Is a brain and a nervous system a necessary condition of being a human being?

I have certainly benefited from the argument put forward in our discussion and am thankful for the effort that Eduardo, Michael, and Robert have put into it. 

Robert George says that “To suppose that embryos are something other than human beings---rational animal organisms of the human species---is to undercut the ground for believing that infants, severely retarded persons, and comatose individuals are human beings.” He says, “Infants possess, as do embryos, the primordia (which are most fundamentally epigenetic) for self-directed development to the point at which they can immediately (though intermittently, of course, due to the need for sleep) perform characteristically human mental acts.  They possess in radical (=root) form the basic natural capacity that will in the course of development unfold to the point at which, if all goes well, they will be able to engage in conceptual thought, deliberation, and choice.  It is the possession of the basic natural capacity (shared by all human beings, even if blocked in the severely retarded), and not immediately exercisable capacities (possessed by some human beings but not by others, and possessed by some to a greater degree than by others), that determine the kind of substance a human being is, namely, a rational animal organism.”

There is, however, another possible distinction. Infants, severely retarded persons, and comatose individuals possess a nervous system and a brain; embryos do not. If it is argued that a human organism becomes a human being only when it develops a nervous system and a brain, the failure to recognize embryos as human beings does not undercut the ground for believing that infants etc. are human beings.

How well developed that nervous system and brain should be to regard a fetus as human is the subject of extensive debate. It would place humanity anywhere from the end of the first trimester to the end of the second trimester or early in the third. See generally Daniel A. Dombrowski and Robert Deltete, A Brief, Liberal, Catholic Defense of Abortion 43, 71-72 (2000). I am not arguing in favor of the brain/nervous system position though I do think it fits with the intuitions of many that the moral claims of a fetus are greater in the second and third trimesters than the first. I am suggesting that using this approach would not undercut the grounds for believing that infants are human beings and presents a challenge to efforts to demonstrate independent of authority that embryos are human beings as opposed to being "mere" human organisms.