This diagram shows the components of a flagellum that must all work together to give a bacterium motility. There are 20 major protein components that compose the flagellum itself, with another 20-30 required for it to grow and function.
A protein-based molecular motor at its base composed of many different proteins working together rotates the flagellum in the bacterial membrane using the proton motive force. Protein rings support the flagellum in the cell membranes and cell wall. A secretory system works to export the flagellin monomers that the hollow filament is composed of, allowing it to grow from its tip. Bacterial flagella continuously grow into their environment, and their length is controlled stochastically: if they get too long they break.
The difficulty in explaining the evolution of complicated cell structures such as the flagellum comes from the fact that if a single protein in the complex ceases to function, the entire structure ceases to function. Thus, it is difficult to imagine a priori a stepwise evolutionary path to the current structure, a problem originally conceived from looking at the complexity of the mammalian eye.