Your Inner Fish

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Your Inner Fish Chapter 4 Summary & Analysis

Summary
Analysis
Though teeth might not be the most glamorous anatomical structure, looking at teeth is an integral part of learning about the lifestyle of an animal because teeth tell scientists what an animal most likely eats. For humans, we have a mix of blade-like teeth for cutting meat and flat teeth for grinding plant or meat material. Our upper and lower jaws also fit together precisely (a fit called occlusion) to break up food with maximal efficiency.
Teeth are a fountain of information for paleontologists that must rely on very little physical evidence to make conjectures about the entire ecosystem of the past. From the simple knowledge of how an animal eats, Shubin can reconstruct a complex picture about how and where that animal lived.
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Teeth are one of the most common finds for paleontologists, because an easily-preserved hard mineral called hydroxyapatite makes up much of the outer layer of the tooth. Teeth are especially helpful for mammal fossils, as mammal species have distinctive teeth. While reptiles all have similar teeth that they replace many times over their lifetime, mammals have teeth that occlude and are only replaced once. During the time period from 225-million to 195-million years ago, paleontologists see a progression from dog-sized reptiles with fairly simple teeth to small animals with mammal-type teeth that fit together inside a smaller jaw.
Mammals have relatively complex teeth that are specialized for each species. From the basic template of reptile teeth, mammal teeth seem to develop in ways that let that mammal get the most nutrition and survive in their environment. Based on the fossil record, there seems to be a direct line from reptiles with simple teeth to mammals with more complex teeth.
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Shubin began studying these early mammals at Harvard, under Farish A. Jenkins, Jr., who specializes in looking for the fossils of early mammals. With the help of expert fossil finders Bill Amaral and Chuck Schaff, Shubin learned how to spot the distinctive signs of bone in the ground. At first, this was incredibly difficult for Shubin. Schaff, a traditional “cowboy” type despite his New York upbringing, showed Shubin how to search for fossils without wasting effort.
Shubin directly learns from the experience and expertise of other scientists who help him learn how to find fossils. Shubin previously acknowledged the element of luck involved in finding fossils, but here he honors the skill and years of study that men like Jenkins and Schaff have put in to their lives as fossil hunters.
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Once Shubin is able to see the bone in the midst of desert rocks, he realizes there are fundamental rules to fossil hunting. Rule one: go to rocks that seem most likely to have fossils, based on geological search images or past experience of productive sites. Rule two: don’t follow in the footsteps of other fossil hunters, but search new terrain. Rule three: Only one person looking for fossils per area of the site. The more Shubin practiced looking for fossils, the easier it got to identify fossil from rock in all sorts of terrain and lighting conditions.
Though the process of finding fossils may have seemed random or chaotic to Shubin at first, here he simplifies the process into 3 straightforward rules. Shubin’s rules break down the huge task of sweeping an entire desert landscape for fossils into more manageable chunks. Just as Shubin looks for simplicity and order in the fossil record, he also looks for simplicity and order in the search for the fossils themselves.
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Jenkins’ site in the Arizona desert was full of tiny animals with bones no more than an inch or two long. Their teeth were even smaller, but Shubin was fascinated by the signs of occlusion in tiny mammals 190 million years old. It gave him a humbling perspective on the development of complex human anatomy.
The specialized fit of these tiny teeth allows Shubin to draw a line from these early mammals to human life today. The development of human complexity is even more incredible when Shubin understands where mammals started.
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Back in school after working with Jenkins all summer, Shubin decides to lead his own expedition. With limited funds, Shubin needs somewhere fairly accessible, yet with the right type and age of rocks. With the help of Paul Olsen, a professor at Columbia University, Shubin settles on a swath of 200-million-year-old rocks in Nova Scotia, Canada. Amaral and Schaff come along for a two-week dig.
The theoretical concerns of where Shubin will find the most useful fossils are balanced by the practical concerns of funding. Shubin brings in another person on the journey to find early mammalian fossils, further proving that science is always collaborative.
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Back in Boston, Amaral works as the fossil preparator for the rocks that Shubin’s team found in Nova Scotia. He uncovers a tiny reptile jaw from an animal called a trithledont that shows signs of wear on the cusps of the teeth – evidence of occlusion. Shubin is incredibly impressed with Amaral’s find and learns that some of the most important discoveries actually occur once paleontologists leave the field.
Though trithledonts are reptiles in most of their physical characteristics, they have a mammalian jaw – marking them as an intermediate stage between reptiles and mammals, just as the Tiktaalik was an intermediate between water animals and land animals.
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Shubin returns to Nova Scotia in the summer of 1985, hoping to find more trithledont fossils, but is disappointed to find that the dig site from the previous year is now weathered away from the tide. Still, Shubin and his team decide to investigate nearby rocks on the beach. One day, Shubin and Amaral get stuck on a spit of volcanic rock because the high tide blocks their path back to the base camp. Though volcanic rock was previously thought to be too hot to support fossil preservation, Amaral notices a whole area of small fossil fragments.
