The reconstruction of a once-living landscape in northern Greenland from 2 million years ago, deduced from bits of DNA bound to minerals…
I originally published When Does a Human Life Begin? 17 Timepoints here at DNA Science in 2013. My intent was to inform those who confuse embryo with fetus with baby by presenting how biologists describe human prenatal development – beginning at fertilization. Human gestation is on average 38 weeks, not 40, according to biology.
I rerun “17 Timepoints” periodically to counter assaults on woman’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 reran “17 Timepoints” with the updated headline (which I didn’t write) Viewpoint: ‘The fetus is 1/25th of an inch’ — Texas abortion ban bungles the science on when human life begins, contends biologist and professor.’
And along the way, various right-to-lifers have responded to my post with insults to my expertise, but no sign of actually understanding the biology. So it goes …
Now the rerun of “17 Timepoints” is in response to the leaked Supreme Court document threatening Roe v Wade, published in Politico and written by Josh Gerstein and Alexander Ward. To paraphrase Ronald Reagan, here we go again.
From October 3, 2013
I’m the author of several college-level 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 5-point 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.”
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 to the oocyte, one jettisons its tail and nuzzles inside the much larger cell, which becomes a fertilized ovum. That’s conception.
4. Pronuclei merge. The DNA in these packets from each gamete replicate, and then the pronuclei meet and merge, within 12 hours. The intermingling chromosomes form a new human genome. Following the first mitotic division, some genes from 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-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. Sometimes that happens, and multiples result.
6. Day 5. The new genome takes over as maternal transcripts wane. Cells continue to divide, forming a hollow ball. A smidgeon of cells, the inner cell mass (icm), that separates and lodges on the interior surface will become the embryo proper, as the hollow ball contorts into the extra-embryonic membranes. (The icm is what all the fuss about human embryonic stem cells is about — but the stem cells arise in a glass dish, they don’t actually come from an embryo, never have.)
7. End of the first week. The embryo implants in the uterine lining.
8. Day 15. The primitive streak forms. This marking along one side is the first inkling of a nervous system. Some nations ban experimenting on human embryos from this point.
9. The gastrula forms throughout week 3. Tissue layers arise, first the ectoderm and endoderm, then the sandwich filling, the mesoderm. Each layer eventually becomes specific body parts.
10. Day 18. The heart beats.
11. Day 28. A strip along the back of the embryo, the notochord, closes. Within it forms the neural tube, which gives rise to the spinal cord; a bulge at the top comes to contain the brain. If the tube doesn’t close completely, a neural tube defect results.
12. End of week 8. The embryo becomes a fetus, all structures present in rudimentary form.
13. Week 14 or thereabouts. “Quickening,” the flutter a woman feels 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. This is the earliest point of viability. Obstetricians call this week 23.
16. Puberty. Sexual maturity is the Darwinian 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 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.
Having a functional genome, tissue layers, a notochord, a beating heart … none of these matter if the organism cannot survive where humans naturally survive.
Technology has taken us to the ends of the prenatal spectrum, yet not provided too much for the middle, other than fetal surgeries for a handful of conditions and attempts at mimicking a uterus. We can collect and select gametes to guide conception, collect and select very early embryos to avoid those genetically destined to a terrible medical fate.
Although the gestational age at which a premature infant can survive has crept ever younger, it hasn’t by much, not since I starting thinking about these things back when I was a stage #16. In high school I worked in a lab that processed material that would arrive in dixie-cup-like containers, the “products of conception” after abortions. The cups held floating tiny tissue pieces, nothing like the brutal images on the placards held up in front of Planned Parenthood clinics to antagonize women making the hardest choice of a lifetime.
I would really love to retire this post.