I would define a human being as an individual which is human. If there was no context to suggest more precise definitions, I would define "human" as "member of the human species" and "individual" as "single living organism." I would argue that this is the definition which best matches up with what people generally mean when they say "human being."
You want to talk about self awareness and sense of self. That's fine, but I don't think that these are the first things that people mean when they say "human being." For example, I think that most people would say that someone in a coma is a human being. Again, if your definition does not allow this than that is okay in the sense that we can discuss it, but you need to make it clear.
Especially if you challenge someone else to prove that something is a human being. It would be most charitable to assume a common definition for such a claim, not your personal specialized one.
There is a never a point from the moment of conception until that the DNA blueprint that defines all human development that is to come is not present, and is not unfolding.
The humanity of a young boy entering kindgergarten, of the old man delirious in the final stages of his cancer, or the man in his prime waging a war, are very different from each other.
But the difference at any stage of ones life, between the baby in the birth canal, and the baby taking her first breath, between the child in utero sucking her thumb, or sucking on her thumb on her first day on the outside, are progressive, and organic, with no breaks in the sequence in which one can point to say that this is a human life and this is not.
Because science itself shows that the same organism that exists at conception carries on existing until the moment of death.
Human life ultimately is not defined by science though. Our humanity is defined through an act of supreme faith.
We can either chose to have faith that every human life is of infinite and inherent worth, regardless of any exterior judgments of the worth of that life, or we can chose to believe that the worth of a human being is contingent on our judgements of its worthiness.
Reason alone would conclude that the world that sees all human life as of infinite value and of inherent worth is a better world in which we pick and chose which lives are worthy of life, and which are of no inherent value to us.
