We have a hell of a lot of evidence that learning, planning, processing sensory input, sending out signals to move muscles etc. are all happening in the brain.
> Could it be those with extensive amputations of gut and an articial heart could be "less human" in a broader sense than just mass?
If that's the case, it's pretty subtle. People with an artificial art don't act much differently than those with a normal heart; compare that with people who've had frontal lobotomies.
> But you seem awfully sure there is consciousness. Perhaps there is no such thing and you only think there is?
This line of argument generally rapidly devolves into semantics and quibbling over definitions. Giving a formal definition of consciousness is difficult. You either end up with a definition like "that phenomena where people think and consider themselves a single entity", in which case it's trivially true that human beings have consciousness; or you define it as something vague and metaphysical like "having a soul" in which case it's unprovable whether humans are conscious or not.
I like this passage from the Macmillan Dictionary of Psychology:
"Consciousness - The having of perceptions, thoughts, and feelings; awareness. The term is impossible to define except in terms that are unintelligible without a grasp of what consciousness means ... Consciousness is a fascinating but elusive phenomenon: it is impossible to specify what it is, what it does, or why it has evolved. Nothing worth reading has been written on it."