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Your Inner Fish by Neil Shubin.
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Many features of the human body are just complex versions of those in simpler creatures that, at first glance, seem totally unlike us.

In Your Inner Fish, Neil Shubin, a professor and paleontologist who studies fish fossils, explains that understanding how a shark’s head, a reptile’s brain, and a fish’s fins developed helps make sense of complicated and confounding human anatomy.

Shubin’s story starts in the Canadian Arctic, where he and colleagues discovered a key link in the chain from the earliest creatures to humans: a 375-million-year-old fossil fish, Tiktaalik, that developed features for living on land. Tiktaalik’s rudimentary joints, including a head free of the shoulder, are precursors to those of amphibians, reptiles, birds, mammals—and humans.

Casting new light on the human family tree, ancient fossils like Tiktaalik, as well as embryos and DNA, provide clues to a story of human development stretching back 3.5 billion years.

Fossil Clues to Human Development

Fossils are one of three major types of evidence for how human bodies developed and how they work; the others are embryos and genes.

To understand the origins of land animals and their connection to humans, Shubin set out to find evidence of the first limbed animal, or fish that walked on land.

In 2004, after four expeditions over six years in the Canadian Arctic, he and his team found a fossil skeleton of a transitional creature between fish and land animals. Like a fish, it had scales and fins with fin webbing, but like a land animal, it had a flat head with eyes on top and a neck. Also, the fins contained bones corresponding to salamander-type shoulder, elbow, and wrist joints, giving it the ability to propel itself on land.

The land-fish, called Tiktaalik, is as important to human history as the African hominid fossil Lucy. Through Lucy, we trace our primate history; Tiktaalik tells us our history as fish. The story of the development of human anatomy through small changes over millennia can be read in fossils, as well as in our genes through DNA—starting with our “inner fish.”

A Skeletal Pattern for Limbs

This book shows how scientists can trace bones, teeth, the DNA recipe, and the biological process for building organs from early creatures to humans. These similarities show that the world’s diverse creatures are variations on a theme.

Fish like Tiktaalik carried the pattern or blueprint for our hands and feet, which would continue to develop and be refined over hundreds of millions of years through a progression of fish, amphibians, and reptiles.

For example, in the 1800s, scientists found that nearly all animals with limbs (wings, flippers, bones, or hands) have the same structural limb design, although the shape and size of the bones vary. In this shared limb design, one bone connects with two, which attach to an array of small knucklebone-type “blobs,” which connect to digits.

Tiktaalik’s fin was a primitive version of a limb—a wrist bone with spaces for four other bones. Its hand-like fins likely enabled Tiktaalik to move along the bottom of streams or ponds and flop its way across mudflats.

A DNA Recipe for Limbs

Just as echoes of our bone development can be seen in earlier animals, our genetic recipe also traces back to other creatures.

All cells contain the same DNA. But organ and tissue cells develop differently because certain genes (stretches of DNA) in the cells are active while others are turned off.

In an embryo of any animal, the genetic switches for making limbs activate between the third and eighth weeks, and limbs start developing. First, tiny buds protrude from the embryonic body, then the tips develop into paddles. A patch of tissue in the paddles’ tips, called the ZPA, controls development of the bone pattern of limbs by varying the concentration of a molecule in the cells building the limb.

In the 1990s, researchers identified the mystery molecule dictating limb development, which they called the Sonic hedgehog gene (named for a video game character). Experiments manipulating the gene to produce limbs in chicken, mouse, shark, and skate embryos showed that all appendages, whether fins or limbs, share the...

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Your Inner Fish Summary Your Inner Fish Guide Introduction

Many features of the human body are just complex versions of those in simpler creatures that, at first glance, seem totally unlike us.

In Your Inner Fish, Neil Shubin, a professor and paleontologist who studies fish fossils, explains that understanding how a shark’s head, a reptile’s brain, and a...

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Your Inner Fish Summary Your Inner Fish Guide Chapter 1: Your Inner Fish

Finding important clues to our human past in fossils seems improbable when you consider that:

  • 99% of species are extinct.
  • Only a fraction of these are preserved as fossils.
  • We find only a fraction of all fossils.

