Unfortunately, you never really laid out what you wanted proven. As such, you got exactly what you asked for. Were you asking when "life" begins? Life begins when something has the code necessary to drive its existence. At conception, a zygote has everything it needs to drive its entire "life." Or were you asking when the immortal soul is attached to the life? Scripture makes frequent referrence to God knitting us in the womb Himself, and that He "knew" us even then, and even before then. Not that He was aware of our existence, but that He "knew" us (also translated as "choosing" us). It never implies that there is a time, even a single second, that He regards us as any less then a child of His. Dispute that with Him, if you wish.
Now, humanity is defined by the presence of the soul, as that is what makes us unique. The expressions of that soul, eventually, are the characteristics of humanity. Unfortunately, you cannot determine the presence or absence of a soul by those characteristics, as they are limited by the flesh. Brain damage, developmental changes, any number of things can prevent that expression without disqualifying a person as human. As such we only have one Godly definition for humanity, and that is the womb of a woman.
Your "wood and barn" analogy is a poor one, I am afraid. Wood does not contain the information to become a barn. It was never meant to be a barn. At the moment of conception (actually, just a touch later when the "crossover" occurs) the conceptus is a completely unique cell. Biologically, it has every ounce of information it will ever need to become a genetically unique creature. Rather than "wood and barn" you are looking at a "blueprint" of something that has never been built before, and will never be built again. Even identical twins are unique, to some extent. Even more important though, is that this genetic uniqueness is just a fleshy reflection of the Spiritual uniqueness that God has granted. Remember, Scripture states that this a "chosen one" of His even before this process begins. Once that process has begun, it is His.
What I am saying is not vastly different than what Justin was saying. His appeal to you is, in fact, identical to the one I use with atheists (something you should take to heart). It is appealing because it contains the appearance of humility, and lets people off from their wrongheadedness gently. There is a perception of arrogance around Christians deciding when life begins, or when a conceptus is human. The real arrogance and the real sign of mankind's hubris, though, is when it started deciding when God does or does not love one of His creations.
