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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.
4. Pronuclei merge, within 12 hours. After fertilization, the packets of DNA from male and female — the pronuclei — approach, merge, and the intermingling chromosomes pair and part, as the first mitotic division looms. A new human genome forms. Following that first division, some genes from the new genome are accessed to make proteins, but maternal transcripts still dominate development.
5. Cleavage. Divisions ensue. The cells of an 8-celled embryo (day 3) have not yet committed to becoming part of the embryo “proper” (one with layers) or the supportive membranes. Such a cell can still, on its own, develop. An 8-celled embryo whose cells are teased apart could lead to an octamom situation.
6. Day 5. The new genome takes over as maternal transcripts are depleted. The inner cell mass (icm) separates from the hollow ball of cells and takes up residence on the interior surface. It will become the embryo proper, distinguishing itself from the remaining part of the ball fated to become the extra-embryonic membranes. The icm is what all the fuss about human embryonic stem (hES) cells is about — the stem cells aren’t the icm cells, but are cultured from them.
7. End of the first week. The embryo implants in the uterine lining.
8. Day 16. The gastrula. Tissue layers form, first the ectoderm and endoderm, then the sandwich filling, the mesoderm. Each layer gives rise to specific body parts.
9. Day 14. The primitive streak forms, classically the first sign of a nervous system and when some nations set the deadline for no longer using human embryos in experiments.
10. Day 18. The heart beats.
11. Day 28. The notochord closes, and within it the neural tube forms, preliminary to the spinal cord, while the bulge at the top will come to house the brain. If the tube doesn’t close completely, a neural tube defect (anencephaly, spina bifida, and a few others) results.
12. End of week 8. The embryo becomes a fetus, all structures present in rudimentary form. Attention anti-choice advocates: before this the prenatal human is not a fetus.
13. Week 14 or thereabouts. “Quickening,” the flutter a woman feels in her abdomen that will progress to squirms and kicks from within.
14. Week 21. A fetus has a (very slim) chance of becoming a premature baby if delivered.
15. Birth.
16. Puberty. The Darwinian definition of what matters on a population and species level, when reproduction becomes possible.
17. Acceptance into medical school. I don’t know where this came from, a joke about Jewish mothers, but in some circles it might now apply to acceptance into preschool. Or when one’s grown offspring leave home.
My answer? #14. The ability to survive outside the body of another sets a practical, technological limit on defining when a sustainable human life begins. That limit may of course change.
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