Two bones of your middle ear used to be part of the jaw

The malleus and the incus, which pass sound across the middle ear, are the articular and the quadrate, the jaw joint of a reptile. Mammals bite with a different pair of bones. The old pair was free to hear.

A reptile's jaw joint is between the articular bone, on the lower jaw, and the quadrate, on the skull. Mammals bite between two other bones, the dentary and the squamosal, and the old pair sits in the middle ear: articular became malleus, quadrate became incus, and both now vibrate between the eardrum and the stirrup. The idea is old, from nineteenth-century embryology. What made it unbearable to doubt was the fossils. Early mammal relatives show the two joints at once, the new one taking the bite, the old one still attached to the jaw but already involved in hearing, and then, in later animals, detached. The transition is a sequence of skeletons, not a story told backward from the anatomy lab.

The benefit is mechanical. Three bones in a chain can amplify a faint pressure in air into a motion the fluid of the inner ear will notice. A jaw joint cannot do that job and chew. The ancestors did not redesign the head in one step. They chewed with a new hinge while the old hinge was still in the way, and the old hinge, relieved of the bite a little at a time, became an instrument. The instrument is now small enough to lose inside a hearing aid's field of view. Its names — hammer, anvil — describe the work. They hide the previous employment.