What is the nature of the new cell that comes into existence upon sperm-egg fusion? Most importantly, is the zygote merely another human cell (like a liver cell or a skin cell) or is it something else?
Just as science distinguishes between different types of cells, it also makes clear distinctions between cells and organisms. Both cells and organisms are alive, yet organisms exhibit unique characteristics that can reliably distinguish them from mere cells.
An organism is (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 stresses the interaction of parts in the context of a coordinated whole as the distinguishing feature of an organism. Organisms are “living beings.” Therefore, another name for a human organism is a “human being”; an entity that is a complete human, rather than a part of a human.
Human beings can be distinguished from human cells using the same kind of criteria scientists use to distinguish different cell types. A human organism is composed of human parts (cells, proteins, RNA, DNA), yet it is different from a mere collection of cells because it has the characteristic molecular composition and behavior of an organism: it acts in an interdependent and coordinated manner to “carry on the activities of life.”
Human embryos from the one-cell (zygote) stage forward show uniquely integrated, organismal behavior that is unlike the behavior of human cells. The zygote produces increasingly complex tissues, structures and organs that work together in a coordinated way. Importantly, the cells, tissues and organs produced during development do not somehow “generate” the embryo (as if there were some unseen, mysterious “manufacturer” directing this process), they are produced by the embryo as it directs its own development to more mature stages of human life. This organized, coordinated behavior of the embryo is the defining characteristic of a human organism.
While, under some circumstances, human cells can assemble into primitive tissues and structures, under no circumstances do mere human cells produce the kind of coordinated interactions necessary for building a fully integrated human body. They do not produce tissues in a coherent manner and do not organize them so as to sustain the life of the entity as a whole. They produce tumors; i.e., parts of the human body in a chaotic, disorganized manner. They behave like cells, not like organisms.
The conclusion that human life begins at sperm-egg fusion is objective and based on the universally accepted scientific method of distinguishing different cell types from each other and on ample scientific evidence. Moreover, it is entirely independent of any specific ethical, moral, political, or religious view of human life or of human embryos.
A neutral examination of the evidence 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 one-cell stage forward are indeed living individuals of the human species; i.e., human beings.
To say that a zygote or embryo is a genetically distinct human organism (a human life) does not require religious doctrine or dogma; the scientific method can establish that.
But to say that a zygote or embryo is a mere clump of cells or tissue requires that ideological, philosophical, or political doctrine/dogma be given precedence over science.