NOTE: Here we define "human being" as a biological concept,
not to be confused with the philosophical concept of "person."
By its simplest definition, a "human being" is a member of the species Homo sapiens.
While there is much debate over when a human organism becomes a "person," there is not much debate over when a human organism begins biologically:
"Biologically speaking, fertilization (or conception) is the beginning of human development. Fertilization normally occurs within several hours of ovulation (some authors report up to 24 hours) when a man’s sperm, or spermatozoon, combines with a woman’s egg, or secondary oocyte, inside a woman’s uterine tube (usually in the outer third of the uterine tube called the ampulla)."
Related Blog Posts:
Do human beings begin at fertilization?