Where did our three little ear bones come from?

[sue_frantz]

One of the many things I love about teaching psychology is that I can learn something new about the field—about our humanness—just about anywhere. I am currently reading Skeleton Keys by Brian Switek (2019), a science writer and bone geek. Exploring the origins of our bones, this book is a fascinating history. Any history that starts a few hundred million years ago—as this one does—reminds me how improbable our existence is. It is improbable that mammals exist, that primates in particular exist, that humans exist, and, lastly, that I, specifically, exist. With an incomprehensible timeline that is measured in millions of years, I can’t help but think—in the greater scheme of things—how small I am. While that millions-of-years perspective didn’t stop me from being irritated with some of my fellow drivers on my morning commute, I did think about that dinosaur who one day felt irritated with their fellow dinosaurs when travelling to wherever dinosaurs travelled. You have my empathy, dinosaur.

In a brilliant example of burying the lede, I’m actually writing about where the three little bones in the middle ear come from, as I just learned from Skeleton Keys. Stick with me.

Protomammals—a group of animals who were precursors to mammals—had jaws comprised of a number bones. Visit the Wikipedia page for Dimetrodon, a protomammal that lived almost 300 million years ago. On that Wikipedia page, scroll down to the drawings of the skull. In the lateral view, notice the quadrate bone at the back of the upper jaw and the articular bone in the back of the lower jaw. Over time—and by “time” I mean millions of years—those bones shrunk in creatures that followed Dimetrodon, but did not disappear. The quadrate evolved into the incus (anvil), and the articular evolved into the malleus (hammer). The stapes (stirrup) had a different origin, but same idea. It was a small bone on top of the hyoid bone in the neck of protomammals (Maier & Ruf, 2016).

Press your fingers into the skin right in front of your ear. Open and close your jaw. This is where your upper and lower jaws meet. Those tiny bones of the middle ear are right behind that joint.

References

Maier, W., & Ruf, I. (2016). Evolution of the mammalian middle ear: A historical review. Journal of Anatomy, 228(2), 270–283. https://doi.org/10.1111/joa.12379

Switek, B. (2019). Skeleton Keys. New York City: Riverhead Books.