Evolution

‎"Some very simple animals have nothing more than light sensitive spots, that enable them to tell the difference between light and dark. But if a patch of such spots formed in even the shallowest of pits, one edge of the pit would throw a shadow and so reveal the direction of light. If the pit got deeper and started to close, then light would form a blurred image. Mucous secreted by the cells would bend the light and focus it. If this mucus hardened it would form a proper lens and transmit a brighter and clearer image.

All these different fully functional stages at different levels of complexity are found in living animals today. Some single-celled creatures have one of those light-sensitive spots. Flatworms have a small pit containing light spots, so they can detect the shadow of a predator. A snail’s blurry vision is good enough to enable it to find its way to food. And the octopus has an eye with a proper lens and can see as much detail as we can. So the structure of the human eye does not demand the assistance of a supernatural designer. It can have evolved gradually, with each stage bringing a real advantage - as Darwin’s theory demands."
- David Attenborough