While I am not an expert in evolutionary biology, what I do know about the
visual systen would indicate that the earliest forms of life evolved the ability to
detect light (after all that's what the visual system does for a living) followed
by the ability to detect changes in light that would correspond to something moving
in the "visual field". Eventually, organisms developed the apparatus to "process" the
information (edge detectors etc.) in order to form images. Evidence from frogs,
fish and lower animals would lean toward this explanation since their visual systems
are composed primarily of cells that are involved with detecting motion and light.
However, if you take into consideration the visual capabilities of squid and octopi,
you might be led to believe that object and motion detection developed somewhat
in parallel. The jury is still out.
If I remember my high school biology correctly, planaria are among the organisms
that are only able to detect light. And for the record...detecting motion is one of
the easiest things the visual system does...interpreting it is the hard part.
Division of Neuroscience
Baylor College of Medicine