Three distinguished and brilliant professors:
- Patrick Lee, the McAleer Professor of Bioethics at Franciscan University of Steubenville (author of Abortion and Unborn Human Life)
- Christopher O. Tollefsen, College of Arts and Sciences Distinguished Professor of Philosophy at the University of South Carolina,
- Robert P. George, McCormick Professor of Jurisprudence at Princeton University (co-author with Tollefsen of Embryo: A Defense of Human Life)
have responded to the idea that there is no scientific consensus regarding the beginning of human life.
They point out that “there have been countless scientific monographs and scholarly articles—in embryology, developmental biology, and genetics—explicitly affirming that a human being at the earliest stage of development comes to be at fertilization.”
They cite three among many possible examples:
“Human life begins at fertilization, the process during which a male gamete or sperm unites with a female gamete or oocyte (ovum) to form a single cell called a zygote. This highly specialized, totipotent cell marked the beginning of each of us as a unique individual.” “A zygote is the beginning of a new human being (i.e., an embryo).”
—Keith L. Moore, The Developing Human: Clinically Oriented Embryology, 7th edition. Philadelphia, PA: Saunders, 2003. pp. 16, 2.
“Fertilization is the process by which male and female haploid gametes (sperm and egg) unite to produce a genetically distinct individual.”
—Signorelli et al., Kinases, phosphatases and proteases during sperm capacitation, CELL TISSUE RES. 349(3):765 (Mar. 20, 2012)
“Although life is a continuous process, fertilization (which, incidentally, is not a ‘moment’) is a critical landmark because, under ordinary circumstances, a new, genetically distinct human organism is formed when the chromosomes of the male and female pronuclei blend in the oocyte.”
—Ronan O’Rahilly and Fabiola Mueller, Human Embryology and Teratology, 3rd edition. New York: John Wiley & Sons, 2000, p. 8). [emphasis added]
They write that any other examples could be cited, some of which may be found here.
Lee, Tollefsen, and George write:
That is the authority of science. On request, we can cite dozens more examples. The authorities all agree because the underlying science is clear.
At fertilization a sperm (a male sex cell) unites with an oocyte (a female sex cell), each of them ceases to be, and a new entity is generated.
This new entity, initially a single totipotent cell, then divides into two cells, then (asynchronously) three, then four, eight and so on, enclosed all the while by a membrane inherited from the oocyte (the zona pellucida).
Together, these cells and membrane function as parts of a whole that regularly and predictably develops itself to the more mature stages of a complex human body.
From the zygote stage onward
this new organism is distinct, for it grows in its own direction;
it is human—obviously, given the genetic structure found in the nuclei of its cells;
and it is a whole human organism—as opposed to what is functionally a part of a larger whole, such as a cell, tissue, or organ—since this organism has all of the internal resources and active disposition needed to develop itself (himself or herself) to the mature stage of a human organism.
Given its genetic constitution and epigenetic structure, all this organism needs to develop to the mature stage is what human beings at any stage need, namely, a suitable environment, nutrition, and the absence of injury or disease. So it is a whole human organism—a new human individual—at the earliest stage of his or her development.
This is why it is correct to say that the developing human embryo is not “a potential human being” (whatever that might mean) but a human being with potential—the potential to develop himself or herself (sex is established from the beginning in the human) through the fetal, infant, child, and adolescent stages and into adulthood with his or her identity intact.
You can read the whole thing here.
To see a unique presentation on the development of the human body in the womb, from conception to birth, see this fascinating TED presentation from Alexander Tsiaras, chief of scientific visualization in the department of medicine at Yale University, who uses micro-magnetic resonance imaging to visualize this process: