I originally posted When does a human life BandGin? 17 time points here at DNA Science in 2013. My intention was to inform those who confuse embryo, fetus and baby by presenting how biologists describe human prenatal development – beginning with fertilization. Human gestation lasts an average of 38 weeks, not 40, according to biology.
I raise “17 time points” periodically to address abuses of women’s reproductive rights – which unfortunately happens with disturbing regularity.
In 2017 I reposted when The Federalist published “Life Begins at Conception, Says Department of Health and Human Services”.
Then in September 2021, Genetic Literacy Project revived “17 Timepoints” with updated title (which I didn’t write) Viewpoint: ‘The fetus is 1/25th of an inch’ – Texas’ abortion ban is shaking up science about when human life began, says a biologist and professor.
And along the way, various rights to life responded to my message with insults to my expertise, but no sign of any real understanding of biology. So, how are you…
Now, the rerun of “17 Timepoints” is in response to the leaked Supreme Court document threatening Roe vs. Wade, published in Policy and written by Josh Gerstein and Alexander Ward. To paraphrase Ronald Reagan, here we go again.
Since October 3, 2013
I have authored several college-level textbooks on human genetics, human anatomy and physiology, and introductory biology. I’ve been in this business for decades.
Life science textbooks from mainstream publishers don’t explicitly state when life begins, because it’s not just about biology, but about philosophy, politics, psychology, religion, technology, and emotions. . On the contrary, textbooks list the characteristics of life, leaving the interpretation to the reader. But I can see where the fallacy comes from that textbooks define life as beginning at conception – it requires a page break. Consider a report from the Association of Pro-Life Physicians. After a 5-point list of the characteristics of life taken from a “scientific textbook”, the analysis of this group ends with “According to this basic definition of life, life begins at fertilization, when a sperm s ‘units with an oocyte’.
I have thought a lot about the question of when a human life begins. So here are my picks of when a biologist might say a human organism is alive. I save my opinion for the end.
1. Life is a continuum. Gametes (sperm and oocytes) connect the generations.
2. The germline. As oocytes and sperm cells form, their imprints – epigenetic changes to the parents’ genomes – are lifted.
3. The fertilized egg. Of the hundreds of sperm that survive swimming to the egg, one throws up its tail and nestles inside the much larger cell, which becomes a fertilized egg. It’s the design.
4. The pronuclei fuse. The DNA of these bundles from each gamete replicates, then the pronuclei meet and fuse, within 12 hours. The intertwined chromosomes form a new human genome. After the first mitotic division, some genes in the new genome are accessed to make proteins, but maternal genetic information, in the form of RNA transcripts, still guides development.
5. Cleavage divisions ensue. The cells of an 8-cell embryo (day 3) have not yet committed to being part of the “proper” embryo (one with layers) or supporting membranes. Such a cell can still, by itself, develop. Sometimes this happens and multiples result.
6. Day 5. The new genome takes over as the maternal transcripts decrease. The cells continue to divide, forming a hollow ball. A few cells, the inner cell mass (icm), which separates and lodges on the inner surface will become the embryo proper, as the hollow ball twists into the extra-embryonic membranes. (ICM is the source of all the hype about human embryonic stem cells – but stem cells appear in a glass dish, they don’t actually come from an embryo, ever.)
7. End of the first week. The embryo implants in the uterine lining.
8. Day 15. The primitive line is formed. This marking along one side is the first hint of a nervous system. Some nations ban experiments on human embryos from this point on.
9. The gastrula forms throughout week 3. Layers of tissue appear, first the ectoderm and endoderm, then the filling of the sandwich, the mesoderm. Each layer eventually becomes specific parts of the body.
ten. Day 18. The heart beats.
11. Day 28. A band along the back of the embryo, the notochord, closes. Inside is formed the neural tube, which gives rise to the spinal cord; a bulge at the top comes to contain the brain. If the tube does not close completely, a neural tube defect results.
12. End of week 8. The embryo becomes a fetus, all structures present in a rudimentary form.
13. Week 14 or so. “Acceleration”, the throbbing a woman feels that will evolve into squirming and kicking from within.
14. Week 21. A fetus has a (very slim) chance of becoming a premature baby if delivered. This is the first point of viability. Obstetricians call this week 23.
16. Puberty. Sexual maturity is Darwin’s definition of what matters to populations and species, when reproduction becomes possible.
17. Social milestones. Acceptance into (a) preschool (b) college or (c) medical school; marriage; a career begins; when adult offspring leave home.
My answer? #14. The ability to survive outside the body of another sets a practical and technological boundary for defining when sustainable human life begins. This limit can of course change.
Having a working genome, layers of tissue, a notochord, a beating heart…none of that matters if the organism can’t survive where humans naturally survive.
Technology has taken us to the ends of the prenatal spectrum, but not too much for the middle, aside from fetal surgeries for a handful of conditions and attempts to mimic a womb. We can collect and select gametes to guide conception, collect and select very early embryos to avoid those genetically destined for a terrible medical fate.
Although the gestational age at which a premature baby can survive is getting younger and younger, it hasn’t changed much, not since I started thinking about these things when I was stage #16. In high school, I worked in a lab that processed material that arrived in dixie-cup containers, the “products of conception” after abortions. The cups contained tiny floating pieces of fabric, nothing like the brutal images on signs held up outside Planned Parenthood clinics to antagonize women making the toughest choice of their lives.
I would really like to remove this post.