NOTE: words such as "you" and "your" which appear below are not intended to refer to the Original Poster in this Message Thread.
When does human life begin? Heh, any paleontologist can tell you that hominid life began millions of years ago, and anatomically modern human life began more than 100,000 years ago. Or, of course, any biologist can tell you that an individual human life begins with fertilization of an egg.
So? There is still one more question to be asked, that pro-lifers cannot answer:
"WHY DOES IT MATTER?"
Your mere say-so that it matters is irrelevant. And your mere say-so, that somebody else (such as God) said it matters, is also irrelevant. FACTS are relevant, not mere unsupported opinions.
Now, because you have Free Will, you might say something like, "My life matters to me, because I choose it to matter." This is actually a valid argument --but it only applies to you. You cannot make that argument for someone else, such as someone bent on committing suicide, for example, because normally you are in charge of your life only, and you are normally not in charge of the other person's life. So you can try to convince the other person that his or her life matters to that person, or matters to some other person, but you cannot force that bent-on-suicide person to exercise Free Will to agree with you.
With the preceding in mind, now consider a human that does not have any Free Will, such as a man that is brain-dead and on full life-support. This particular human life exists, but that existence certainly cannot matter to himself; a dead brain means he hasn't any ability to have any opinion of any sort. Well, then, now we get to ask, "Does this brain-dead human life matter to anyone else?" It might or might not, after all --let's suppose it matters to YOU. But a second and possibly more relevant question is, "Who is in charge of this brain-dead human life?" --let's suppose this is NOT you. What are your options, should the person-in-charge decide to "pull the plug" on that brain-dead human life? As mentioned above, you could try to convince the person-in-charge that the brain-dead-human life matters. What if the response was, "Prove it!" ? Can you think of any other way to prove that that brain-dead human life matters to you, than by "taking over" and becoming the person-in-charge of that brain-dead human life?
The preceding now brings us to the Abortion Debate. An unborn human life does not have Free Will; its actions are entirely of the stimulus/response biological-robot variety. Free Will requires more brain-power than it possesses, especially at the early stages of pregnancy. Therefore it cannot exercise Free Will, and claim that its life matters to itself. Meanwhile, the person-in-charge of that unborn human life is the mother; does that life matter to the mother? The evidence is, sometimes it doesn't matter to the mother (or it negatively matters), and that's why abortions are sought. Similar to the bent-on-suicide case mentioned above, you can try to convince the mother that that unborn human life positively matters (and you most certainly need more facts than mere say-so!), but you cannot force her to exercise Free Will to agree with you.
Furthermore, if you want to claim that that unborn human life matters to you, how would you prove it? You certainly are not able to become the person-in-charge; "pregnancy transplants" simply don't exist and may be practically impossible (due to complexity of placenta/uterus interface). I've suggested in other Messages that you could "put your money where your mouth is", and pay all the prenatal and other medical costs associated with carrying a pregnancy to term, plus paying all the costs that are incurred after birth. Surprisingly few pro-lifers (or anti-abortionists) have expressed interest in this method of proving that every single unborn human life matters. Tsk, tsk. Why should anyone else pay to support what you believe?