Resolving the question of when human life begins is critical for advancing a reasoned public policy debate over abortion and human embryo research. This article considers the current scientific evidence in human embryology and addresses two central questions concerning the beginning of life: 1) in the course of sperm-egg interaction, when is a new cell formed that is distinct from either sperm or egg? and 2) is this new cell a new human organism—i.e., a new human being? Based on universally accepted scientific criteria, a new cell, the human zygote, comes into existence at the moment of sperm-egg fusion, an event that occurs in less than a second. Upon formation, the zygote immediately initiates a complex sequence of events that establish the molecular conditions required for continued embryonic development. The behavior of the zygote is radically unlike that of either sperm or egg separately and is characteristic of a human organism. Thus, the scientific evidence supports the conclusion that a zygote is a human organism and that the life of a new human being commences at a scientifically well defined "moment of conception." This conclusion is objective, consistent with the factual evidence, and independent of any specific ethical, moral, political, or religious view of human life or of human embryos.
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