Q: Human life obviously begins at conception because the fertilized egg (the conceptus) will develop into a human being.
A: Will it? How do you know this? What if the mother is run over by a truck? Clearly, the fertilized cell will only develop into a human being if and only if it gets the right conditions. A male spermatozoan or a female ovum will also develop into a human being given the right conditions; does this mean life begins with the formation of the gametes?
Q: But the gametes cannot develop into a human being on their own, they are incomplete, whereas the conceptus is complete.
A: Can the conceptus develop into a human being on its own? Is it really complete? If we placed a conceptus on the palm of your hand and left it there, would it develop into a human being on its own?
Q: Hmmmph! Then look at it from another angle. You would have to agree that conception is the cause of a beginning, and is thus the beginning of life.
A: Make up your mind. Is conception a beginning, or the cause of a beginning? If conception is the cause of a beginning, then what about the cause of the beginning of conception - is that not also a beginning? If you ask me, your "conception" is merely a conception of your brain.