Some of our bones are easy to see. The zygomatic bones that give our cheeks shape, the delicate phalanges of our fingers, and the bony bulbs that are our kneecaps all stand out from beneath the flesh.

Chew on this: Millions of years before the emergence of true mammals, an early ancestor had a tiny, saddle-shaped bone connected to the jaw that was thought to belong to mammals alone. That bone, ...