Arguing from Science
The "classic" arguments from the other side are collapsing under the weight of science. "No one knows when life begins" and "It's a blob of tissue" are frankly on the wane, especially in the context of surgical abortion, which is how the vast majority of abortions are done today.[2]
Still, establishing the evidence of the beginnings of human life will ground your argumentation in science, giving you a firm foundation for additional arguments and preempting the charge that you are basing your position on faith or religious belief.
Cite the Facts
Here is a thumbnail sketch of the scientific evidence of the existence of human life before birth. These are irrefutable facts, about which there is no dispute in the scientific community.[3]
At the moment when a human sperm penetrates a human ovum, or egg, generally in the upper portion of the Fallopian Tube, a new entity comes into existence. "Zygote" is the name of the first cell formed at conception, the earliest developmental stage of the human embryo, followed by the "Morula" and "Blastocyst" stages.[4]
Is it human? Is it alive? Is it just a cell or is it an actual organism, a "being?" These are logical questions. You should raise them, and then provide the answers.
The zygote is composed of human DNA and other human molecules, so its nature is undeniably human and not some other species.
The new human zygote has a genetic composition that is absolutely unique from itself, different from any other human that has ever existed, including that of its mother (thus disproving the claim that what is involved in abortion is merely "a woman and her body").[5]
This DNA includes a complete "design," guiding not only early development but even hereditary attributes that will appear in childhood and adulthood, from hair and eye color to personality traits.[6]
It is also quite clear that the earliest human embryo is biologically alive. It fulfills the four criteria needed to establish biological life: metabolism, growth, reaction to stimuli, and reproduction.[7]
Finally, is the human zygote merely a new kind of cell or is it a human organism; that is, a human being? Scientists define an organism as a complex structure of interdependent elements constituted to carry on the activities of life by separately-functioning but mutually dependant organs.[8] The human zygote meets this definition with ease. Once formed, it initiates a complex sequence of events to ready it for continued development and growth:
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 definitive body, birth, childhood, adolescence, maturity, and aging, ending with death. This coordinated behavior is the very hallmark of an organism.[9]
By contrast, while a mere collection of human cells may carry on the activities of cellular life, it will not exhibit coordinated interactions directed towards a higher level of organization.[10]
Thus, the scientific evidence is quite plain: at the moment of fusion of human sperm and egg, a new entity comes into existence which is distinctly human, alive, and an individual organism - a living, and fully human, being.[11]