When does human life begin?

I have often wondered whether trees, flowers and grass, or even bacteria and the like may have some form of consciousness so primitive we cannot even concieve of it. It seems like a basic property of life--and one would imagine varying degrees of conciousness, decending in order from humans and perhaps dolphins to say, monkeys, dogs, then lizards, insects, etc. Might not even a lowly amoeba have some vague, undeveloped form of consciousness?

So, perhaps "life" starting at conception--or before--is a real possiblity, though it would be impossible to understand or define. Such a weak and primitive "consciousness" would only gradually begin to resemble something we could recognize--as the brain and spinal cord developed--and there would be no clear defining moment at which a developing embryo or fetus became "a human life"--it would be something more like that paradox about the ship that was replaced plank by plank, which never at a single moment could be said to have changed from "the old ship" to "the new ship."

Anyway, just some thoughts.