Three Tiny Bones
1Eighth grader Ikaika Brennan had an earache that would not go away, so his doctor sent him to an ear, nose and throat clinic. While she checked his ear, Dr. Ottoline Mwangi-Ross showed him a model of the middle ear. Sound makes the eardrum vibrate, and three tiny bones called the hammer, the anvil and the stirrup pass the vibrations inward. Then she surprised him: "Two of those bones started out as part of your jaw." She lent Ikaika a set of notes from the medical school where she teaches, so he could see what she meant.
2The notes first compared adult animals. In an adult mammal, each half of the lower jaw is a single bone, and the jaw hinges directly against the skull. Adult reptiles and birds have several bones in each half of the lower jaw. Their jaw hinges between two small bones at the back of the jaw, one in the lower jaw and one in the skull, and their middle ear has only one bone, shaped like a rod.
3Next the notes described embryos. Early embryos of fish, reptiles, birds and mammals all grow ridges along the sides of the neck called pharyngeal arches. In the first arch, rods of cartilage form to support the growing jaws: one for the upper jaw and a long one for the lower jaw. In fish, reptile and bird embryos, bones form at the back ends of these rods, where they meet, and become the jaw hinge. In mammal embryos, the back ends of the same rods form the hammer and the anvil. For a while they sit at the back of the jaw, and then they separate from it and move into the middle ear. Later, most of the rest of the lower-jaw rod disappears.
4Opossums show one step of this in a newborn. A Virginia opossum is born after only about 13 days of development and keeps growing in its mother's pouch. At birth, its mammal jaw hinge has not formed yet, so the baby opens and closes its mouth at a hinge between the bones that will become its hammer and anvil. A few weeks later the new hinge forms, and those two bones move into the ear.
5The notes ended with dolphins. Adult dolphins breathe through a single blowhole on top of the head, and their skin is smooth and hairless. In a dolphin fetus, two nostrils first form near the tip of the snout, as in most mammals, and a few whisker hairs grow on the snout; the whiskers usually fall out soon after birth. A museum at the medical school keeps preserved dolphin and pig fetuses of different sizes. Students there measured how far back each nostril sat, as a percent of the head's length from the snout tip. Their results are in Table 2.