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

“Getting Ahead” focuses on the development of the human head. In graduate school, Shubin had difficulty memorizing the structure of the head’s cranial nerves. Inside the head, there are twelve different cranial nerves, which link the brain to muscles and organs inside the skull. These nerves allow the brain to control our muscles and in turn allow us “to bite, to talk, and to move our eyes and whole head” (111). Many of the nerves follow a direct path from the brain to the organ and only perform one function. However, two nerves—the trigeminal nerve and the facial nerve—have complex routes, linking to a variety of seemingly unrelated muscles. The trigeminal nerve connects both to our jaw and to our ear, and controls a variety of functions including chewing. The facial nerve controls facial expression, and follows a twisting path through the skull.

The trigeminal and facial nerves are complex because of a stage in human embryonic development. As an embryo grows, its head originally forms as a “big glob” on one end of the embryo (115). The small glob of cells develops into four different swellings and folds, which Shubin refers to as arches. Within each arch, different parts of the human head develop. Jaw bones and two ear bones—the malleus and incus—develop in the first arch, while the second arch forms “the third small ear bone (the stapes), a tiny throat bone, and most of the muscles that control facial expression” (116). The trigeminal and facial nerves have convoluted paths and functions because each nerve serves the tissues formed in its arch: the trigeminal nerve the first arch, and the facial nerve the second.

The arches are important for embryonic development because they reveal that a clearly defined pattern governs the structure of our heads. Similar arches are found in the embryos of fish and sharks, whose arch creases develop into gills. There is a particular similarity between our arches and those of sharks, as the first arches of both humans and sharks form jaws. Shubin traces gill arches back to a species of worm called Amphioxus, which makes use of the same gill arches found in sharks and human embryos to “filter out little particles of food” from water (128). 

This chapter investigates the mechanisms by which a single-celled embryo is able to develop into a fully formed body. Shubin describes how the bodies of creatures follow a precise structure, with most animal bodies symmetrically organized “front/back, top/bottom, and left/right” (129). Further, despite their eventual differences, the embryos of most creatures are often remarkably similar at their early stages of development. During the 1800s, the scientist Karl Ernst von Baer probed the extent of similarities between embryos through observations of chicken embryos. Von Baer’s experiments revealed that all of an animal’s organs develop in one of three different layers of tissue located in the embryo: the ectoderm, the endoderm, and the mesoderm (135). 

Following von Baer’s experiments, other biologists sought to understand which of an embryo’s parts dictate how the embryo should build its body. The biologist Hans Spemann, working in the early 20th century, sought to discover whether the “information to build whole bodies” was housed in all or only some of an embryo’s cells (139). Spemann tested this experimentally by dividing a newt egg in two. The result was twin newts, proving that “some cells have the capacity to form a whole new individual on their own” (140). Spemann’s experiments were further developed by the biologist Hilde Mangold. When Mangold grafted a portion of one embryo’s cells onto another, the resulting embryo formed two whole bodies. Based on this experiment, Mangold concluded that embryos contain a small piece of tissue called the “Organizer,” which controls the development of the entire body (141).  

Shubin concludes the chapter by discussing a variety of experiments to identify the genes that control the Organizer and other elements of body development. One experiment conducted on mutated flies identifies the Hox gene, which exists in every “animal with a body” and keeps our bodies ordered in a front-to-back direction (144). Another experiment by Richard Harland identifies the Noggin gene. Noggin acts similarly to the Organizer, and leads to the development of a second head when injected in an embryo. Finally, an experiment conducted by biologist Mark Martindale shows that the same genes in sea anemones and humans are involved in organizing our “head-to-anus axis” (150).