Human Embryology -Ontologically Awkward Thoughts


Ontogeny recapitulates phylogeny: foetal development goes through the same stages as evolution would have -Haeckel’s theorem. It’s a phrase -one of the few- I remember from my embryology classes in medical school. It’s a bit simplistic, of course, and long since discredited but nonetheless illustrative of some evolutionary phylogenetic similarities. It demonstrates the potential for variation in the developmental effects of various controller genes. It also demonstrates just how far embryology has come since the days when lecturers were forced to bore their classes with soporifically-presented unpronounceable and mysterious Latin names.

In those earlier than early days, I had no inkling that I would end up as an obstetrician and be involved, however vicariously, in the process. Unfortunately, I used to fall asleep in the embryology classes -it’s where I learned to nap with my head carefully balanced upright on my shoulders. I didn’t care what I was missing; in those years, I suspect it was not very much. Nowadays, though, everybody seems to be curious, and I often get asked about baby development. Happily, I don’t have to resort to showing the parents my old, retired embryology text book with its overly complicated diagrams that puzzled me even then. Now I have a colourful chart on the wall of my examination room that illustrates -simply, and in an appropriate, real-life size- the developing foetus at various gestational weeks.

Those diagrams don’t even attempt to show any of the amazing features in evolutionary development, though, so I am constantly on the lookout for aspects of it that were not appreciated in my student days. Of course, in those heady times I well might have missed some stuff…

For example, the ear-bones of mammals (humans are mammals, remember) have an interesting history. There are three tiny little bones in the inner ear -ossicles- that serve to modulate any sound energy arriving there and to dampen the vibrations from the ear drum. Reptiles and various other creatures only have one, the stapes (‘stirrup’ in Latin). Where did our two extra bones come from? Well, those same reptiles (also including birds, dinosaurs, etc.) have a few extra bones in the joints of their jaws which we mammals have co-opted for our inner ears: the incus (‘anvil’ in Latin) and malleus (hammer). Nature doesn’t waste things -the innovation is called exaptation: the process of using structures that have already been introduced by evolution and re-purposing them. And yes, I guess that means we have to chew differently than dinosaurs, but hey…

Other things persist in us with diminished or unknown function. I’m thinking here of such structures as the appendix. Most of us only know about it if we have to have it removed. Mine had to be taken when it ruptured in first year medicine while I was in the anatomy lab -my only regret was that we hadn’t toured that far in our cadaver yet so it was still terra incognita for us. But we all figured it was once a digestive organ (I mean look where it is, eh?). There is a suggestion (C. Choi in Live Science, 2009) that even now, the appendix might contain important bacteria that can reconstitute gut flora after such things as antibiotics have changed the normal distribution and population of the micro-organisms. We know that normal bowel bacteria have many functions, including a role in the immune system, so it would be nice to think the appendix might help refurbish our defenses. Re-arm us. Unfortunately, that may mean I’m all on my own now… Naked, sort of.

And, of course, there is the caecum (Latin for ‘blind pouch’) to which our appendix is attached. In some herbivores it’s much larger and it is a hotbed of bacteria that can hydrolyse cellulose. Without them, grass is about as nutritious as cardboard and with our little dead end package, I’m assuming it’s why we no longer enjoy grazing very much.

I’m intrigued by the coccyx as well. It is the lower end of the spinal column and is what forms the tail in other animals. Believe it or not, the human foetus has a little tail between about 31-35 days into development (I had to cull this one from Wikipedia, I’m afraid, because my old embryology text was fashionably non-committal on the subject. Uncomfortable, even.). I also learned from the same source that because it is where some muscles still attach, the coccyx hasn’t yet disappeared entirely. Maybe it’ll be gone in a few millennia -if we’re still extant as a species, that is… and don’t discover that we need a tail again.

Finally, I have to confess I have a personal interest in wisdom teeth. Since I don’t have any, and never did, I have always felt cheated. Diminished, as it were, by the thought that I was given neither the wisdom to which I, as a human, was entitled, nor the teeth as a little bonus. A dentist once tried to reassure me that not everybody has them and, in an attempt to impress me that she had recently graduated with honors from dental school, that it is probably related to the functioning of a gene called PAX9. This only depressed me further, however, and immediately suggested to me that either I didn’t get the gene, or that mine was malfunctioning. She tried a different tack (attempting, no doubt, to distract me from what she was doing in my mouth). Third molars (she had to explain that she was using dental-speak for ‘wisdom teeth’ when I started to salivate excessively) were probably how our ancestors dealt with plants they were eating. They were grinders. But I was lucky, she continued, probing deeper into my gum, because we changed our diets and evolved smaller jaws. So nowadays there’s no room for the extra teeth. They are vestigial and have to be pulled out, in other words. My persisting skepticism must have been obvious, however, and I could see her mask change shape. Given the drill in her hand and the impatience in her eyes, I pretended to be convinced and dropped the subject. I still worry, though.

Embryology, like its eponymous subjects, has evolved over the years. It has become more interesting and certainly accumulated more frills. DNA and ever-advancing technology have dragged it into social media and otherwise polite conversation -our ancestry titillates us like never before. I mean I could probably even stay awake in the classes nowadays. But all the same, the skill I developed for napping unawares shouldn’t be dismissed as vestigial; it is still useful. I look upon it as an exaptation of my neck muscles and inner ear, an evolutionary strategy for surviving some of the appurtenances that continue to bedevil us in the modern age: there are still concerts I would rather not attend. And as for meetings…

 

 

 

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