What does it mean to be human?

Throughout Tezuka's work, the subject of "humanity" continuously comes up.  It is first mentioned in reference to making Mimio, a rabbit, into a human.  The scientists experimented on Mimio to try to see if they could in fact "turn him human," however once Mimio has the ability to talk, dress, and act like a human, he is constantly told he isn't one and can never be one.  They seem to base humanity not merely on intelligence (as Mimio clearly is incredibly smart) but also on outward appearance.  When he makes the mistake of hitting Uncle Bill on the head in an attempt to save the blueprints, he is scorned for making a mistake that apparently no true human would have done.  However, aren't humans prone to making mistakes to begin with?  Wouldn't it be natural for a human to make the same mistake Mimio did in the midst of all the confusion and fighting happening in that moment?  Doesn't that genuine mistake prove that Mimio isn't some faultless, genius, almost robotic science experiment?  Wouldn't that flawlessness end up making Mimio seem even less human and distanced from normal people?  It seemed a bit odd that they would immediately attack Mimio's humanity when he messed up.  I could see them being angry, but not necessarily to the cruelty of undermining Mimio's identity.
By the end of the manga, however, when Mimio shows his empathy and love of humanity by sacrificing himself to finish the tunnel, that's when his humanity is brought back to light.  As Mimio lies dying in the hospital, he asks John if he's finally human, to which John replies "you're greater than any human...Mimio...I'll never forget you..." (148).  John's final response to Mimio's question seems almost like a way out.  I don't know whether John actually believes, even now after all that has happened, whether Mimio is in fact "human" or can be considered one.  John seems to make Mimio jump from "less than human" in his mind to "greater than human."  Neither of which is actually just "human," which is what Mimio has been craving to be seen as since he was created.