I've seen this argument before, and don't accept it simply because virtually everything is defined as an "arbitrary" point in time. Quite frankly I can't think of anything that's defined by a single, instantaneous event off the top of my head (but I'm sure you or others could if I were to press that point).
The point is though that I see nothing wrong with defining human (or any sexually reproduced) life as beginning at fertilization. Fertilization being defined (for the purposes of this post) as when all the chromosomes in question have fused to form a new genome. Whenever that is in the microcosmic world is rather irrelevant, because I know of no biochemist, microbologist, or physician that would argue that "one" event takes more than an hour after the initial invasion of the sperm through the egg's membrane. An hour at most.
I put the word one in quotes in that last sentence because you're correct palmer, that it doesn't happen "instantaneously", but that biological fact is irrelevant to this conversation, unless somehow we could have the ability to abort a fertilized egg before the chromosomes fully assembled, something that's simply impossible, by all practical means. I suppose it could biologically be done, in a lab, but why? The only thing I could think of that could come close in the real world would be the "morning after pill", but even that doesn't actually stop the process of proper chromosomal formation, it merely prohibits implantation.
Now you may return to your original point that it's merely an arbitrary time point selected, thus not really "scientific", but as you admitted in your first post, your 8-10 week point is arbitrary as well. You may say, "It's not only arbitrary but scientific because of the formation of neurons". Well, what's wrong with the scientific fact that all humans have 46 (or sometimes 47) chromosomes, and thus anytime that such a cell is formed, it's a unique human life? Why can't that be used to "scientifically" validate the arbitrary "life begins at fertilization" argument, just as you use a scientific fact to validate your arbitrary "8-10 week" argument?
You either support abortion or you're against it. There is no middle ground. There is no reason to draw the line anywhere other than at fertilization, or at actual birth, 9 months later. Anywhere inbetween is just hazy, feel good rationalization and comprimise. But when talking about babies, there can be no comprimise (or at least there shouldn't be).
My definition does not do that. It relies on the human form and human functions to help protect humanity. I aim specifically at protecting rather than definitions because empathy is a protective emotion that will protect unborn children which laws will always fail to do. You may call that hazy feel-good rationalization, but empathy, like other emotions is not triggered by a microscopic physical change occuring at some arbitrary point in the course of an hour. It is, like all emotions, a "hazy feel-good" phenomenon in the brain. That's simply the reality of emotions.
You have just two choices, deny the reality or role of empathy or supress it, or use it and show that empathy is possible at the earliest stages of humanity and harness the power of emotion to protect those earliest human forms.