Critical Response to Your Inner Fish

Neil Shubin, a paleontologist and anatomy professor, wrote Your Inner Fish following his assignment to teach a first-year anatomy class at the University of Chicago. The author posits that paleontologists are uniquely suited to teach human anatomy because the explanations for human anatomy can be found in the anatomy of other animals. In his book, Shubin argues that missing evolutionary link is Tiktaalik, a 375 million year old fish, who explains the evolutionary shift from fish to land-living animals. In a deviation from popularly evolutionist theory, Shubin’s book delivers the important message that ancient fish bones provide valuable information about the human form, and taking a page from Darwin's legacy, he explains the evolutionary process and how humans got to the form that they are today.

In 2004, Shubin (2009) and his research team discovered a fish with a flat head (later named Tiktaalik, which is Inuktitut for “large freshwater fish”) embedded in 375-million-year-old rocks on Ellesmere Island (p. 4, 21-22). Aside from other notable anatomical differences from present-day fish – conical heads, no neck and the like – this fish had scales, but also had a flat head and neck. The fish also had the evolutionary framework for an upper arm, forearm and wrist. Tiktaalik even had shoulder, elbow and wrist joints like those found in humans. The most important aspect of this discovery is that all of this anatomy in Tiktaalik was covered by a webbed fin, yet clearly corresponded back to the arm structure of modern land animals (Shubin, 2009). This discovery was touted as the intermediate between two different kinds of animal – fish (lives in water) and tetrapod (lives on land) (Shubin, 2009). This connection, however, is only the beginning of the scientific discoveries for the author.

Shubin reviews the human body, piece by piece, and seeks to explain its evolutionary history. He also seeks to connect humans back to their fish origins. Shubin (2009) first reviews the anatomy of the hand, with its joints, fingertips and nails. Although the author classifies the hand as “quintessentially human,” he also relies on other scientific research in the field (conducted by Sir Richard Owen) to connect the human arm to those of other animals, through the presence of one bone in the arm -- the humerus (Shubin, 2009, p. 30). Shubin (2009) argues that this bone, “articulated by two bones, which attach to a series of small blobs” serves as the basis for the “architecture of all limbs” (p. 31). He explains that the only difference between a human arm and, for example, the wing of a bird, is the shapes and sizes of the bones, along with the corresponding number of blobs, fingers and toes (Shubin, 2009). Shubin (2009) again looks to the fish for history of the human limb pattern, relying on the single bone pattern of the lungfish, similar to the ulna in humans. However, Shubin (2009) made the true connection between humans and fish again looking to Tiktaalik, who was found to have an actual wrist. The similarities between humans and fish did not end here for the author.

Shubin compares the symmetry of the human body to the bodies of other animals. Like humans, he identified that fish have a front and a back, a top and a bottom and a left and a right. He also identified that limbs, irrespective of which species of embryo, all looked substantially similar during the development and maturation process. These similarities were explained by other scientists -- Karl Ernst von Baer and Christian Pander -- through the identification of three germ layers, which were the same in every animal, and which were ultimately responsible for the formation of organs in animals (Shubin, 2009). The scientists also identified other parts of the body, and commonalities between animals. A sequence of genes, named “homeobox,” was virtually identical in every species studied by the scientists (Shubin, 2009, p. 108). The “Hox gene” was identified in every animal with a body, and is responsible for the structure of the body, including the proportions of the body (Shubin, 2009). Interestingly, the commonalities do not only extend to physical attributes for animals, but they also extend to an animal’s basic senses.

Looking back at the theory that the activation of certain cells will stimulate the development of a given area, this theory can similarly be applied to the development of the human sense of smell. According to Shubin (2009), fish (among other species) share the same nasal structure as mammals – one internal nostril and one external nostril. The author goes on to examine the number of odor genes housed in the nostrils to trace the evolution of the sense of smell from land animals to water animals. While fish still have the odor genes, they are described as underutilized in these animals. This is also true for mammals like dolphins and whales, except that their genes were modified and are used to create a blowhole for breathing (Shubin, 2009). The same can be said for humans and their use of these genes.

The body system that was the most interesting in the book was Chapter 10 (Ears). Shubin explained that there are three parts to the ear – the external, middle and inner ear. The eardrum (part of the inner ear) is attached to three small bones – the malleus, stapes and incus (Shubin, 2009). The author goes on to describe the anatomy of the human ear, and how the organ functions to carry sound. The structure of the human ear is different from mammals, in that mammals have the three bones attached to their eardrums, reptiles and amphibians have one bone, but fish – the evolutionary missing link – has none. Shubin notes the work of Karl Reichert, German anatomist, and his studies on the gill arches. Reichert’s studies on the skulls of embryos revealed that the gill arch formed the malleus and incus in mammals (Shubin, 2009). He also found that there is a rod (called the “hyomandibula”) in fish, which corresponds to the stapes in the human ear (Shubin, 2009). This chapter provided a causal connection between human and fish, even it appeared that none would be available.

Interestingly, the scientific findings in this book are not purely academic issues. They also have implications for human health and well-being. Chapter Three (Handy Genes) of the book examines the DNA of different species, and through anthropological analysis, identifies genetic commonalities between the animals. In this chapter, Shubin (2009) explains that each living thing begins life as a single, microscopic cell. The cells divide and become “specialized,” with each cell having a specific area of responsibility. The skeleto-development process occurs as certain genes are “turned on and turned off” during this development. Shubin reviewed the research of other scientists who experimented with embryonic development and skeletal formation in chickens to identify the genetic molecules that produced limbs. These researchers were able to manipulate the development of digits using a complex tissue transplant process (Shubin, 2009). The tissue was named “the zone of polarizing activity” (or ZPA), and cells in this zone were believed to release the molecule that controls limb formations (Shubin, 2009, p. 50). This genetic molecule (subsequently named the “Sonic hedgehog gene”) was found in every limbed species, and manipulating the gene in these animals was found to produce the same effect. The implications for human health and well-being is tremendous, and holds the possibility for genetic engineering to cure developmental malformations.

Overall, the book provided a new concept on human evolution. While many have argued that humans evolved from primates, this is the first book in which it is argued that humans actually evolved from fish.

Reference

Shubin, N. (2009). Your inner fish: The amazing discovery of our 375-million-year-old ancestor. London, UK: Penguin.