It's not a single step, it's just one sign post on a long highway (you can't show all of them). Every step along the way is pretty straightforward and immediately useful.

Eyes are an interesting example, because they are seemingly incredibly complex systems, yet eyes have evolved independently dozens of times. For example, octopuses and humans have equivalently complex eyes which have entirely different origins and have slight differences (for example, human eyes have a blind spot due to the nerve fibers being on top of instead of behind the light receptors).