The eye doesn't work without the brain, and the brain doesn't work without the heart, and the heart doesn't work without the rest of the body.
Vastly oversimplistic, and only causing *apparent* problems by jumping in very late in the proceedings.
"A team of researchers, including Stanford scientists, has discovered that certain single-celled infectious bacteria can tell the difference between light and dark, and actually increase their infectiousness 10-fold when hit by sunlight." 2007
"It was already known that
Synechocystis cells move towards a light source that is shone at them from one side, which implies that the cyanobacteria can “see” where the light is. But how can such a tiny cell accurately detect where light is coming from?
Schuergers et al. tracked how
Synechocystis moved in response to different light conditions, and found that the secret of “vision” in these cyanobacteria is that the cells act as tiny spherical lenses. When a light is shone at the cell, an image of the light source is focused at the opposite edge of the cell. Light-detecting molecules called photoreceptors respond to the focused image of the light source, and this provides the information needed to steer the cell towards the light. Although the details are different, and although a
Synechocystis cell is in terms of volume about 500 billion times smaller than a human eyeball, vision in
Synechocystis actually works by principles similar to vision in humans." 2016.
Useful working with light long before a brain, or a separately developed eye.
All the core material required for further evolution by natural selection, in the event of improved sensing of light being an advantage (Over and against the energy penalties required.)