1. Human life begins at conception (geography notwithstanding) because:
2. All the genetic material a person has or needs is present at conception, barring any defects
3. All humans have been created in the image of God, therefore
4. All humans have dignity – they don't get dignity, they aren't given dignity, they don't keep dignity. It is inherent to having been created in God's image. It is, a corrolary to the "I am" statements in Scripture.
I call this a corrolary because we have been created in his image, so dignity becomes something of a communicable (communicated) attribute...we are like God in having dignity...unlike an incommunicable attribute (like omniscience, omnipresence, etc) attributes that aren't communicated to humanity.