Science
Which is the smallest bone in the human body?
- The stirrup, in the ear
- The little toe
- The wrist bone
The answer is The stirrup, in the ear. The stirrup - about three millimetres, and it carries sound.
The stapes, or stirrup, in the middle ear - about three millimetres long and named for exactly what it looks like. It is the last of three tiny bones that carry vibration from the eardrum to the inner ear, and the three together form a lever system that amplifies the signal enough to move fluid, which is much harder to shift than air.
The other two, the hammer and the anvil, are just as small. All three are fully grown at birth and never get any larger, which is unique among human bones.
Their origin is one of the tidier stories in comparative anatomy. In reptiles, the equivalent bones are part of the jaw joint. Over the transition to mammals two of those jaw bones migrated into the ear and took up hearing instead, and the sequence is visible in the fossil record. We hear with repurposed jaw.