Nonetheless, for hundreds of years, scientists like Shubin have been uncovering fossils of worms and fish that, combined with clues from DNA studies, go a long way toward explaining the structure of our bodies.

Not just curiosities, fossils are an important part of the story of our development. This chapter explains how scientists determine where to look for fossils, how they categorize what they find—and how Shubin knew where to find the fish that foreshadowed human limb development.

Fossil Clues to Human Development

In 2004, on Ellesmere Island in the Arctic, Shubin’s team found a 375-million-year-old fossil fish with a neck, a flat head, and fins capable of propelling it on land, making the creature a key link in the evolutionary chain from fish to land animals.

But understanding its significance as a clue to human development several billion years later requires understanding how scientists find and interpret fossils in the first place....

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Shortform Exercise: Rethinking Life’s History

Paleontologist Neil Shubin argues that uncovering our human origins in fish and other creatures doesn’t detract from our uniqueness, but makes our existence that much more incredible. Having developed from a common blueprint, we’re part of all life, not separate from it.


What’s your reaction to the above statement and why?

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Your Inner Fish Summary Your Inner Fish Guide Chapter 2: Patterns in Skeletons

The next three chapters of this book show how scientists can trace bones, teeth, the DNA recipe, and the biological process for building organs from early creatures to humans. These similarities show that the world’s diverse creatures are variations on a theme.

The human hand first took shape in the fins of primitive fish. This chapter traces the origins of our hands, through fish fossil discoveries in the 1800s, the early 1900s and, finally, in the last several decades by Shubin and his colleagues.

A complex array of bones, muscles, tendons, nerves, and vessels in our hands enables intricate movement—for instance, 10 muscles and six bones working together enable us to twiddle our thumb and tilt our hand. But our hand does more than that—with it, we connect with others through touch and make our thoughts concrete by creating art, music, meals, architecture, and more.

Yet the human hand derives from a pattern common to vastly different animals. In the 1800s, anatomist Sir Richard Owen found that nearly all animals with limbs (wings, flippers, bones, or hands) have the same structural limb design, although the shape and size of the bones vary. In this shared skeletal...

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Your Inner Fish Summary Your Inner Fish Guide Chapter 3: How Genes Develop Limbs

Just as echoes of our bone development can be seen in earlier animals, our genetic recipe also traces back to other creatures. DNA research answers questions about our development that fossil study can’t answer because variables can be manipulated in animal embryos to see what happens.

DNA contains the “recipe” that builds human or animal bodies from an egg. By experimenting with DNA in animal and shark embryos, scientists have shown that all appendages, whether limbs or fins, develop the same way from the same DNA recipe. This chapter looks at the experiments, starting in the fifties and sixties, that led to this understanding.

Although the human body is made up of hundreds of different kinds of cells, which make our bones, organs, nerves, and tissues function differently, all cells contain the same DNA.

Organ and tissue cells develop differently because certain genes (stretches of DNA) in the cells are active while others are turned off. The active genes make a protein affecting what a cell looks like and how it behaves.

Understanding the on-off mechanism explains what genes build which parts of the body and how they work, whether building a limb or a fin.

...

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Your Inner Fish Summary Your Inner Fish Guide Chapter 4: Teeth Answer Many Questions

Like bones, our teeth also connect us to other living things, past and present, and are thus important to understanding our bodies. This chapter examines how our teeth trace back to early creatures in two ways:

1) Our mix of different types of teeth that fit together (occlude), giving us the ability to efficiently eat a wide variety of things, comes to us courtesy of a tiny 200-million-year-old mouse-like creature.

2) Further, the unique material contained in our teeth, bones, and other tissues—hydroxyapatite—can be traced to fish.

Humans and other animals also benefit in other ways from the biological process that originally developed teeth. It was modified over time for making different kinds of skin structures—scales, feathers, skin, and mammary glands.

Lessons From Teeth

Teeth aren’t very exciting and don’t get much attention in anatomy classes, but they tell scientists a lot about animal lifestyles and diets dating back to the earliest fossils. Because they’re so hard, teeth are often the best-preserved animal part revealed in fossils.

Reflecting our omnivorous diet, humans have a mix of incisors for cutting meat and molars and premolars for chewing meat and plant...

