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
Along The Backbone 