When I’m teaching, I frequently find myself caught between a fascinating digression and the need to keep things simple. For example, if I’m teaching my Human Anatomy class about the lower jaw, I always have to fight an urge to launch into the complexities of lower jaw evolution.
“See? This is the mandible,” I say, “and it’s just one piece of bone.” But all the while, I am clamping my teeth down firmly on my tongue because the exciting truth is that the human mandible isn’t just one bone: it’s really composed of two symmetrical bones, which are called the “dentaries” and are fused together at the midline, but most other mammals have unfused dentaries, and in fact all other vertebrate classes (and the ancestors of all mammals!) have a mandible that is composed of several bones, which include the dentary, the angular, the surrangular and…
You see how it is? The paired dentary thing might be a bit interesting to my pre-nursing students, because human fetuses have that condition. But they could care less about the number of bones in the mouth of a carp and which of those elements are retained by toads and snakes.
At least, not if it isn’t on the test.
So, for the purposes of my class, the human mandible usually remains a single bone, which certainly isn’t called the dentary.
These moments are sad and a little frustrating, but it is often clear what the correct course is. Keep it simple. Maybe hint at the underlying complexity and hope that some intrepid soul will ask me about it later.
However, there are times when the digression is more philosophical. These are almost always much more confusing to the student, but also, I think much more important to understand. No, it won’t be on the final exam, or the MCAT, but I think it will be important in some more nebulous way.
The prime example of this kind of dilemma is the concept of function. My whole course is about the relationship between structure and function. Long bones can act as long levers. Big round cells can house lots of cellular machinery for making lots of proteins. Thin, flat cells act as a permeable barrier because molecules do not have far to diffuse. The list of examples of form following function is endless.
The problem is that “function” does not exist, or at least, many of the things we call functions really aren’t, and the things that really are functions are only functions if we agree on some very special language.
Steven Jay Gould and Elizabeth S. Vrba said it best way back in 1982, and Gould went on at some length about this topic in his essay on spandrels.
Never mind what “spandrels” are, even if they are fascinating.
What Gould and Vrba were trying to say was that function is a concept that presupposes a design, a conscious purpose. When you make a machine, each gear has a function. You put each one there for a reason. You might have multiple purposes in mind for that gear, but there is a still a finite list of functions in your mind. Unfortunately in science, we cannot know the mind of the Designer (if it exists), so we cannot know whether any particular form has a function at all, or how many it has.
Take a rock. What is its function? It depends entirely on context. If it is a chip of flint in a small bag of primitive tools, we could say that its function is to cut things. If it is a big round cobble, it could have the function of beating someone on the head, or pounding grains. Does it have two functions? But can we even say that much? What if it’s just sitting in a river, untouched by human hands? Is its function to provide shelter to some lucky crayfish? What if there is no crayfish? Does it have all of these functions or none?
All we can say about the rock is that it has several potential properties or effects, not that it has a function — unless, according to Gould and Vrba, it has been shaped by natural selection. But even then, the rock (or a cell, or a bone…) might have many functions.
So imagine that you’re one of those hapless students who has found themselves in my class room. You’re just trying to understand the function of the skeletal system in terms of lever mechanics when all of a sudden, the crazy professor just stops talking. He freezes, eyes glazed, and smoke starts coming out of his ears.
The problem is that unlike the example of the dentaries, this crazy, abstract, philosophical problem actually matters.
In fact, getting it wrong can impede science ..and not just pie-in-the-sky, putative-ancestrous, namby-pamby evolutionary science. It could mis-route the flow of actual, physical, grant dollars!
How? Let me tell you the story of the glial cells.
The word “glial” means gluey. It’s Greek, but that’s another digression. Glial cells are found in nervous tissue, such as the brain. And when they were originally discovered, it was decided that their function was just… glue. They just held the neurons together. But after a century, it was discovered that they have important physiological roles (or effects!) to play in the brain and other nervous tissue. They can destroy pathogens. They can produce cerebrospinal fluid. They can control the nutrients, wastes and toxins in the environment of the neurons. They might even help neurons form connections.
So, I argue, when scientists said things like “the function of the glial cells is to hold the brain together” they were cutting themselves off from further discoveries. They had found the function. Why search for more? Had they asked instead, “what is the effect of these cells on their environment?” they would have found very different answers.
Not enough? Perhaps you feel that the early neurobiologists are getting a bad rap. Ok. What is the function of the compound DHEA?
Chemically, DHEA (dehydroepiandrosterone. (Gesundheit)) is similar to cholesterol, estrogen and testosterone (and lots of others hormones, but I digress). In fact, it is one of the chemical intermediates in the biosynthetic pathway from cholesterol to estradiol, the “active form” of estrogen. But it doesn’t just exist fleetingly in the cell as it is turned from A to B. It actually circulates in the blood stream. Moreover, it doesn’t just get turned into other hormones; it has a biological effects of its own. Plus, DHEA has some activity at the receptors for estradiol and testosterone.
So, it seems that DHEA is sometimes released “on purpose”, and sometimes “by accident” – because it is an intermediate – and sometimes it has a direct effect, and sometimes an indirect effect, and sometimes it is turned into other compounds that have other related effects. The best way of understanding DHEA is not in terms of functions or intentionalities, but as a network of multiple effects that reach some sort of balance under different circumstances. If you try to understand it all in terms of functions, plans, designs, you’ll get hopelessly lost.
DHEA has an important biological role, but it may have no function. And looking for “the function” will get in the way of understanding how the whole system works.
This actually happens all the time in biology and medicine. Drugs have unintended side-effects because they target systems that have multiple functions. Or they unintentionally target multiple systems with different functions. In fact, you can buy DHEA in your local health food store, but it’s completely unclear what benefit that might convey. So, students should understand how complex this issue is.
And yet… It’s just a whole lot easier to tell the students that the functions of an organ are X, Y and Z. Certainly, it’s easier to test them that way.
So I don’t let them in on this alternate view on functions. They’ll learn the complexities (if not the philosophical subtleties) in an advanced physiology course.
But sometimes, when I’m teaching them about the ear, I do tell them that the bones we use for hearing evolved from the bones of the jaw…