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20/03/2017 When Does a Human Life Begin? 17 Timepoints | DNA Science Blog 1/10 PLOS.ORG PUBLICATIONS Search PLOS Blogs Diverse perspectives on science and medicine STAFF BLOGS BLOGS BY TOPIC ć ć ABOUT PLOS BLOGS CONTACT ± About This Blog Search This Blog Previous Next j k When Does a Human Life Begin? 17 Timepoints Posted October 3, 2013 by Ricki Lewis, PhD in Uncategorized 169
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20/03/2017 When Does a Human Life Begin? 17 Timepoints | DNA Science Blog 2/10 A human fertilized ovum. (Spike Walker, Wellcome Images) I rerun this most­read post about when human life begins every time that the discussion resurges, which is usually in the shadow of proposed restrictions on women’s reproductive rights. Strong feelings always seem to trump biological facts. Confusion among politicians appears to be apparent concerning when certain events begin or structures appear; whether to track development from fertilization (conception) or the last menstrual period; and even the distinction between an embryo and a fetus. A 4 or 6 week prenatal human is not a fetus — the difference is not arbitrary, it has biological meaning. From October 3, 2013 I’m the author of several college­level textbooks, on human genetics, human anatomy and physiology, and intro biology. I’ve been in this business for decades. Life science textbooks from traditional publishers don’t explicitly state when life begins, because that is a question not only of biology, but of philosophy, politics, psychology, religion, technology, and emotions. Rather, textbooks list the characteristics of life, leaving interpretation to the reader. But I can see where the disingenuous idea comes from that textbooks define life as beginning at conception — it requires a leap off the page. Consider a report from the Association of Pro­ life Physicians. After a 5­point list of life’s characteristics from “a scientific textbook,” this group’s analysis concludes with “According to this elementary definition of life, life begins at fertilization, when a sperm unites with an oocyte.” Being a biologist, a textbook author, and a mother, I’ve thought a great deal about the question of when a human life begins. So here are my selections of times at which a biologist might argue a human organism is alive. I’ll save my opinion for the end. 1. Life is a continuum. Gametes (sperm and oocyte) link generations. 2. The germline. As oocytes and sperm form, their imprints – epigenetic changes from the parents’ genomes – are lifted. 3. The fertilized ovum . Of the hundreds of sperm surviving the swim upstream to the oocyte, one jettisons its tail and nuzzles inside the much larger cell, which becomes an ovum, completing its own meiosis. A fertilized ovum = conception.
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