One of the most frequent motifs that Shubin employs is that of an “inner fish” that exists within the human body (8). The image first occurs in Chapters 1 and 2, when Shubin describes his hunt for the fossil of Tiktaalik. Shubin explains that Tiktaalik is significant because it contains traits of both fish and amphibians, and provides deep insight into how fish first came to evolve limbs and walk on land. For Shubin, one of the most fascinating elements of Tiktaalik is the fact that it contains the bone structure of a limb within its fins. This same bone structure—a single long bone, two interlocking bones, and a wrist bone—is found inside the limb of every limbed animal on Earth, from reptiles to birds to humans. As Tiktaalik is the earliest living creature to have limb bones, scientists can trace the history of human limbs back to Tiktaalik. For Shubin, this is akin to having an “inner fish” within our bodies.

The notion of an “inner fish” extends beyond skeletal structures. In Chapter 3, Shubin describes his colleague Randy Dahn’s experiments with shark embryos. Dahn discovers that Sonic hedgehog, the gene that builds limbs in human embryos, plays a similar function in building shark fins.