In today's nytimes.com Robert George of Princeton and Christopher Tollefsen of the University of South Carolina argue for the full personhood of the human embryo in their new book Embryo: A Defense of Human Life, reviewed here. ““To be a complete human organism,” they write, “an entity must possess a developmental program (including both its DNA and epigenetic factors) oriented toward developing a brain and central nervous system.” The program begins at conception; therefore, so does personhood.”
Their’s is a purely scientific argument. Full humanity is not located in a soul, but in a biological program. And great potential is there; viz., the possibility of developing a brain and central nervous system. I think, from a purely scientific standpoint, they are correct. If there is no human soul, then of course full humanity begins at conception. If it does not, then it’s truly weird to think that personhood begins somewhere along the way.
As a Christian theist I believe persons have souls. See, for fun, neuroscientist Mario Beauregard's The Spiritual Brain. But I think it's interesting to see how atheists and Christian theists could both agree that abortion is taking the life of a defenseless person; i.e., abortion is murder. The work of George and Tollefsen gives us the pure science. "George and Tollefsen reason that the embryo is fully human and its life therefore inviolable, because its program is self-contained."
Note: the nytimes review finds problems with the reasoning of George and Tollefsen. For me, the point remains that, if persons did not have souls, then full personhood would begin at conception, because if it did not then a "person" would somehow, even suddenyly, emerge along the way. What, then, would be the thing that happened to change a mass of protoplasm into a "person?"