Neil Shubin, the author and narrator, opens the book with a story about his experience teaching a human anatomy course at the University of Chicago, even though his degree and research has been primarily in paleontology. The summer after he taught this course, he discovered a fossil fish from 375 million years ago that reframes the transition between fish and land animals. Fossils are the only way to see the past of every animal alive today and understand the development of the human body.
In the summer, Shubin goes to rocky cliffs of the Arctic Circle to look for fossils. The ancient fish he finds—when they are from the right period during the transition between water and land creatures—give valuable insights into the early stages of human skull, neck, and limb development. The fossil record generally follows a progression from the oldest fossils in the deepest rock layers to the most recent fossils in the higher layers. Based on the layers where fish and amphibians have been found, Shubin should look for rocks that are 375 million years old if he wants to find fossils of animals that bridge the divide between water creatures and land creatures.
Shubin starts looking in his hometown of Philadelphia, Pennsylvania with one of his paleontology students, Ted Daeschler. They find a small shoulder bone of a hynerpeton, an early amphibian from the Devonian Period whose fossils have also been found in Alaska and the Yukon. Shubin and Daeschler began looking to mount an Arctic expedition to a region of the Canadian Arctic that has similar rocks to Pennsylvania. A field expedition to the Arctic presents many logistical challenges, but Shubin, Daeschler, and Farish A. Jenkins lead a team through these tough conditions. In 2004, Steve Gatesy, a member of Shubin’s team, finds a fossil fish of a species that has never been seen before. Over the next two years, Shubin and his team examine the fossil and find that it straddles the barrier between water animals and land animals. Shubin and the team decide to name the specimen Tiktaalik roseae.
Chapter Two focuses on hands, one of the most complex anatomical structures in the entire animal kingdom, and a hallmark of the human species. In the 1800s, the anatomist Richard Owens found that all land animal limbs have the same basic bone structure as the human arm, even if the limb looks radically different on the surface. Most fish have a very different structure in their fins, but certain fish have a very simple limb structure that matches land animals. Fossil preparators Fred Mullison and Bob Masek discovered that Tiktaalik is one of these fish. Eventually, fish descended from Tiktaalik probably moved out of the water altogether and became the first amphibians.
Shubin then moves to discussing genes and embryonic development of hands. Randy Dahn researches shark and skate embryos, looking for the genes that control protein production to form a fin to better understand the genetic information that directs fin and limb development in all animals. Limbs grow during the third to eighth week after conception, with a small bud of tissue called the zone of polarizing activity (ZPA) at the extreme tip of the end controlling all development. Researchers looking further into how the ZPA works found that there are more genes, called hedgehog genes, that control the development across the front-to-back axis of the whole body. These genes are almost identical in animals as different as flies, frogs, and mice. Dahn proved that the same genes are active in sharks and skates, though these fish have radically different limb-like structures than land animals. Genes therefore connect all living creatures.
Chapter Four highlights teeth, a special area of research for paleontologists because the hard material teeth are made of is especially likely to become fossils. The type of teeth an animal has also tells scientists much about that animal’s lifestyle because teeth determine what kind of food an animal can eat. Mammalian teeth are far more complex than reptilian teeth. Shubin explains that he first became interested in fossil finding by finding early mammalian teeth. It took a lot of work for Shubin to learn to identify possible fossil sites in the field, but with the help of his advisor Jenkins, and expert fossil hunters Bill Amaral and Chuck Schaff, Shubin was finally able to find tiny mammalian teeth in the Arizona desert. Bill and Chuck later accompany Shubin on an expedition to Nova Scotia and find a reptilian jawbone that has mammalian style teeth, showing the developmental path from reptiles to mammals.
Shubin then introduces the complex human head, full of nerves that seem to follow insane paths. Four nerves in particular have a circuitous route through the body that stems from the development of human ancestors. As an embryo, the human head is a collection of four blobs, called arches. The different body systems, such as the inner ear and the throat, formed out of these four arches correspond to where those tricky nerves go. Shark embryos have these same arches and their nerves follow the same pattern, with the exception of the ear. Looking for the origins of the human head in worms that have a primitive backbone, the same arches form cartilage rods that help the worm filter water through its body.
