I always thought the first sensation was the separation of darkness from light. From there subtleties and exceptions consciousness grew ever more complex. Every time a difference was found a new layer of consciousness was developed.
Today we can perceive so many differences and weigh their outcomes without fully understanding their roots.
The first sensation was probably chemical rather than light. Organisms long predate photosynthesis. The early earth had plenty of energetic molecules floating in the water.
In this case I mean that the first sensor is probably something that responded to gradients of molecules in the water, so that the proto cell could move in the direction of more food.
So I'd say that the first sensation was smell/taste. Though that is, of course, speculative.
