Why is there Oxygen in Our Atmosphere?

The Story of O2 in Science.
Atmospheric oxygen is produced by photosynthesis, but the production of gaseous oxygen tends to be balanced by its removal by respiration. Apparently it's actually the burial of organic material which allows oxygen to accumulate in the atmosphere.

Visual Cells

There are two principal types of visual cells in animals: ciliary-type, typically found in vertebrates and other deuterostomes; and rhabdomeric-type, found in insects, molluscs, and other protostomes. Recently there's evidence that prebilaterians, the common ancestors of deuterostomes and protostomes, may have had both types of cells and that indeed both types of cells can still be found within both groups.
See Jellyfish vision starts with cAMP signaling mediated by opsin-Gs cascade in PNAS and Evolutionary biology: Light on ancient photoreceptors in Science.