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Your Inner Fish Summary Your Inner Fish Guide Chapter 5: The Pattern in Our Heads

The complicated assemblage of bones, muscles, arteries, and nerves that comprises our head—and makes our eyes, ears, and nose function—lacks a discernible pattern or logic to anyone trying to learn it. However, this chapter explains that while it seems complicated, our head’s structure is based on a simple plan found in sharks, with echoes of even earlier structures in headless worms.

A Pattern in Our Heads

The head’s components are difficult to see because they’re contained by the skull, which consists of plates, blocks, and rods.

  • The plates fit together, covering the brain.
  • The blocks are a platform holding the brain, with arteries and nerves running through it.
  • The rods are bones in our jaw, ears, and throat; they start out as rods but change shape.

The skull has compartments for our brain, eyes, parts of our ears, and nasal system. There are also muscles allowing us to move our head and eyes. Twelve cranial nerves make things function. Some have simple paths—for instance, attaching to the eye (optic nerve) or ear (acoustic nerve). But four cranial nerves have complicated functions and routes. The most confusing are the trigeminal and facial nerves.

The...

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Your Inner Fish Summary Your Inner Fish Guide Chapter 6: Body Design

Humans share their basic body design with nearly all other creatures. It starts with embryos, which go through the same early stages of development, regardless of the animal type. This chapter explains how embryos develop key body structures.

Animals as diverse as humans, fish, lizards, birds, amphibians, and mammals all have symmetrical bodies of the same design, with a front/back, top/bottom, and left/right, plus a head, spinal cord, and organs in specific places. Heads and feet point forward in the direction we move and the butt points the opposite direction.

When you look at embryos there are many more similarities among animals than differences.

The Common Plan

In the 1800s, biologist Karl Ernst Von Baer was struck by the similarities in embryos. Further, he discovered that all organs in an embryo, whether it’s a chicken, fish, mammal, or amphibian, originate in one of three layers of tissue called germ layers. For example, every type of animal’s heart forms from the same layer. He concluded that all animals develop by the same stages.

For the first few days after conception, an embryo is a ball of cells called a blastocyst, which attaches to the wall of the...

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Your Inner Fish Summary Your Inner Fish Guide Chapter 7: The How and Why of Bodies

Cells have mechanisms for communicating, sticking together to create specific materials like bone, and trading proteins. Without these abilities, cells couldn’t build bodies. These mechanisms predated the various body plans and patterns we can trace back through DNA and fossils.

They raise key questions about bodies beyond their design:

  • What is a body in the first place?
  • When did bodies first appear?
  • How did they come about?
  • Why did they come about?

This chapter explores these questions by tracing the development of bones, examining simple bodies such as sponges, and looking at life forms without bodies—all of which provide clues to the construction of human bodies.

What Is a Body?

Not every clump of cells or bacteria working together constitutes a body. Bodies have several defining characteristics:

  • The parts work together to create a greater whole.
  • Some parts are necessary for life—for instance, if you remove cells from vital organs, the body would die.
  • The parts (brain, heart, stomach, and so on) have different functions.
  • The division of labor extends down to the smallest components: genes, cells, and proteins.
  • ...

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Your Inner Fish Summary Your Inner Fish Guide Chapter 8: Developing a Sense of Smell

Humans, mammals, birds, reptiles, amphibians, and fish share the same basic system for detecting odors, which, like our other systems, has ancient origins. Drawing on both paleontology and DNA research, this chapter shows how we can trace our sense of smell to its start in primitive fish.

Humans can detect 5,000 to 10,000 different odors, as the brain responds to a host of molecules suspended in the air. Here’s how it works.

As we breathe, we pull odor molecules into the nose, where they’re trapped in the mucous lining in a patch of tissue with millions of nerve cells. The nerve cells bind to the air molecules and send signals to the brain, which identifies them as a smell. Each air molecule connects with a receptor in the nose that’s tuned to that type of molecule. A particular odor involves many different molecules and therefore many different receptors sending signals to the brain. An odor is like a chord made up of different notes with one combined sound. The brain reads combined receptor signals as one smell.

