[procaryote]

You could have 2 - 4 for other sensory cells, like heat or touch, and then be a single mutation away from sensing light.

Iirc, some snakes have some heat sensing cells arranged in pits to give them a directional heat sense. If you did that with visible light instead of IR those pits would be what halfway to an eye would look like

[wolfhumble]

> sensory cells, like heat or touch

Like light sensitive cells these would need a similar 1 - 4 +++, so the point would really be the same.

[procaryote]

Not at all... If the question is "how could an eye evolve" it's reasonable to start from a baseline of a complex organism that has sensory cells but lack eyes or even light sensitive sensory cells.

A cell sensing light can use the same type of nerves etc as other sensory cells, so there is no need to explain how a cell sensing light + a nerve + a central nervous system evolves in one step.

If you wonder how a nerve could evolve for example, that is a different question. If you have two cells and they can exchange chemical signals, which is useful in itself, you have the start of a nerve.