipse. This simple fact was ingeniously used in an experiment to study how the processing deals with the rapidly changing incoming signals.
Our eyes move several times per second. If we were aware of what our eyes were seeing we'd have difficulty making sense of such rapid movements. As it is we don't sense such movements, and one theory held that the signal processing in our vision system deleted certain scenes to keep the image steady in our brains. But when subjects were shown a horizontal line too quickly to be sensed, they nonetheless then saw a circle as an ellipse.
In other words, even those scenes of which we are not aware have an effect on the scenes that we do see. Our vision system is even more complex than we thought, and the evolutionary narrative, that a few mutations created and modified a few genes from which arose fancy new vision capabilities, has become that much more absurd.