Per the "American Heritage Dictionary of Science," there are four defining characteristics of life: (1) growth, (2) reproduction, (3) response to stimuli, and (4) metabolism. Numerous scientific texts document that these attributes are all present at or just after fertilization, which is the earliest stage of human development. In keeping with these facts, the medical textbook "The Developing Human: Clinically Oriented Embryology" states that "fertilization" creates "a new combination of chromosomes that is different from that in the cells of either of the parents," and this "is the beginning of a new human being."