Your Inner Fish: A Journey into the 3.5-Billion-Year History of the Human Body

A recurring idea in many of Your Inner Fish’s chapters is how evolution often involves a process of “repurposing”—adapting existing organs or bodily structures to perform new and necessary functions (214). Often, people conceive of evolution as a process in which creatures create new body parts or organs from scratch. Instead, Shubin demonstrates that evolution proceeds by a slow adaptation of existing bodily structures until they finally form an entirely new organ. Shubin primarily explores this idea of repurposing in Chapter 8, when discussing the structure of middle ears. Mammalian middle ears differ from those of our amphibian ancestors in that mammal middle ears contain three tiny bones, whereas amphibian middle ears only contain a single bone. These bones vibrate when sound enters the ear, helping the brain to perceive and interpret sound. The extra two bones in mammals’ ears allow mammals to hear a wider range of high-frequency sounds. However, mammals did not evolve these extra bones by creating two new bones which did not previously exist. Instead, the fossil record reveals that mammals adapted two bones in amphibian jaws, making them smaller and repurposing them for use in the ear.

Shubin describes numerous other bodily structures that have been repurposed for new functions. In Chapter 4, he shows that the process that produces teeth also creates other organs and tissues. Teeth form when two layers of skin tissue fold-in on each other. The fold creates a pocket in the skin into which tooth development proteins are secreted. Shubin explains that this process of organ creation is the same for any organ that “develop[s] within skin: scales, hair, feathers, sweat glands, even mammary glands” (105). Shubin compares this to a “factory” in which a newly discovered production process (such as creating plastics) is used to create a number of new products (106). As Shubin shows throughout Your Inner Fish, much of our bodies’ functions would not be possible without repurposing existing organs.