Fossil finding depends on much more than careful research, as Shubin cannot control the environment where he has to find his fossils. Yet that lack of control can also lead to strokes of luck, such as Amaral’s accidental find in volcanic rock that no fossil hunter ever would have checked according to the accepted norms and regulations of fossil finding.
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Shubin and Amaral dig out the volcanic rock site, finding that there are patches of sandstone that protected the fossils from the volcanic heat. They find several more trithledonts, which provide valuable clues to the progression from reptilian teeth to mammalian teeth. Though trithledonts do not have true occlusion, their upper and lower teeth scrape against each other like scissors, showing an intermediary step to the fit of the mammalian jaw. After trithledonts, the fossil record shows an explosion of new mammal species that have many different kinds of teeth specialized for different kinds of chewing to expand their possible food sources.
Sandstone is a sedimentary rock, the perfect type for finding fossils. The trithledonts that Shubin and his team find there show the long time frame for development of species. It would be ridiculous to assume that new body structures could pop up in a matter of generations. The changes that most benefit the animals, such as teeth that increase the number of food sources available to an animal, are the ones that remain and continue to develop.
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Teeth and Bones – the Hard Stuff. The most immediately special feature of teeth is how hard they are compared to other organs. The mineral that makes the outermost layer of teeth extra-hard, hydroxyapatite, is also found in lower concentrations in bones and the inner layers of the teeth. This mineral distinguishes human hardness from the hard exoskeletons of other animals. Shubin then turns to investigating where hydroxyapatite came from.
Hydroxyapatite is found in both bones and teeth, bringing together two different body systems with a fundamental similarity even if they look different on the outside. This mineral also shows a progression from the exoskeletons of insects or crustaceans to the inner hard skeletons of more complicated creatures like mammals.
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The most common fossils from the ancient oceans are conodonts, first discovered in the 1830s by Russian biologist Christian Pander. Conodonts are small, shelly organisms covered in spikes that have been found on every continent on Earth. At first, no one knew what conodonts actually were. Finally, a professor of paleontology at the University of Edinburgh found a slab of rock in university storage that showed a primitive jawless fish with the distinct impression of conodonts in its mouth. Conodonts are teeth.
The fossil record is incredibly useful, but also includes huge gaps. The case of conodonts is one area where fossils actually confused scientists because they did not yet have all the information. Yet through generations of study, building on work from past scientists, these questions can be answered. Conodonts are now so well catalogued that they are used as index fossils: fossils which help paleontologists pinpoint the specific geologic age of the rocks they are looking at.
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Part of the struggle in identifying conodonts as teeth was that the teeth were the only hard part of these ancient jawless fishes’ bodies, and were therefore the only part of the jawless fish that was preserved as a fossil. These fish most likely developed teeth in order to break through the hard exoskeletons of potential prey. Once animals developed hydroxyapatite-rich teeth, the mechanism for creating hard structures out of hydroxyapatite then became a method of protection. Fish called ostracoderms actually have a disk-like shield of bone covering their head that is entirely made of the tissue that human teeth are made of.
Using one simple mineral, animals can do many complicated things depending on what will help them most. Animals who are on the high end of the predator-prey chain need teeth that increase their ability to eat other animals. Animals lower in the predator-prey hierarchy need protection from more dangerous creatures. The same type of body structures can be used in many different ways.
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Teeth, Glands, and Feathers. Teeth are also special due to their specific method of development. Teeth are made of two layers of tissue that fold together, with the outer layer becoming the enamel and the inner layer becoming the dentine and pulp of the tooth. This two-layer process is also used in the development of all body structures that form within skin, such as scales, fur, hair, or feathers. Shubin compares the process to a new assembly line process; once teeth were developed, animals reused the same system for creating teeth to create many different body parts. This underlying process links organs as different as feathers, teeth, or even mammary glands.
Shubin uses the development of teeth to show two things – first, that a complicated structure like a tooth can come from a relatively simple method of folding together two tissues, and secondly, that this process is the same between many different body systems across many various animals. The feathers, teeth, or mammary glands that come from this process develop in birds, mammals, and reptiles, showing that these different species must have shared an ancestor in the past from which they all inherited this process.
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Shubin recaps what the book has argued so far, tracing how the same organ can be found in many different creatures. Chapter 1 focused on finding versions of human organs in ancient rocks. Chapter 2 compared the bones of fish and humans, while Chapter 3 looked at the genetic similarities in the development of those bones. Chapter 4 highlights teeth to once again show the deep similarities between different body parts and animals.
Shubin’s recap simplifies even his own book, helping break up the many different points that he is trying to make so that they will be easily understood. The recap also unifies these separate chapters with the common theme of following one organ (hands, teeth, etc.) in many animals.
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