When does a human being begin to exist? That question is at the very heart of the abortion debate, yet it is far from susceptible to a sure answer. This much is beyond serious dispute: biological life begins at fertilization, when the female's egg is united with the male's sperm. But does a collection of cells constitute a human being? Some biologists believe that fertilization does mark the beginning of humanity, since the fertilized egg is a distinct and unique genetic entity. This belief shores up the antiabortion argument of Catholic bishops as well as those of secular pro-life groups. John T. Noonan, a professor of law at the University of California at Berkeley, explains the church's theological position this way: "Once conceived, the being was recognized as man because he had man's potential. The criterion for humanity, .thus, was simple and all-embracing: if you are conceived by human parents, you are human."
Others argue that human life does not start until a week or so after conception, when the fertilized egg has traveled through the Fallopian tube and implanted itself in the wall of the uterus. "We are able to discern [the embryo's] presence and activity beginning with implantation," wrote Dr. Bernard Nathanson, former chief of obstetrical services at New York City's St. Luke's Hospital, in his 1979 book Aborting America. "If this is not 'life,' what is?"
Others pinpoint the beginning of human life when the heart of the embryo begins beating, around the fourth week of pregnancy, or when the central nervous system has developed to the stage where simple reflexes are evident, around the sixth week. By the eighth week, the embryo is undergoing the transition to a fetus and is definitely recognizable as a human being—a stage that some defend as the beginning of human life. Says Dr. Maurice J. Mahoney of the Yale University School of Medicine: "For me, humanness requires that some process of development has taken place which gives the embryo a human form, so that it has a nervous system, a heart and circulatory apparatus, and indications of human shape."
Protestant Theologian Paul Ramsey, a professor of religion at Princeton, declines to identify the precise moment when life begins. But he argues that science now offers evidence of human characteristics in the fetus far earlier than once believed. "I do not say human life begins with conception," says Ramsey, "but science has given us ample factual grounds for believing that the unborn child is an independent human being within the time span [that is, six months] in which the law now says this unborn child can be killed."