The most basic question of the pro-life pro-choice debate is "when does human life begin?" Because if abortion kills a human being then almost everyone would consider it wrong. If it kills something else, then most people would support it. The way I see it, there are four ways to look at the question.
Looking at what I've written so far, I decided I'd better break this into three or four parts at least. This is part one, which explores the question from a purely scientific angle. Subsequent posts will focus on religious views, philosophy, history and logic. If you have something to say on the matter, please comment. If you have different views, let's discuss, but let's stay civil and on topic.
ScienceWhen does human life begin according to science? At conception. This is settled science, and there is no wiggle room. At conception a new and completely self contained organism is created. Despite Bill Nye's stupid shout out, the uterus is not anything like a 3D printer. A 3D printer constructs an object by placing bits of material on a surface, like building blocks. The uterus does not build a baby bit by bit.
On the contrary, the zygote, from the first moment it is created, constructs itself into a fully formed baby, without direction from the uterus (and in fact, the uterus takes direction from the fetus). All the child needs are the same things you and I need - food oxygen and warmth - to continue living. It is a living organism, separate from the mother. And human? Again settled science. Of course. It has human DNA. A human can only beget another human, just as a dog can only beget dogs and a cat beget cats. The scientific name for this is the law of biogenesis and it has been known since before DNA was discovered.
Want some references? I can give you lots more, but here are some from science textbooks.
"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 Müller, Human Embryology and Teratology, 3rd edition. New York: Wiley-Liss, 2001. p. 8.]
"Every time a sperm cell and ovum unite a new being is created which is alive and will continue to live unless its death is brought about by some specific condition." [E.L. Potter and J.M. Craig, Pathology of the Fetus and the Infant, 3rd edition. Chicago: Year Book Medical Publishers, 1975. p. vii.]
"The development of a human begins with fertilization, a process by which the spermatozoon from the male and the oocyte from the female unite to give rise to a new organism, the zygote." [Sadler, T.W.Langman's Medical Embryology. 7th edition. Baltimore: Williams & Wilkins 1995, p. 3]
"The chromosomes of the oocyte and sperm are...respectively enclosedwithin female and male pronuclei. These pronuclei fuse with each other to produce the single, diploid, 2N nucleus of the fertilized zygote. This moment of zygote formation may be taken as the beginning or zero time point of embryonic development." [Larsen, William J. Human Embryology. 2nd edition. New York: Churchill Livingstone, 1997, p.17]In any event, it is clear from science that abortion at any stage of development kills a human organism. I'll come back to this point later.