In the ongoing debate about cloning human embryos for research, and about destroying them in order to harvest their stem cells, it is important to keep some basic facts in mind. Our moral analysis must be built upon fundamental scientific truths. If we obscure the facts, then we will not think clearly or act responsibly about these issues.
Every human being begins as a single-cell zygote, grows through the embryonic stage, then the fetal stage, is born and develops through infancy, through childhood, and through adulthood, until death. Each human being is genetically the same human being at every stage, despite changes in his or her appearance.
Embryologists are united on this point. Consider the following statements from standard textbooks: "Human development begins at fertilization .... This highly specialized, totipotent cell marked the beginning of each of us as a unique individual" (Keith L. Moore and T. V. N. Persaud); "Almost all higher animals start their lives from a single cell, the fertilized ovum (zygote).... The time of fertilization represents the starting point in the life history, or ontogeny, of the individual" (Bruce M. Carlson); "Although life is a continuous process, fertilization is a critical landmark because, under ordinary circumstances, a new, genetically distinct human organism is thereby formed.... The embryo now exists as a genetic unity" (Ronan O'Rahilly and Faiola Muller).
Normally, the embryo comes into being through sexual conception, in which the female egg cell is fertilized by a male sperm cell. In sexual reproduction the new individual gets half of its chromosomes from the nucleus of the sperm cell and half from the nucleus of the egg cell. The new organism thus produced is genetically distinct from all other human beings and has embarked upon its own distinctive development.
In addition to this normal process, we have developed laboratory techniques with which to manipulate the procreation of new human organisms. One of these techniques stages the encounter of sperm with egg in a laboratory dish rather than in a woman's body. This is in vitro fertilization (IVF). Another technique is an asexual one in which no sperm is involved. Instead, an egg has its nucleus removed and replaced by a nucleus from another type of cell--a body cell. The egg is then stimulated by an electrical charge, creating a living human zygote. This is cloning, a process in which the body cell that donated the replacement nucleus supplies the chromosomes of the new human organism.
Whether the new organism is produced by fertilization or by cloning, each new human organism is a distinct entity. Twins are genetic duplicates of each other, but no one would deny that each is a distinct human individual. Similarly, a clone would be a genetic duplicate of another human being, but there is no denying that it would also be a separate individual.