33 pages • 1 hour read
Neil ShubinYour Inner Fish: A Journey into the 3.5-Billion-Year History of the Human Body
Nonfiction | Book | Adult | Published in 2008A modern alternative to SparkNotes and CliffsNotes, SuperSummary offers high-quality Study Guides with detailed chapter summaries and analysis of major themes, characters, and more.
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BetaChapters 10-11Chapter Summaries & Analyses
Chapter 10 Summary: “Ears”
In Chapter 10, Shubin considers the biological history of our ears and ability to hear. Mammalian ears are divided into three sections: the outer, middle, and inner ears. The middle ear consists of three small bones—the malleus, incus, and stapes—whose vibrations allow us to hear noises. In the early 20th century, anatomist Ernst Gaupp published a study showing that the malleus and incus in mammals function as jaw bones in reptiles. Later, the paleontologist W.K. Gregory built on Gaupp’s study by analyzing a set of fossils, showing that over time, the malleus and incus evolved from being jaw bones in reptiles to middle-ear bones in mammals. Similarly, the stapes bone began as a jaw bone in sharks and fish and developed into a middle-ear bone in reptiles and mammals. As animals evolved to walk on land, they needed a heightened sense of hearing. In response, Mammals repurposed existing bones in the jaws of reptiles and fish and modified them for hearing (215).
The inner ear consists of a group of “gel-filled” tubes twisted into a “snail-shell shape” (217). The gel moves in response to both physical movement and noises, in turn allowing the brain to sense “sound, position, or acceleration” (217). This capacity makes the inner ear similar in structure to neuromast organs, or sacs embedded in fish skin allowing fish to detect changes in water currents. It has yet to be discovered which of these structures “came first” (224); however, the development of both organs is controlled by a gene called Pax 2, providing further evidence of an evolutionary connection between the two.
Chapter 11 Summary: “The Meaning of It All”
In Chapter 11, Shubin explores the importance—or “meaning”—of the human body’s evolutionary relatedness (229). The “biological “law of everything” that “lies at the root of how we compare different creatures” states that all living things descend from “parental genetic information” (230-31). However, as living things reproduce, mutations occur in their genetic code, slowly creating new biological characteristics—such as humans’ ability to walk on two legs. This process is called “descent with modification” (232), by which animals develop into new and unique species while still sharing some traits with their ancestors. Biologists can observe living organisms and group them based on similar characteristics, reconstructing the evolutionary “family tree” (232). They can then test whether this reconstructed tree of evolution is accurate by comparing it to the fossil record and charting which animals evolved at which points in time.
Due to this process of descent with modification, the human body is essentially a variety of structures borrowed from other animals and jury-rigged to “walk on two legs, talk, [and] think...” (246). Since these structures were not designed to be used for human purposes, “our bodies fall apart in certain predictable ways” (246). For instance, human metabolisms are better suited for our hunter-gatherer ancestors, who needed to quickly gain fat to compensate for periods of little food supply. As modern humans eat more regularly and are less active than our ancestors, our hunter-gatherer metabolisms have lead to an epidemic of obesity.
Several human ailments can be traced to our similarities with fish and other creatures. Hiccups result from a “central pattern generator” in our brain stems that originally evolved to assist tadpoles to breathe through their gills (254). Hernias, meanwhile, partially occur due to vestiges of sharks’ gonads within our bodies.
Chapters 10–11 Analysis
A key concept that Shubin explores in these two chapters is how evolution often repurposes existing bones or organs for new bodily functions. Rather than creating new organs from scratch, animals often gain new abilities by altering existing bodily structures. Shubin demonstrates this evolutionary repurposing through exploring the history of the human ear. One of the defining traits of mammalian ears is the presence of three small bones in the middle ear, whose vibrations allow mammals the ability to hear high-pitched frequencies. Reptiles and other amphibians, in contrast, only contain one middle ear bone. By examining the fossil record, biologists discovered that mammals did not create their two extra middle ear bones out of thin air. Rather, mammals repurposed two reptilian jaw bones for use as ear bones. Over the course of millions of years, these two bones became progressively smaller, and slowly moved position from the jaw to the ear. Likewise, mammals’ inner ears are similar in structure to neuromast organs embedded in fish skin. While it remains unknown which of these two organs evolved first, it is likely that one of them repurposed the other for a new function.
Just as the act of repurposing provides animals with new traits and abilities, so it can also result in the development of new illnesses. Shubin illustrates this by comparing the human body to a “hot-rod [VW] Beetle” (246), with parts jury-rigged to perform unintended functions. This evolutionary repurposing means that the body develops certain weaknesses and “falls apart” (246). One example is hernias, which stem from our evolutionary connection to sharks. Gonads in humans begin their development in the same place that they do in sharks: near the chest cavity. While in sharks the gonads remain in one location, in humans, they must descend through the body to become testicles or ovaries. The result of this long descent is a weakened tissue wall, which can lead to the development of hernias later in life.
