https://prolifepages.wordpress.com/2012/09/19/when-does-human-life-begin/
Dianne N. Irving, M.A., Ph.D. has one of the more concise and precise summations of the question of when human beings first come into existence in her lifeissues.net article:
Cloning:
When Word Games Kill
http://www.lifeissues.net/writers/irv/irv_06wordgame.html
“It might surprise many that there has been an unaltered scientific consensus for half a century that a real, already existing, live, whole human being begins as a human embryo (or zygote) immediately at fertilization. This is a scientific fact – not an “opinion”, or a religious or theological belief. And public policy debates and decision making should not continue to escape that unavoidable scientific fact.”
The testimony of the top experts from around the world assembled for hearings before a U.S. Senate Judiciary Subcommittee Hearings on the question, “When Does Human Life Begin?”, April 23-24, 1981 all came to the same, unanimous, unambiguous answer — Human life begins at conception. A few quotes:
https://prolifepages.wordpress.com/when-does-life-begin-senate-judiciary-subcommittee-hearings-april-1981/
Dr. Micheline M. Mathews-Roth, Harvard medical School, gave confirming testimony, supported by references from over 20 embryology and other medical textbooks that human life began at conception.
* “Father of Modern Genetics” Dr. Jerome Lejeune told the lawmakers: “To accept the fact that after fertilization has taken place a new human has come into being is no longer a matter of taste or opinion … it is plain experimental evidence.”
* Dr. Hymie Gordon, Chairman, Department of Genetics at the Mayo Clinic, added: “By all the criteria of modern molecular biology, life is present from the moment of conception.”
* Dr. Richard V. Jaynes: “To say that the beginning of human life cannot be determined scientifically is utterly ridiculous.”
* Dr. Landrum Shettles, sometimes called the “Father of In Vitro Fertilization” notes, “Conception confers life and makes that life one of a kind.” And on the Supreme Court ruling _Roe v. Wade_, “To deny a truth [about when life begins] should not be made a basis for legalizing abortion.”
* Professor Eugene Diamond: “…either the justices were fed a backwoods biology or they were pretending ignorance about a scientific certainty.”
For those who might prefer a more thorough, scientific explanation, please see Dianne N. Irving, M.A., Ph.D.’s article:
When do human beings (normally) begin?
“scientific” myths and scientific facts
http://lifeissues.net/writers/irv/irv_01lifebegin1.html