Is it possible for eyes to have evolved? Many people have said, what is the point of 'a half-evolved eye'? The eye needs many, precise, separate parts to make it work. How did evolution 'know' the final design of the eye before it reached it? Surely if eyes evolved, it means there must have been millions of generations of blind creatures with useless, half-finished eyes?
Or does it?
Charles Darwin said this on the evolution of the eye:
if numerous gradations from a perfect and complex eye to one very imperfect and simple, each grade being useful to its possessor, can be shown to exist; if further, the eye does vary ever so slightly, and the variations be inherited, which is certainly the case; and if any variation or modification in the organ be ever useful to an animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, can hardly be considered real
--Darwin, C (1859). On the Origin of Species. London: John Murray.
The evolution of the eye could be the result of one evolved state, or independently in many clades.
No one knows for sure.
I have no idea how eves cold have evolved, or what a semi evolved eye could be like? But now that you ask this question, i can think of some possibilities. For example consider the following functionalities:
- Ability to detect absence or presence of light.
- Ability to differentiate between colours.
- Ability to distinguish between different intensities of light.
- Ability to feel light as we are able to feel heat transmitted through radiation.
All the above abilities can be very useful as can be deduced by some animals to perform the functions of light by other means such as sonar waves in bats, and sensing of heat waves in mosquitoes.
There is also this scientifically established phenomenon of dermo-optical- perception, or the ability some people have to see through touch or with their skins.
You start with a simple patch of light-sensitive cells. It slowly gained shape through random mutations. Any half-blind creatures would be killed.