I noticed recently on the blog of an internet acquaintance of mine (i.e., someone I know only through the internet, but have interacted with quite a few times; an atheist by the way), that he questioned how anyone could say life began at conception (this is in the context of the abortion debate). He admitted that drawing the line at birth is arbitrary, but he felt that any other line, including conception, would be just as arbitrary.
That inspired me to explain one completely non-theistic argument I believe can be made for life beginning at conception. First, I agree that drawing the line at birth would be too arbitrary. After all, two children can be at the exact same stage of development in the womb, but one will be born one week ahead of his or her due date whereas another one comes a week late. Some children are born extremely prematurely, so that doctors have to hook them up to respirators in order to survive. Yet someone who willingly murders such a child outside the womb would be accused of being a criminal whereas anyone drawing the line at birth would have to say there is nothing wrong with aborting a child at that exact same stage of development but still in the womb. Being inside or outside the womb doesn't relevantly differentiate between how much of a "life" exists in that little body.
So why do I advocate drawing the line at conception? Because at conception a process of bodily change begins that does not end until our death. Left to their own devices, an individual sperm or egg will never develop into a human being. But once conception occurs, that zygote/embryo has begun the process of developing, aging and changing. This process never ends until we die. After conception we start developing body parts. We start growing. The body begins the never-ending process of undergoing changes.
Can this process be interrupted so that the child is never born? Of course. A dear friend of mine lost a child before birth. But the fact that the process can be interrupted is irrelevant. After all, it can be interrupted once we are outside the womb as well. Any number of diseases or other abnormalities can kill us, but that does not mean we are not human. It does not mean we do not have life. Can the development process be stopped in the womb? Yes. But it can also be stopped outside the womb. So this cannot serve to make any relevant distinction.
Once this process begins, our body never stops undergoing this process of change. You can not even make a relevant distinction by claiming that while in the womb we are developing whereas afterwards we are degenerating. After all, we continue to grow outside the womb (our ears actually never stop growing until the day we die). A baby's skull is not fully developed when it exits the womb. And who can forget puberty? That certainly is a new stage of development after birth.
My point is simply this. Conception is the only non-arbitrary line that can be drawn. Before that, this process of change has not begun. After that it has, and it never stops until we die. Any other line would be completely arbitrary. Therefore, the only logical conclusion would be to treat life as beginning at conception.