@rexpetrus @southerncloud @ OP
At its most basic form, sure, when one clump of cells combines with another clump of cells they form a new clump of cells with new, unique abilities. It has the potential to become a human, sure, but is it a human? and more importantly, does it matter? For the sake of a tumblr post I will keep this short, but in essence, biology tells us that procreation is not a uniquely human trait, even if specific human DNA is. But in a biological perspective, the DNA of a plant and a human has the same intrinsic worth, that is, the worth it can give to the environment around it, and as such the destruction of a plant seed and the destruction of a fetus are an equal loss and/or gain.
But, then, what makes a human different from a plant or animal, or even a rock? That is not a biological issue, but a social one, a philosophical one. And yes, this can be a theological one as well, but the important notion is that theology does not exist in a vacuum. By all definitions, it is a social study. Thus, the worth of something is defined by the society within which it exists - again. Not purely biological. So, then we must ask, what defines a human? You could go with our classic “I think, therefore I am.” Or maybe more specifically “I consent, therefore I am.” The ability to consent to something - not just sexual intercourse, but anything - is something intrinsic to the human being, that is, that we are not driven by purely instinctual urges and hold each other accountable for following them if they negatively impact another. Complex thought, rational decisions - those are unique to humanity, as far as we know. Again, for the sack of a tumblr post I am following this line of reasoning - there are many ways to philosophically define a human being.
So. Abortion as an issue is not a biological one, but social. And as a social issue, it has a basis in humanity and the societal laws governing that social group. In our social group, part of that governing law consists of autonomy, that is, the right to one’s own body. And thus we come to the meat of the abortion topic. “Your right to extend your fist ends at the tip of my nose.” Or, I like to use the unstoppable force meets immovable object analogy. The unstoppable force of the autonomy of the pregnant person meets the immovable object of the humanity and/or autonomy of the fetus. If the two are considered with equal worth - in the eyes of God, in the eyes of the Universe, in Science, whichever - then there is no fundamental right answer. You cannot stop the force and you cannot move the object.
But, we are not at a standstill. Ethically, this problem does not exist in a vacuum. In our society, when you pass away, unless you have given explicit permission your organs cannot be taken for donation, and violations can cost jail time - even if the organs will save lives. We choose to say - good or bad, ethical or not - that the potential life saved by the donated organ is not worth the autonomy of the deceased person. Not the only example, but the one i will share. We can apply this same principle to the discussion of abortion and make the reasonable claim that, in our current society, it is not worth the potential life of the child to violate the autonomy and choice of the mother. Again, whether you agree or not, that is the logic set in precedent by other laws we have in place.
In summary, yes, the conception of a zygote does indeed happen how you described, but the biological elements of this are far less important than the social ones. And the social governing laws we have in place, as set in precedent by the autonomy laws of the deceased, say that we consider the autonomy of a current situation to be more important than that of a potential one.
*As an additional note, many dead things can grow. growth as humans perceive it is just complex multiplication, really. A mountain can grow. rain can grow. anything accumulating matter, really. Biological definitions of a human are so far beyond conception that it is impossible to pin them to one discussion, philosophical definitions even moreso.
**additional note number two, the biological defense of the abortion argument is often paired with the biological defense of the transgendered person argument (sex equals gender and the like). Which, of course, has already been disproven multiple times by biologists. In other words, and do forgive me if this is too forward….maybe your understanding of biology is not as complete as it could be. People in glass houses shouldn’t throw stones and such.
***additional note number three, it’s not pro abortion, it’s pro choice. In that the autonomy of the mother - the right to choose what to do with her body - is valued more than the potential life of a fetus.