Every computer program is, in its essence, something that takes some sort of input, executes backstage computation, and returns an output. For example:

• A calculator: equation -> [math] -> solution
• A chess AI: board state -> [knowledge of rules, game tree analysis, random number generation] -> single move
• Facebook: mouse click -> [server query, web styles] -> webpage

Programs that don’t return output do exist, but obviously the inputted information should be stored for later query and eventual output. If not, why bother computing anything? Just think of a pocket calculator with no screen. Silly. Similarly, some programs don’t take input, but then their outputs are predetermined regardless of the current conditions. A calculator without a keypad would be equally absurd. I/O is vital for anything that wants to interact.

More complex systems might have several sensors that continuously update and a plethora of actions it can execute, but it still reduces to sets of inputs and outputs. A self-driving car takes in visuals from cameras, distances and textures from radio, acceleration/rotation from accelerometers, and whatever else they use as input. Then, for output it returns decisions about steering direction, axle rotation rate, braking, turning on lights, honking, inflating airbags. If human passengers are on board, the car system also outputs air cooling/circulating/humidification parameters, turns on and tunes the radio, swings windshield wipers, locks the door during motion. The car’s ultimate goal is to “drive to a destination safely,” but it possesses quite the toolbag of outputs to facilitate its interaction with traffic.

Well, you can reduce the human body to an I/O system too! Our inputs are the five classical senses that we all know and love: sight, hearing, smell, taste, and touch. Let’s talk about each of them.

Vision takes photons as input, detected by photoreceptors on the eyes’ retinas, and from which our brains extrapolate color information for point in visual space. Vision is spatially precise, far-reaching, extremely fast, and rich in detailed information. The brain devotes a huge chunk of resources to visual processing, and it’s totally worth it.

Audition (hearing) is second, gathering waves in air pressure and decomposing them into vibrational frequencies using hair cells in our ears’ cochleas. Using two ears and volume/dispersion patterns, our brains can roughly approximate direction and distance of origin. However, all signals mix additively, so our brains perform guesswork and filter presumed noise to uncover meaning.

Olfaction (smell) and gustation (taste) are fundamentally similar: the recognition of molecules using a set of 100 or so receptors. Being linked to the vital activity of eating and not getting poisoned, smell is evolutionary older than sight and sound, but it never moved past its ancient basis in molecular interactions. Smell is confounded by air diffusion and wind, so we can’t extract timing information. We also can’t spatially locate its source because our nostrils combine into one air chamber, eliminating our ability to hone like with hearing.

Somatosensation, which vaguely translates to “touch,” turns out to have many components. Crude touch, pressure, vibration, fine touch, pain, temperature, and itch, all with their own sensors and nerve endings in our skin. Balance is more similar to hearing (detecting waves of fluid in our ears), and proprioception (self-position awareness is detected in our muscle fibers).

All in all, we have 10 or so types of fundamental external inputs plus a couple more internal ones (baroreceptors, chemoreceptors), which is a fairly broad repertoire. On the other hand, do you know what’s weird? I think that, on the macroscopic sentient scale, human bodies possess one sole output: skeletal muscle. Our bodies’ goal is to “live,” or more precisely, “eat enough, have sex, don’t die until you make babies,” but apparently that just reduces to pulling parts of our bodies around around. Think about it.

Navigating the world is essential, and we perform it using our handy (and foot-y?) four limbs, steered by their complex sets of muscles. We need locomotion to move toward our food, manual dexterity to move food to our mouths, jaw muscles and tongue to chew, and throat muscles to swallow. Locomotion becomes important again when we when and where deposit our waste and how to move away from it afterwards.

Humans are fundamentally social beings, and social output is almost entirely dictated by skeletal muscle function as well. Speaking is accomplished by pushing out air (using chest muscles) past vocal cords (using throat muscle puppeteers) and modifying the vibrating airflow (using tongue and lip shape). Facial expressions use 40-ish face muscles and rely on genetic programming in everyone’s brains to communicate meaning. Gesticulation is just repurposed locomotion.

Sure, the efferent nervous system has plenty of internal outputs: including hormone release, fluid secretion, smooth and cardiac muscle contraction, and all sorts of cellular responses, but that’s all internal regulation. As far as I know, we don’t hope to interact meaningfully with the outside world by sweating more, drooling using our left or right glands, beating our heart quickly, excreting pheromones in special sequences, or farting in Morse code. The one exception may be male erection, which is modulated by blood flow patterns…

It’s intriguing that once the vertebrate/mammalian/primate body got past the unicellular scale, skeletal muscle was its default solution for almost every external output task. Moreover, like elastic rope, muscle can only exert contractile force. Why don’t we use expanding sacs to push on objects? Why don’t we have ink-ejecting glands, color-changing pigments, swim bladders, built-in bioluminescence, fine control of pheromones, rotating axles, spring-loaded clasps, or forward-pointing flamethrowers? Perhaps muscle worked so well for outputting our brains’ computational power and intent that there was no selective pressure to evolve anything else.

Revel in our marvelous and versatile muscles. Thanks to the nature of our biology, the best way to care for our muscles is to exercise them, so move forth, sing, and dance. The brain may define us as humans, but our skeletal muscles bring us to life.