The sciences of embryology and genetics are clear that human life begins at fertilization. There are four empirical attributes of life (growth, reproduction, metabolism, and response to stimuli), and all of these are present at fertilization. Also, the genetic composition of humans is formed during fertilization, and as the textbook "Molecular Biology" explains, this genetic material is "the very basis of life itself." In accord with these facts, the textbook "Before We Are Born: Essentials of Embryology and Birth Defects" states: "The zygote and early embryo are living human organisms."