Now, life has a peculiar property that separates it from other matter in the universe: It grows and develops, according to an innate plan and potency. Living things change -- specifically, they change themselves -- over time. Life is, in fact, organic rather than mechanical. It grows and changes without losing its identity; it is not constructed from the outside, like a car on an assembly line.
This fact is crucial to answering the question of when an individual human life begins. When identifying life, we must look for an internal unity and capacity, rather than the simple possession of accidental features. Archbishop Rigali recognizes this when he asks for two criteria: distinction from the parents, and the ability to grow and develop under the right conditions. In her paper, Dr. Condic puts it more technically:
In considering the question of when the life of a new human being commences, we must first address the more fundamental question of when a new cell, distinct from sperm and egg, comes into existence: when during the interactions of sperm and egg do we observe the formation of a new cell with both a material composition and a developmental pathway (i.e., a pattern of cell behavior) that are distinct from the cells giving rise to it? These two criteria (unique composition and behavior) are used throughout the scientific enterprise to distinguish one cell type from another . . . .
These criteria -- distinction from the parents and the intrinsic ability to develop along human lines -- are based in reason, not the tenets of any religious faith. To reject them for any other criteria -- such as the possession of some specific human faculty or feature -- is, as Condic puts it, "logically akin to linking the beginning of 'personhood' to the eruption of teeth in an infant or to the onset of menses in an adolescent -- they are arbitrary, variable, and not indicative of any fundamental change in the entity under consideration."
So, taking this internal orderedness as our criterion, why do we say that conception is clearly the beginning of human life? How can a scientific layman understand and make the arguments necessary to defend this position? Conception is the point at which the male gamete, the sperm, fuses with the female gamete, the ovum or egg, and introduces its half of the genetic information to the half already present in the ovum. Adult humans have two complete sets of the genome: 23 pairs of chromosomes, for a total of 46. Each gamete has only one set, so it is genetically only half of what is necessary for a human to live. Of course, an organism is no more reducible to its genetics than a computer is to its programming, so it is also important to note that, in addition to this genetic completeness, the zygote at this point also has all of the additional structures and compounds necessary to proceed along the human developmental pathway.
(Column continues below)
Subscribe to our daily newsletter
Conception is the point when the new organism begins to exist because it is the first moment at which a single organism capable of developing as a member of its species -- in this case, the human species -- comes into being. Before conception, neither a sperm nor an ovum can develop into anything at all on their own. After conception, there is no point that could be identified as the beginning of a new organism, because every significant stage after conception is only the transition from one physiological stage to another. None of those steps could be considered the "beginning" of development, anymore than we consider puberty -- a major developmental stage -- to be a stage that defines human life (however much we may joke about it). It's crucial to distinguish mere developmental changes from ontological changes. If development had not already begun -- that is, if the organism which is developing didn't already exist -- we wouldn't see the subsequent stages.
The fact that the potential for human growth is intrinsic to the organism is also crucial: The human identity derives from the person's own nature, not from the environmental conditions in which it is placed. The developing embryo's need for nutrients and a protective environment within its mother are sometimes raised as an objection to the embryo's personhood. But the need for a favorable environment and the need for food are conditions that no organism can evade, no matter what its stage of development. We need food and shelter appropriate for our age when we are in the womb, as newborns, as adolescents, and as octogenarians. This mere need for food and shelter cannot be a condition for humanity, because it is a need we never escape. Nor does the method of obtaining nutrition -- the placental and umbilical complex -- have a bearing on this issue. Feeding tubes in hospital patients don't negate their humanity; why should a natural feeding tube in the womb?
The point where a life begins is the point where we can say, by reasonable and not arbitrary criteria, that a new organism has begun to exist. We identify it not by looking for the point where the organism can survive on its own, or the point where it no longer needs protection and nutrients. We don't even look for the point where we find unique genetic codes, although that may be a clue. Rather, we identify it by looking for the point where an independent being exists that is innately structured and ordered toward development as a member of the species. We find that point precisely at the fertilization of the ovum by the sperm -- not before, because the gametes are incomplete on their own; and not after, because by those points development of the organism has already begun and the organism must already exist. Conception is the only point.