Original: Their last common ancestor had at most a very simple photoreceptive spot , but a range of processes led to the progressive refinement of this structure to the advanced camera eye - with one subtle difference : The cephalopod eye is '' wired '' in the opposite direction , with blood and nerve vessels entering from the back of the retina , rather than the front as in vertebrates .
Simplified: Their last common ancestor had a simple photoreceptive spot , but a range of processes led to the progressive refinement of this structure to the advanced camera eye .