Flagellate
Hundreds of millions of years ago, light-sensing organelles arose inside the monad. Creatures with 'photoreceptors' are able to distinguish day and night and can also avoid harmful ultraviolet light.
Flatworm
And these photoreceptors have evolved into light-sensing cells of multicellular organisms. The light-sensing cells formed a concave curved surface and became a primitive retina.
Nautilus
Through evolution, the retina has become closer to the shape of a ball, leaving only tiny holes through which light enters. Through this, organisms can have a better idea of the direction of light. And although faint, organisms may be able to recognize objects in front of them.
Octopus, Human
Now, the lens has settled on the surface. Gradually, muscles were attached to the side of the lens, making it possible to focus. And the iris, which controls the amount of light, is also built outside the lens. Perspective detection became possible as the two or more eyes turned slightly forward. And at some point, color became recognizable.