system
Given the following post, respond as if you're commenting on Reddit:
human
How do complex structures (like eyes) evolve where the individual components seemingly only offer increased fitness as a completed system?
gpt
The starting premise is wrong. 

The eye, though you wouldn’t necessarily recognize it as such, began hundreds of millions of years ago with unicellular life forms like green algae, that have a structure known as an “eye spot”. While quite primitive compared to the complex eye, it performs the crucial task of using photosensitive pigments to detect light and transduce it into a signal the organism can react to. This gives a tremendous advantage, for example, by guiding photosynthetic cells towards light so they can grow and reproduce. 

Eventually this spot became a pit as tissue surrounding it grew outwards around it, perhaps as a form of protection, and also giving greater directional sensitivity. 

Fast forward a bit, and the pit is now at the back of a nearly enclosed spherical cavity, which courtesy of basic physics, acts like a primitive pinhole camera, so it can now detect low-resolution images. 

Fast forward some more, and now there is a basic transparent membrane filling the entry hole, separating the contents of this proto-eye from the surrounding environment. Now it is possible to customize the internal contents to aid further refinement of the chemicals and structures, incrementally improving function. Maybe the light-sensitive area expands, and/or other light-reactive molecules emerge and add detection of additional colors of light. 

This transparent membrane eventually thickens, changing the focus of light, and now the eye can see objects over a wider range of distances. This thickened outer layer has become a cornea, which is responsible for the majority of refraction (ability to bend light). 

Moving ahead, the multiple layers begin to differentiate into the fixed-focus outer cornea, and an internal optical layer that adds even more refractive power. This lens becomes controllably flexible, and now we have the ability to see near, intermediate and far objects clearly. 

So in just a handful of traceable steps we have evolved from primitive light sensitivity to detailed vision. Of course these steps took millions of years, billions of generations, untold zillions of “experiments” (most probably failures that dropped out of the running). In fact, it also appears that this process has been repeated independently multiple times throughout history, leading to different successes, which we distinguish by the variety of photoreceptors seen across the animal kingdom.