Why does Zarathustra believe that human beings are a bridge between ape and Overman?
In his Prologue, Zarathustra says this of man: "You have made your way from worm to man, and much in you is still worm. Once you were apes, and even now, too, man is more ape than any ape"(#3). Earlier he had said, "I teach you the overman. Man is something to be overcome". The overman is Zarathustra's, that is Nietzsche's, invitation to man to perpetually overcome himself. What the ape is to man, that is, "a laughingstock or a painful embarrassment," (#3) that man is to the overman. ...
In the third part of Zarathustra, in the chapter titled "On Priests", Zarathustra says that "Never yet has there been an overman". This shows that the overman is a constant invitation to man to overcome himself perpetually. He cannot get to a point where he can say that he has reached his goal. This is why the human being is a bridge between the ape and the overman. The human being should be in constant self-improvement. Those who think the work has been done already he calls the last men. '"We have invented happiness', say the last men, and they blink."(#5).