Why the human zygote is an organism (and why it matters)
In the public debate over embryo-destructive biomedical research, many people dismiss the claim that the human zygote and blastocyst/young embryo (early stages of human prenatal development just following conception) are human beings on the grounds that other cells and tissues—such as a patch of skin cells, or the sperm and egg—are also living and human, yet no one supposes that they are themselves human beings. But these critics are not well-informed of the biological facts. The crucial difference is that zygotes and embryos are organisms, and skin cells, sperm and egg are not. The zygote/embryo is a whole distinct human organism—that is, a human being, a self-developing member of the species Homo sapiens—at a very early stage of life. Other cells are mere parts of larger wholes, not individual organisms themselves.
But the term “organism” requires explanation. Dr. Maureen L. Condic, a Berkeley-educated neurobiologist and professor at the University of Utah School of Medicine, where she teaches human embryology, explains:
An organism is deﬁned as “(1) a complex structure of interdependent and subordinate elements whose relations and properties are largely determined by their function in the whole and (2) an individual constituted to carry on the activities of life by means of organs separate in function but mutually dependent: a living being.” This deﬁnition stresses the interaction of parts in the context of a coordinated whole as the distinguishing feature of an organism.
Based on this deﬁnition, it has been proposed that human beings (including embryonic human beings) can be reliably distinguished from human cells using the same kinds of criteria scientists employ to distinguish different cell types: by examining their composition and their pattern of behavior. A human being (i.e., a human organism) is composed of characteristic human parts (cells, proteins, RNA, DNA), yet it is different from a mere collection of cells because it has the characteristic behavior of an organism: it acts in an interdependent and coordinated manner to “carry on the activities of life.” In contrast, collections of human cells are alive and carry on the activities of cellular life, yet fail to exhibit coordinated interactions directed towards any higher level of organization. Collections of cells do not establish the complex, interrelated cellular structures (tissues, organs, and organ systems) that exist in a whole, living human being. Similarly, a human corpse is not a living human organism, despite the presence of living human cells within the corpse, precisely because this collection of human cells no longer functions as an integrated unit.
So is the zygote an organism? Condic continues:
From the moment of sperm-egg fusion, a human zygote acts as a complete whole, with all the parts of the zygote interacting in an orchestrated fashion to generate the structures and relationships required for the zygote to continue developing towards its mature state. Everything the sperm and egg do prior to their fusion is uniquely ordered towards promoting the binding of these two cells. Everything the zygote does from the point of sperm-egg fusion onward is uniquely ordered to prevent further binding of sperm and to promote the preservation and development of the zygote itself. The zygote acts immediately and decisively to initiate a program of development that will, if uninterrupted by accident, disease, or external intervention, proceed seamlessly through formation of the genitive body, birth, childhood, adolescence, maturity, and aging, ending with death. This coordinated behavior is the very hallmark of an organism.
Mere human cells, in contrast, are composed of human DNA and other human molecules, but they show no global organization beyond that intrinsic to cells in isolation. A human skin cell removed from a mature body and maintained in the laboratory will continue to live and will divide many times to produce a large mass of cells, but it will not re-establish the whole organism from which it was removed; it will not regenerate an entire human body in culture. Although embryogenesis begins with a single-cell zygote, the complex, integrated process of embryogenesis is the activity of an organism, not the activity of a cell.
Based on a scientiﬁc description of fertilization, fusion of sperm and egg in the “moment of conception” generates a new human cell, the zygote, with composition and behavior distinct from that of either gamete. Moreover, this cell is not merely a unique human cell, but a cell with all the properties of a fully complete (albeit immature) human organism; it is “an individual constituted to carry on the activities of life by means of organs separate in function but mutually dependent: a living being.”
[T]he embryo comes into existence at sperm-egg fusion … a human organism is fully present from the beginning, controlling and directing all of the developmental events that occur throughout life. This view of the embryo is objective, based on the universally accepted scientiﬁc method of distinguishing different cell types from each other, and it is consistent with the factual evidence. It is entirely independent of any speciﬁc ethical, moral, political, or religious view of human life or of human embryos. Indeed, this deﬁnition does not directly address the central ethical questions surrounding the embryo: What value ought society to place on human life at the earliest stages of development? Does the human embryo possess the same right to life as do human beings at later developmental stages? A neutral examination of the factual evidence merely establishes the onset of a new human life at a scientiﬁcally well deﬁned “moment of conception,” a conclusion that unequivocally indicates that human embryos from the zygote stage forward are indeed living individuals of the human species—human beings.
Science, then, tells us what the embryo is: an individual human organism, a human being, at the embryonic stage of life. It cannot tell us how the embryo ought to be treated, which is a moral (rather than scientific) question.
But if it is true (as pro-life advocates argue) that human beings as such have intrinsic moral value—that there is a fundamental equality among all members of our species, irrespective of size, age, ability and condition of dependency—then we may not destroy embryonic human beings for their stem cells any more than we may kill and harvest the useful parts of a 10-year-old child for the benefit of others.