Re: How do various body parts or abilities evolve separately?

The prototype "eye" was just a group of cells with photoreceptor
pigments (similar to the rhodopsin present in the rod cells). In the course
of  evolution the "optic mechanism" developed and gave rise to an "image
forming eye". Natural selection has tried various optic mechanisms in
different animal species.
For example,
Jumping spiders have a fixed lens and a moving "screen" (retinular cell
layer)
to clearly focus the image. Bony fishes have a fixed "screen" and a back and
forth moving lens to focus the image. In higher animals (like us) the lens
and "screen" are fixed, focusing is achieved by the changing curvature
(refractory index) of the lens!

Suggested Reading:

    A nice book on evolution that I enjoyed reading "THE BLIND WATCH MAKER"
by Richard Dawkins (1986),Published by Harlow: Longman.