In humans, mammals, birds, reptiles, amphibians, and fish, the sense of smell is handled in the skull. We all have one or more nostrils for pulling in air...

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Your Inner Fish Summary Your Inner Fish Guide Chapter 9: Developing Vision

This chapter explains how eyes work and traces the history of their component parts—this history stretches back to some ancient creatures including flies, jellyfish, and worms.

Animals use many different tissues and organs to see, or capture light—for example:

  • Invertebrates use simple photoreceptor organs
  • Insects have compound eyes
  • Humans have camera-type eyes

Studying different types of eyes suggests how humans’ ability to see developed. The development of the human eye can be compared to that of a car. The creation of parts such as tires and engine components are part of the development of the car as a whole. Our eyes have a history—as an organ and as component parts: cells, tissues, and genes. Studying that history reveals we’re an amalgam of pieces of other creatures.

How Eyes Work

Eyes capture light that’s sent to the brain for interpreting as an image.

Human eyes, like those of most animals, are like cameras. Light entering the eye is focused on a screen (the retina) in the back of the eye, which sends signals to the brain. But first, light travels through several layers: the cornea, iris (which controls the amount of light), and then the lens,...

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Your Inner Fish Summary Your Inner Fish Guide Chapter 10: Developing Hearing

The inner workings of the human ear function like a Rube Goldberg machine. However, this chapter explains that while our ears are complex, parts evolved from simpler creatures: reptiles, fish, and sharks (remember gill arches?).

The ear doesn’t look like much from the outside, but the outer appearance belies its complexity. There are three parts: the external ear, the middle ear with three ear bones, and the inner ear made up of nerves, tissue, and gel. While the external ear is a late evolutionary development (the flap is found only in mammals), the middle and inner parts have antecedents in the bone structure of sharks.

How the Ear Works

In simple terms, the ear works like this:

  • Soundwaves are drawn into the outside cartilage flap, or external ear.
  • They travel into the middle ear and rattle the eardrum.
  • Three small bones attached to the eardrum also shake. One bone is attached to a snail-like bone with a kind of plunger.
  • The plunger goes up and down, moving around the gel in the snail-like structure.
  • The moving gel triggers nerves, which send a signal to the brain. The brain interprets it as sound.

The Middle Ear

Our middle ear bones betray...

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Your Inner Fish Summary Your Inner Fish Guide Chapter 11: The Human Family Tree

With our capabilities in DNA and fossil research, we can see more clearly than ever how our bodies fit into the story of how life developed on earth.

The preceding chapters show there are striking similarities between us and other creatures, both living and long gone. We have connections in our development with microbes, ancient worms, sponges, fish, sharks, tiny extinct rodents, and many more creatures.

Humans have parts that resemble parts of other creatures, we have certain parts in common with every other animal, and we have parts that are unique to us. This chapter shows how scientists can build a human family tree that shows the order in which these features develop.

Tracing the human tree through its many branches even explains some of the quirks in our development, such as hiccups and sleep apnea.

Signature Traits

The most basic law of biology is that every living creature had parents (or in the case of cloning, genetic information from parents). We’re modified descendants of our parents. In other words, all organisms are modified versions of their parents’ DNA.

This pattern of “descent with modifications” means we can trace family lineage by blood...

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Shortform Exercise: Respecting Your Body’s Design

We can trace most ailments of modern life to the fact that our bodies were built to be active predators on the hunt for food, or gatherers and agriculturalists—yet today, most of us spend our days sitting.


How physically active are you during the majority of your day? What effects does this have on your body and how you feel?

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Table of Contents

  • 1-Page Summary
  • Introduction
  • Chapter 1: Your Inner Fish
  • Exercise: Rethinking Life’s History
  • Chapter 2: Patterns in Skeletons
  • Chapter 3: How Genes Develop Limbs
  • Chapter 4: Teeth Answer Many Questions
  • Chapter 5: The Pattern in Our Heads
  • Chapter 6: Body Design
  • Chapter 7: The How and Why of Bodies
  • Chapter 8: Developing a Sense of Smell
  • Chapter 9: Developing Vision
  • Chapter 10: Developing Hearing
  • Chapter 11: The Human Family Tree
  • Exercise: Respecting Your Body’s Design