If we are to pinpoint the moment of beginning of personhood that has the best scientific grounding, out of all the moments that are candidates, that will certainly be the beginning of the single-cell stage. A minute before, there were two spatially-separated haploid cells, each with insufficient genetic information ever to become anything resembing a full-grown person; a minute after, there is a single definable organism with the exact genetic information that it will carry throughout life, at 1 month of gestation, at 4 years, at 60 years.
Here Dr. Maureen L. Condic writes:
Structures capable of new functions are formed throughout embryogenesis. For example, grasping becomes possible once hands have formed. But the fundamental process of development proceeds continuously, both prior to and after hand formation, and the onset of this function reflects an ongoing developmental process. Given the continuous nature of development, to argue that embryos and fetuses become humans once some anatomical or functional landmark such as “consciousness” has been achieved is to assert some kind of magical transformation; i.e., that at some ill-defined point, a non-human entity spontaneously transmogrifies into a human being, without any change whatsoever in its behavior, its molecular composition, or any other observable feature.
I reject this argument. For something actually to transform into a different kind of thing, a change must take place in its composition or in its pattern of biological activity. For example, sperm and egg are two specific human cell types that fuse to produce a distinct cell (the zygote) with unique molecular composition and with a pattern of organismal behavior that is distinct from the behavior of either sperm or egg. A clear, non-magical, scientifically observable transformation from one kind of entity (two human cells) to another kind of entity (a distinct human organism) has occurred. . . .
Building the complex architecture of the brain is a continuous process that is initiated at sperm-egg fusion and proceeds through orderly steps under the direction of a ‘builder,’ that is, a human organism that is present from the beginning. The presence of an agent capable of constructing the mature body, including the brain, is the only sustainable definition of a human being. This agency should not be misconstrued as some kind of mystical or spiritual element that is merely attributed to an embryo or fetus based on personal or religious belief. The fact that the embryo acts as an agent is entirely a matter of empirical observation; embryos construct themselves. . . .
. . . science does not dictate that citizens have a right to equal protection under the law, but. . . . science has clearly determined when human life commences, and this determination legitimately dictates that equal protection under the law must extend to human beings at embryonic and fetal stages of development.
For more scientific sources attesting to the personhood of the zygote, see Appendix 1.
Though the beginning of the single-cell stage has the best scientific grounding among all the moments that are candidates for the beginning of personhood, some will debate against defining “personhood” in this way. And “personhood” or “person” is not the only word that can be debated. While some will maintain that a zygote is “a human being, but not a person,” others, since they can’t deny that it belongs to the species Homo sapiens, will insist, with an equally straight face, that it is “human, but not a human being.”
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