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

In his 2008 book Your Inner Fish: A Journey into the 3.5-Billion-Year History of the Human Body, paleontologist and biologist Neil Shubin explores the evolutionary history of various anatomical structures. As Shubin explores the histories of everything from our limbs to our eyes and ears, he shows how closely related humans are to all of Earth’s living creatures.

The first two chapters of Your Inner Fish describe one of Shubin’s most important scientific contributions: the discovery of the fossil Tiktaalik, an ancient fish that lived around 375 million years ago. Tiktaalik is significant because it provides insight into how ancient fish evolved to walk on land.

Chapter 1 takes place during the 1990s, when Shubin discovers the fossil of an ancient fish fin that appears to contain the structure of a shoulder bone. Hoping to find a fully preserved specimen, Shubin plans an expedition to an area of the Arctic likely to contain the desired fossil. Though the first expedition proves fruitless, Shubin finally discovers the fossil of Tiktaalik during a 2004 expedition. Chapter 2 shows how the ancient Tiktaalik provides information about human bodies—in particular, our limbs. Though Tiktaalik is a fish, it contains the bone structure of an animal limb embedded within its fin. Tiktaalik is the first known creature to evolve a limb, and thus shows how an “inner fish” is embedded within the human body (8).

The following chapters of Your Inner Fish consider other parts of human anatomy, delving into each structure’s evolutionary history. In Chapter 3, Shubin explores the similarities between human DNA and that of creatures as diverse as chickens, flies, and sharks. Shubin describes a number of biological experiments performed on chicken embryos. These experiments reveal the existence of a gene called Sonic hedgehog, which controls the embryonic development of limbs in all limbed creatures.

In Chapter 4, Shubin explores how the fossil record provides insight into the development of mammalian teeth. While teeth had existed for millions of years, mammals developed a unique jaw structure suited for consuming the “diverse diet” necessitated by their environments (98). Chapter 5 considers the embryonic development of the human skull. Though skulls appear to be internally complex, Shubin explains that their structure follows a precise pattern governed by a series of four arches that form in the embryo. These embryonic arches are shared with every living thing on earth, including sharks and worms.

Chapters 6 and 7 explain how the human body can be constructed from a single-celled embryo. In Chapter 6, Shubin describes a number of biological experiments performed on salamander and fly embryos. These experiments reveal the existence of the so-called Organizer, a small patch of tissue that tells the embryo how to build a body structure. Chapter 7 describes how creatures with bodies first evolved millions of years ago. Experiments on living single-celled organisms, called choanoflagellates, reveal that these organisms contain all of the molecular tools necessary to form a multi-cellular body. Shubin argues that an atmospheric rise of oxygen provided ancient single-celled organisms with the environment needed for building a body.

Chapters 8, 9, and 10 focus on the evolutionary history of our sense organs. In Chapter 8, Shubin reveals the genetic code that grants us the ability to smell, with a single gene corresponding to each of the odors our noses can sense. Chapter 9 explains how our eyesight is made possible by a molecule called opsin, which exists in the eyes of every creature, regardless of eye structure. Chapter 10 centers on the ear, describing how mammals evolved their middle ear bones from the jaw bones of fish and reptiles. By repurposing these jaw bones, mammals gained the ability to hear higher-frequency sounds.

In Chapter 11, Shubin discusses why it is important to understand the evolutionary history of our body parts. Many illnesses arise due to the “inner fish” inside of us (245). For instance, human hiccups occur due to the vestiges of tadpole breathing systems. Similarly, hernias happen because the location of sharks’ gonads influences the structure of the human scrotum. Shubin concludes by discussing mitochondria, ancient bacteria that continue to exist within our cells.