Abstract

The question of when human life begins is a critical starting point for a reasoned debate on the challenging topics of abortion and human embryo research. While there is considerable disagreement over the legal and philosophical questions of whether and when a developing human embryo is a human person, the biological question of when human life begins is unambiguously resolved by the current scientific evidence. The human zygote, or one-cell embryo, arises from the fusion of a sperm cell and an egg cell and can be distinguished from the gametes based on standard scientific criteria: differences in molecular composition and cell behavior. Embryonic development begins immediately following sperm-egg fusion; within minutes, the zygote initiates a molecular cascade that is contrary to the primary function of gamete cells and that uniquely promotes the health and maturation of the zygote as a developing human organism. Thus, human life begins at the well-defined “moment” of sperm-egg fusion. Common arguments against this conclusion are briefly refuted in light of modern scientific findings. Importantly, the conclusion that a human zygote is a human being rests entirely on the scientific evidence and is independent of any specific legal, ethical, political or religious view of human embryos.