From the head, Shubin moves to explaining the entire human body plan. Many animals have the same basic body plan with a front-back, left-right, and top-bottom axis. Shubin saw these similarities in his thesis work on embryonic limbs. The three germ layers that turn into all the anatomical structures of humans are also responsible for the same body systems in all other complex animals. The first germ layer, ectoderm, creates structures on the outside of the body, like skin. Mesoderm create middle structures like the skeleton. Endoderm creates structures on the inside of the body such as the organs. Scientists over the years did incredible work to find out how each layer knows what to become, eventually discovering the Organizer gene in DNA that controls an animal’s body plan. Called Hox genes, these genes are found in every animal with a body. The more Hox genes an animal has, the more complex its body plan will be.
It seems like humans have simply added on to a recipe for bodybuilding that started all the way back in single-celled microbes. To count as a “body,” a collection of cells has to work together to make a greater whole and have a division of labor among the cells. There is a fine balance of communication between the cells of a body that arose from the earliest animals with bodies, all the way back in the Precambrian Era. These primitive bodies were made out of the connective glue that holds human body cells together and lets them communicate. Structural molecules in the bones are especially important for allowing the whole body to work together. Even sponges, animals with the most primitive bodies of all, have most of the cell connection, communication, and scaffolding systems that humans have. Going even further back in evolutionary history, it seems the first bodies were formed by single-celled microbes that resemble the cells of sponges. They probably formed together to avoid being eaten by larger microbes, and were able to stick together because oxygen levels on the Earth were finally high enough to support a “body” of cells that needed more food.
Chapter eight focuses on the development of the human nose. Smell is one of the most primitive senses, with millions of odor molecules that bond to individualized chemical receptors in the human brain. While ancient jawless fish have relatively few chemical receptors in their brains, modern fish, amphibians, reptiles, and mammals each add on more receptors until reaching the incredible number of smells that the average mammal can detect. Yet though humans have the same receptors for smell that other mammals have, some have been rendered useless by generations of mutations because we are more dependent on our sense of sight.
Moving on to vision, fossils of eyes are rarely found because they are soft tissue that is not usually preserved. So scientists look to the vast range of eye types found in modern organisms. The human eye uses the same basic light gathering molecules (called opsins) that are found in invertebrates. The two types of eyes in vertebrates and invertebrates are made up of the same components. There is even a worm that has both kinds of eyes. After studying flies born with a mutation that caused them not to have eyes, scientists realized that the same gene controls eye production in almost all animals.
In Chapter Ten, Shubin examines the human ear, which gets far more complicated on the inside than it seems on the outside. Two of the three human ear bones seem to have developed bones that form part of the jaw in reptiles and fish. This hypothesis was somewhat confirmed by the discovery of “mammal-like reptiles” that have very small jawbones that recede back towards the reptilian ear. In Tiktaalik, Shubin can see the upper jaw support bone that became the ear bone in reptiles after the transition to land animals. The inner ear is very connected to the eyes, providing humans with a sense of balance. The antecedent to that organ is found in the neuromasts of fish, who need a way to feel the current going past their bodies. This connection between eyes and ears is upheld by genetic research that has identified two genes responsible for forming the inner ear—which are also partly involved in the eyes of primitive animals like jellyfish.
Putting all of this together, Shubin goes back over all the ways that the human body carries the history of life on Earth in its various anatomical structures. Returning to the biological “law of everything,” Shubin explains that every living thing in the world has parents. This means that scientists can trace the development of anatomical structures (descent by modification) by figuring out how different species are “related” through common ancestors. The deep similarities among all animals then become more and more unique as subsets of animals that have the most in common show exactly when in the history of life groups such as reptiles, amphibians, mammals, and finally humans became distinct from one another. The biological record also gives hints about why certain illnesses and injuries are prevalent in humans. Knee injuries, obesity, hiccups, hernias, and mitochondrial diseases all point to the ways that human bodies repurpose body systems from other animals. Shubin ends the book looking towards the future, as scientists continue to unravel where the human body came from, and where it might develop in the future.