The same thing the atom is for. But without the proton, neutron, and electron all working together at once, you have nothing. A proton does not evolve into a neutron, or vice-versa. So a Protozoa might be a key ingredient, but there is no proof that it evolves into the other parts that make up the cell. Without all the pieces of the puzzle, whatever is missing must be assumed, or “believed”. You are deceiving yourself if you think evolution is a fact and not a belief.