I'm not arguing the "human" bit. Sparty said science tells us it's life at conception, I'm simply arguing it doesn't say that. Not definitively anyways.
And my "simply cells dividing" was not inferring anything other than what I said. Once the sperm and egg join to make a single cell zygote, the cells begin to divide but not for a day or two. Sparty also said there was a detectable heartbeat in 3 weeks, though I have read 3 articles that claim the heart and brain don't begin to develop until the 5th week, so I wonder whats beating and where it's sending blood?
As far as the last part of your post, I'm not really going to get in to this argument. I don't like abortion, I think it should be a last resort; not a get out of jail free card that many use it as. But I also don't believe we have the right to choose for someone else what happens to their body. So I'm pro-choice, not to be confused (as it so often is) with pro-abortion.
I replied to him simply because he was using the name of science to argue when life begins, which is not scientifically proven. It's just interpretation of what the definition of life is, etc. Look at the flip side for a moment, no matter how much scientific data the right views on global warming, they don't believe in it either. They argue that science has not proven it, it's simply being interpreted that way.