Along The Backbone

Podcast Teaser: Of all the vertebrate animals, only mammals have muscles of facial expression … why?

Transcript: You are more than just a pretty face.  Your face contains facial muscles that allow you to express and emote to fellow human beings and, some recent data indicates, even to your dog.  But in e-mail, Facebook, and other electronic media that is text-based, we often encounter first-hand how important facial expressions are and how often we are misinterpreted without these visual cues.  To prevent ourselves from being misunderstood, we have given electronic birth to the now ever-present smiley-face emoticons.

But did you ever stop to think about all of the vertebrate animals that lack muscles of facial expression?  Think about it: when is the last time a fish winked at you?  When have you seen an alligator genuinely smile?  What about a frog frowning and expressing deep sorrow?  You haven’t, of course, because most vertebrates except mammals lack facial muscles.

I often ask my students this question: why do mammals have muscles of facial expression?  The usual answers revolve around communication – that mammals are good communicators and need muscles of facial expression to get their various emotional points across.  It’s a nice hypothesis until you consider that a substantial amount of evidence from genetics, brain structure, and the fossil record show quite convincingly that the earliest mammals were nocturnal, and probably spent a good deal of time in hiding from dinosaurs.  As the old Monty Python saying goes, in the darkness nods are as good as winks to blind bats.

But we have another conundrum aside from why facial muscles evolved in mammals, and that is where did these muscles come from in the first place?  Evolution, like a lazy engineer, often doesn’t invent new structures but instead borrows, steals, and augments anatomy from already existing anatomical architecture.  One good thing about muscles and tracing their evolution is that the nerves that supply them with the stimuli to twitch and pull are very conservative.  In other words, no matter how mother nature sculpts the muscles of vertebrates into different forms and functions, the same old chemoelectrical supply lines come along for the ride.  So, we can trace the nerves and their branches to diverse muscles and muscle groups in various vertebrate animals, and using thorough comparative studies we can then determine which muscle groups are related from sharks to shrews.

So, what do comparative studies tell us about facial muscles in mammals?  During embryonic development, the jaws develop from an arch of cartilage that folds forwards.  Behind the jaw arch, a second arch develops called the hyoid arch.  Just as there are muscles that develop with the jaw arch that help close the jaws, so there are muscles that develop with the hyoid arch that accomplish similar ends.  In a shark, the hyoid arch muscles help compress the throat and push struggling prey towards the stomach.  This function of the hyoid arch muscles continues into most other vertebrates, and part of this muscle group is often called the constrictor colli.  These hyoid arch muscles are all innervated by the same nerve, and so we can follow their development very precisely in all vertebrates.

In mammals, the lower hyoid arch muscles do something very fascinating during development: they expand from the hyoid arch onto the face!  It is these muscles that become our facial muscles.  In fact, since these muscles were first identified in human cadavers long ago during the early days of anatomy, the nerve that innervates them is called the facial nerve.  This always throws my students for a loop because it does seem rather odd that the throat-constricting muscles in a shark would be innervated by the facial nerve.

So, we know where the facial muscles are coming from, but we still haven’t tackled why during mammal evolution their lower hyoid muscles would have expanded onto their face.  The answer seems to be rooted in that most fundamental of mammal products: milk.  When most mammals are born, they instinctively search for the mammary glands of their mother, often contained within teats, and begin to suckle.  To suckle effectively, one needs to form the mouth into a gasket around the teat to create the appropriate suction for extracting the life-giving fluid contained within.  To assist baby mammals in obtaining milk from their mothers, there seems to have been strong selective pressure for the expansion of muscles previously associated with swallowing and throat compression.  So far as we can tell, milk production and the evolution of facial muscles go hand-in-hand.

Once a foundation of facial muscles was established in mammals, the way was paved for smiles, winks, frowns, and smirks.  But it is fascinating to consider that what has become an indispensible part of human expression, began with a simple sip of milk.

