To be published in Heartbeat Magazine in Fall 2025
We experience our sense of touch and sound as distinct from each other, but they are both rooted in the same cellular systems. As multicellular organisms, we gained sensation of our immediate vicinity—our skin blooming with sensors that respond to direct contact. The world was small, only extending nanometers past the surface of the skin. Then our cells evolved, extending out in length and sensitivity, until our world included the air and light around us. Our sense of sound is just that—a stretching of our sense of touch, feeling the back and forth motion of the air with remarkable sensitivity. Our hearing organ is built from three parts—outer, middle, and inner. In tandem, these parts open up a world of sound that shapes how we interact with everyday life.
The first thing sound hits is our outer ear, or pinnae. The pinnae is a uniquely mammalian trait. As mammals, our ears stick out of our head, catching sound as it whizzes by. This would have been especially useful during the Jurassic period when early mammaliaformes roamed the Earth under the feet of large dinosaurs and needed to get around under cover of night. The shape of our pinnae preferentially reflects and filters for frequencies that fall within the range of human speaking, limiting excess signal. The positioning of our ears gives us the ability to discern where sound comes from with remarkable accuracy. Since sound hits each of our ears at slightly different times, our brain processes that difference by mapping the sound to the space around us. Bats are an especially remarkable example of this with their disproportionately large pinnae that allow them to navigate their surroundings in the dark as they echolocate.
Past the pinnae and through the ear canal, the airwaves then hit our eardrum, which marks the start of our middle ear. In the human middle ear, there are three small bones called "ossicles". The vibrations from the eardrum are transferred to these ossicles—the malleus, incus, and stapes, in that order—amplifying the force of the vibrations on the inner ear. Like the pinnae, having three ossicles is uniquely mammalian. Birds and reptiles have a single ossicle, limiting the range of pitches they can hear. Lizards, for example, hear between 100 and 5,000 Hz, while humans can hear pitches from as low as 20 Hz to as high as 20,000 Hz with our eardrum and triple ossicle combo. Although these bones sit in a small cavity just behind our ears, they actually originate from bones in the jaws of our ancestors. We can see the remnants of this fork in our evolution by observing our reptilian friends, like snakes, place their jaws against the ground to feel surface vibrations that might warn them of approaching predators.
The stirrup meets the cochlea, the final stop in our sound reception saga. The cochlea is a fluid-filled, spiral-shaped canal, lined with specialized mechanoreceptors. Mechanoreceptors are neurons that send signals to the brain in response to physical sensations like touch, pressure, and motion. The ones in our cochlea are called hair cells because their tops have rows of long, hair-like protrusions called stereocilia. These rows are stacked together, arranged from short to tall, so each hair cell looks like a packed crowd on the bleachers at a high school football game. The vibrations in the middle ear cause the fluid in the cochlea to move the stereocilia back and forth in time with the frequency of sound, triggering a signal that the auditory cortex in our temporal lobe combines with the signals from 3,500 other hair cells to produce the singular sensation of pitch, timbre, and duration. The hair cell is likely the oldest feature of our hearing organ; it's a structure we share with fish. Fish have an organ called the lateral line, a fluid filled tube along their side with hair cells that detect changes in the current flowing around them. In the same way we use hair cells to detect airwaves from afar, fish use their lateral line to detect the ebb and flow of water from afar.
Through our evolution, we have extended our sense of touch to reach across physical boundaries, and with this sense we continually shape our language, art, and culture to connect with each other.