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

Shubin describes the structure and history of human hands. In the 19th century, after studying newly discovered fossils, anatomist Sir Richard Owen realized that the limbs of all land-walking animals follow a similar bone-structure pattern. Regardless of the species, all animal limbs contain the same bones: one single bone forming the upper arm, two bones forming the forearm, a “blob” of little bones forming the wrist, and then the bones that form the limb’s digits (fingers or toes) (43). While Owen believed that such a pattern was the sign of God’s “fundamental design,” Darwin later explained the pattern through his theory of evolution, arguing that all living things with limbs share “a common ancestor” (44).

Though most fish skeletons do not follow this bone structure pattern, several fish species demonstrate how animals evolved limbs from their fish ancestor. The lungfish, for instance, shares with land animals both lungs and a single large bone at the base of its fin. Fossilized fish show even more developed limbs, with the Eusthenopteron following the “one bone-two bone” portion of the pattern, and the Acanthostega gunnari exhibiting “fully formed digits” (48). Until 1995, however, there existed no fish species fossil with a fully formed limb but without digits—a key step in the evolution of limbs.

Shubin made the first discovery of just such a fossil in Pennsylvania—a single fish fin displaying the full bone structure of a limb. After several expeditions in the Arctic, Shubin discovered a complete fossil of the Tiktaalik, whose fin contains the full structure of wrist bones. The bone structure of the Tiktaalik fossil’s fin made it “capable of doing push-ups” (54). This ability allowed the Tiktaalik to jump-up from streams onto the “mudflats along the banks” of its environment (55), potentially avoiding larger fish predators. Shubin argues that the Tiktaalik fossil shows how the limbs of all creatures, including humans, developed, and that within the human body is the structure of a fish. 

Shubin considers what DNA reveals about the history of the human body. All living creatures display a similar embryonic development, growing from a single cell to a complete body. Scientists researching and experimenting on DNA hope to discover how it allows cells to build a complete body—whether that of a shark, a chicken, or a human. Though each of a body’s cells contains a copy of the same genetic code, only portions of this genetic code are “turned on” in each cell, allowing cells to differentiate themselves and perform different functions (63).

Experiments on creatures’ embryos shed light on the development of limbs. Limbs first form as “tiny buds” (66), and then develop the complex structures of bones and digits. When biologists Edgar Zwilling and John Saunders graft a portion of cells from one side of this bud to the opposite side, the embryo develops a “full duplicate set of digits” that perfectly mirrors the original digit set (68). Zwilling and Saunders call these cells the zone of polarizing activity or ZPA, and argue that ZPA molecules fully control the development of a limb’s digits. In the 1990s, a group of scientists experimenting on flies and chickens discover a gene similar to ZPA, which controls the formation of an embryo’s digits. Scientists name this gene Sonic hedgehog, and discover that the gene is active in the same part of the embryo as the ZPA. Further research reveals that the Sonic hedgehog gene exists in “every limbed animal” (73).

In Shubin’s developmental biology lab, scientist Randy Dahn discovers that shark and skate embryos also contain the Sonic hedgehog gene. Dahn is interested in sharks as they are “the earliest creatures that have fins with a skeleton inside” (75). By studying the DNA structure of living shark embryos, Dahn hopes to discover whether the mechanisms by which human limbs form originally evolved in sharks. Dahn indeed discovers the Sonic hedgehog gene in sharks. When he activates the Sonic hedgehog gene on two sides of the embryo’s developing fin, the result is a “mirror-image fin” (77). Dahn then grafts the Sonic hedgehog gene proteins from a mouse onto a developing shark fin. Though shark fins typically consist of identical “skeletal rods,” the presence of the mouse protein leads the shark to develop differently sized bones, analogous to a mouse’s digits (78). The experiment reveals that animal limb digits did not develop from “new DNA”, but rather from repurposing “ancient genes in new ways to make limbs with fingers and toes” (80).