Your Inner Fish

As a graduate student, Shubin had to study the nerves of the human body for an anatomy final. Two of these cranial nerves (nerves in the head) have a very complicated path through the body that becomes much simpler if one knows anything about shark anatomy. The jumble of human nerves is actually a simple plan in fish.

The Inner Chaos of the Head. Head anatomy is difficult to study because the human head is encased in the bone box of the skull. Skulls have three parts: plates covering the brain, a platform block that holds the brain up, and rods in the jaw, throat, and ear. The skull is also three-dimensional, with compartments for different organs that make it harder to visualize how everything fits together.

There are twelve cranial nerves in the human head. Some of these have simple paths to just one organ or muscle in the body, like those that attach to the eye (optic nerve) or ear (acoustic nerve). But four of the cranial nerves have complex functions that take them in “random” paths throughout the head with many different branches. The trigeminal nerve and facial nerve are especially difficult to pin down.

The trigeminal nerve has to do with controlling the muscles that humans chew with and small muscles in the ear, as well as sensations in the skin of the face and the teeth. The facial nerve controls the muscles involved in facial expressions as well as more small muscles in the ear. At first, it seems like these two nerves serve the same function, even crisscrossing over each other at times. Yet Shubin illustrates the sense of these nerves by describing the plumbing of an old building. In order to update old plumbing to modern functions, it is sometimes necessary to “jury-rig” the old pipes and wires to accommodate new needs. Shubin applies the same concept to the history of the nerves in the human head.

The Essence in Embryos. At the embryonic stage, the human head is just a big glob of cells. At three weeks, four blobs begin to form around the area that will be the throat. These blobs, called arches, will become different tissues for the head. Cells from the first arch become the upper and lower jaws, and two of the ear bones. The second arch becomes the third ear bone and the muscles in the face. The third develops into bones, muscles, and nerves in the throat, and the fourth arch becomes the deepest parts of the throat, including the larynx.

Looking at the arches provides a neat trick to understanding the cranial nerves. The trigeminal nerve serves all the body systems formed by the first arch. The facial nerve follows the path of the second arch. The same pattern holds true for the nerves associated with the third and fourth arches.

Shubin also relates the head to the body, following the insight of the German writer Johannes Goethe in the 1800s, who saw that the skull is made up of many vertebrae fused together. Looking at each vertebra as a distinct segment of the human body allows anatomists to see the nerves that are associated with each body system exit the spinal cord in a specific place according to their segment. The same segmental organization exists in the head, but can only be seen at the embryonic stage when the human head is not so complicated.

Our Inner Shark. The arches of the human head look very similar to the gill slits of sharks and fish, but human “gills” are sealed by the plates of the skull before most human babies are born. However, each arch is responsible for many of the same body systems in both sharks and humans. The first arch makes jaw bones for both species, with the only difference being that the human first arch also develops into ear bones. The second arch handles inner ear muscles and throat muscles for humans, while in sharks the second arch creates bones that support the upper jaw. The third and fourth arches focus on gill movement in sharks, while in humans they supply the muscles that allow us to swallow and talk. Mapping these systems on a shark head and a human head creates blueprints that look remarkably similar.

Gill Arch Genes. The first three weeks after conception are a very active time for the arches, with many genes turned on and off as brain tissue begins to develop and specific regions become different from each other. Each arch has a different set of Hox genes active that tell that arch what to become. If a scientist manipulates the Hox genes in a specific arch, it is possible to change the identity of that arch. Experiments on the Hox genes in the arches of frogs were able to create frogs with two “first” arches that developed two jawbones as the frog embryos matured.

Tracing Heads: From Headless Wonders to our Headed Ancestors. Shubin extends the comparison of human heads to shark and frog heads, then further to worm “heads.” Though worms are invertebrates, a specific worm, Amphioxus, has a notochord that acts like a primitive version of a backbone. Though amphioxus has no head, 500-million-year-old fossil impressions of amphioxus bodies have gill arches. This basic structure of the human head stretches all the way back to ancient worms.