IMO, human life begins at implantation (or after implanation is complete). It does not begin at fertilization. This accords with the current medical science definition of pregnancy as beginning with implantation, not fertilization.
In my view both scientists and theologians who either in ignorance (IMO) or out of convenience piggyback on the scientists view of DNA formation being constitutive of the formation of human life are wrong.
Theology is not beholden to science and there’s intense debate in science on many biological issues (ex. definition of a species – this relates btw to how things like Endagered Species acts are implemented … there are literally dozens of different definitions of species that have been proposed by scientists).
Here’s the fundamental problem and we need to separate this issue from the desires we have about abortion.
The argument is that at fertilization (or completion of fertilization), that someone with a unique DNA is formed and thus a new human individual life exists. The problem is that up and till recently it was assumed that identical twins had identical DNA (we now know believe it or not that that’s not quite true) yet no one saw any inconsistency. Also human clones may end up having exactly identical DNA – that doesn’t mean the clone is not a unique individual. Whether someone is a unique or unique new individual is not determined by whether a unique set of DNA has been formed. We also are discovering more and more now that the DNA of an individual is not actually static. And actually parts of your body will include cells with DNA slightly different than other parts of your body. So this whole materialistic (and frankly idiotic) notion of DNA being constitutive of human identity is in my view entirely discredited. I don’t know who thought of this idea but whoever it was, be it theologian or scientist, is just wrong.
Let’s suppose that someone had malformed DNA as some people do. Let’s suppose that in the future we are able to develop technology that replaces this malformed DNA in a grown individual with artificial substitutes with the DNA somehow excreted from the system. That person wouldn’t then suddenly not be human. Maybe according to some whacky biological definition he wouldn’t be, but theologically he would still be human with a human body and human soul.
With those errors out of the way, let me now make my own positive case for my earlier stated view.
It seems to me that the formation of a new human life in a woman involves naturally two things:
Union of sperm and egg in intercourse.
Union of embryo with the mother in the womb (i.e. after implantation is complete).
The first of these involves the union of man and woman. The second, of child and mother. In pregnancy, not only is it preceded by a union of man and woman made fruitful (the seed of life which is the unimplanted zygote) but it is effected by the union between the body of a new human life with the mother (the completion of implantation). One cannot separate these two elements.
So that’s when human life begins. When human personhood begins is another question and I would put that at human brain emergence. So an implanted embryo (or blastocyst) that is yet to develop a brain would be life, the nascent human life that at brain emergence becomes an actual human person with a rational soul. IMO, it is contrary to Catholic doctrine to suppose that a rational soul exists prior to the existence of the brain. This is b/c it is IIRC a dogma that the soul is the form of the body in a way such that the presence and union of soul in body is realized in that same body – IOW, the rational soul can’t be present in a body in some detached way, it must be present in the body itself. But without a brain, it is difficult to see how that could be true. The only way for rational soul at zygote formation to be consistent with Catholic dogma and doctrine is IMO to say that the zygote somehow exhibits in a way unbeknownst to us intellectual activity or at least intellectual potentiality – this isn’t the potential for intellectual potentiality that would need to be there but actual intellectual potentiality (which is present in some in a PVS situation for example but supposedly not in some in a brain death situation). Unless there’s something mysterious about the human zygote beyond current scientific knowledge and which is not present in animal zygotes, this seems clear cut. I think I wasn’t quite clear on the Catholic dogma I referred to there. The Catholic dogma there as I understand it, entails that it is heretical to suppose that a body without a rational soul is indistinguishable in principle from a body with a rational soul. It entails that the presence of the rational soul must be in principle visible in the body, in the actual body itself of which it is the form. I think this complicated issue is why the church and Pope Benedict XVI then CDF Prefect notes:
Certainly no experimental datum can be in itself sufficient to bring us to the recognition of a spiritual soul; nevertheless, the conclusions of science regarding the human embryo provide a valuable indication for discerning by the use of reason a personal presence at the moment of this first appearance of a human life: how could a human individual not be a human person? The Magisterium has not expressly committed itself to an affirmation of a philosophical nature, but it constantly reaffirms the moral condemnation of any kind of procured abortion.
So prescinding from an affirmation of philosophical nature, the church teaches that abortion is regardless what may ultimately be the case there, is to be morally condemned.