References / Further Information

This podcast was chosen by popular demand by the followers of this blog.  Thanks for your continued interest in Along the Backbone.

The ability to open doors depends on two things: 1) being able to grip the door handle and 2) being able to rotate the hand so that the door handle turns.  Could a hungry Velociraptor turn a door handle to get at you, the delectable human in hiding?

Podcast Teaser:  In the science fiction story Jurassic Park, the predatory dinosaurs known as Velociraptor are able to use their hands and arms to open doors behind which delectable people hide.  My students often ask me if this could actually happen, and more generally, how much I liked Jurassic Park.  My responses are, “no,” and “it was good science fiction!”

References / Further Information

Why don’t mammals continuously replace their teeth?  The answer may surprise you.

Podcast Teaser: I hate the dentist.  Well, I like my dentist, but I hate going.  I suspect many of you don’t put a visit to the tooth doctor up on your list of favorite things, either.  You can blame a number of things for the necessity of dentistry: our love of sugar top among them.  But actually the problem is an evolutionary one.  We don’t often stop to think about it, but doesn’t it seem odd that you only get two sets of teeth?  First you have your baby teeth (technically, your milk teeth) and then you get a set of adult teeth.  And you better take care of those adult teeth because when they’re gone they’re gone.  But why is this?  Non-mammals, everything from fish to amphibians to reptiles to birds (well, their ancestors anyways) regularly shed and replace their teeth.  Why should non-mammalian vertebrates have it so good?

References / Further Information

Unlike a lizard where the limbs are sprawled out to the sides, most mammals have drawn their limb bones vertically beneath the body.  What are the functional advantages of such a posture? And what does all this have to do with Dr. Bonnan almost being creamed by an African elephant?

Podcast Teaser: I learned the real meaning of the word “awesome” during a close encounter with an African elephant.  A colleague and I were in an animal park in South Africa, and we had spied a large, lone male elephant walking towards our car.  As I was taking pictures of the elephant, our car was suddenly traveling in reverse with my colleague uttering frantic expletives.  It was at this point I noticed that the elephant was picking up speed and coming right for us.  On attempting to turn the car around, we became stuck, and now our fate was left to a very large mammal.  In my cleverness, I rolled up the car window, as if that would protect me from 6 tons of muscle and bone!

References / Further Information

Many of us enjoy eating meat, but few of us pause to think about how important its pre-meal form, skeletal muscle, is for vertebrate life.  Or why you eat different parts of fish and tetrapods for that matter.

Podcast Teaser: I don’t know about you, but I enjoy a good steak, especially fillet minion.  In fact, many of us enjoy eating meat, but few of us pause to think about how important its pre-meal form, skeletal muscle, is for vertebrate life.  Unless you injure your skeletal muscles, you barely notice them – of course, if you’re a body builder, you probably notice them a lot more.  But the contractions of skeletal muscles across the joints in your skeleton do everything from keeping you upright to preventing nasty falls.  Believe it or not, meat is so universal among vertebrate animals that muscles in one area in a fish do very similar things in the same area in your body.  This is because, long ago and 540 million years away, our common ancestor developed two important traits …

References / Further Information

Transcript Available Upon Request.

It seems only fitting that a podcast series called Along the Backbone should discuss the formation of the backbone in one of lengthiest vertebrates: snakes.

Podcast Teaser: Snakes are lizards.  More specifically, snakes are limbless, eyelidless, earless lizards with megakinetic skulls and well-developed salivary glands that often produce venom.  Among the many standout features of snakes, perhaps the most fascinating is how these vertebrates routinely develop a body that will have 120 or more rib-bearing vertebrae and no limbs.  It turns out that a simple but profound difference in the timing of the expression of developmental genes called HOX genes renders snakes limbless, whereas an increase in the frequency of another set of clock-like genes generates their amazing number of vertebrae.

References / Resources:

Transcript available upon request.