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.
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.”
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.
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 same DNA recipe.
The complicated assemblage of bones, tissue, muscles, arteries, and nerves that comprises our head is based on a simple plan found in sharks, with echoes of even earlier structures in headless worms.
The human head begins forming at the base of the embryo at about three weeks. Four swellings called arches develop in the area that will be the throat. Specific cells in each arch form bone, muscle, and blood vessels.
Cells from the first arch form the upper and lower jaws and two of the ear bones. Cells from the second arch form the third ear bone, a throat bone, and facial muscles. Third-arch cells form bones, muscles, and nerves in the throat that are used to swallow. Finally, cells from the fourth arch form the deep part of the throat, including the larynx and its surrounding muscles and vessels.
Our head reflects the same pattern as those of sharks, fish, and salamanders. The arches of the human embryo look much like the gill slits in the throat area of sharks and fish, although human gill slits are sealed by the plates of the skull before birth. The arches in sharks and humans develop comparable body systems.
These patterns stretch back even further than sharks—to worms that don’t really have heads. A worm called amphioxus lacks a skull but has a notochord—a nerve cord and jelly-filled rod like a primitive version of a backbone. Human embryos also have a notochord, which breaks up to become jelly-filled disks between our vertebrae.
Just as we share common designs for our hands, limbs, and heads with other creatures, we share our basic body design with other creatures as well. It starts with embryos, which go through the same early stages of development, regardless of the animal type.
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.
Every animal’s organs start in one of three layers of tissue called germ layers. For example, every type of animal’s heart forms from the same layer. The layers are:
All animals with a backbone have gill arches and notochords and look the same in the early embryo stages. Distinctive features such as a bigger brain in humans, shells on turtles, and feathers on birds, develop later.
In simplest terms, a body is a group of cells that perform different individual functions (have a division of labor) but together create a greater whole. To form bodies, cells have to be able to: 1) attach to each other to create specific materials like bone, and 2) communicate with each other.
1) Sticking Together: Some of the earliest bodies were multi-celled creatures that lived in the seas 600 million years ago. They were made of the same type of “glue” (collagen and proteoglycan) that allows human body cells to stick together to build materials and organs. In our bodies, this glue is a mix of molecules that differs depending on the organ it’s forming—for instance, a bone versus an eye. Without the molecule mix attaching cells to each other, bodies couldn’t be formed.
In addition to the molecule mix, cells stick together by using various types of molecular rivets. Some work like contact cement gluing the outsides of two cells together. Other rivets bond only to cells with the same kind of rivet, a mechanism that enables cells to organize and ensures that bone cells stick to bone cells, skin cells stick to skin cells, and so on.
2) Communicating: To build bodies, cells must communicate so they know when to divide, make molecules, and die.
They communicate by sending out molecules with messages. A cell sends a signal or molecule, which attaches to the outside of a receiving cell. This sets off a chain reaction of molecules within the cell as the message travels from the outer membrane to the nucleus. As a result, the cell receiving the information changes its behavior.
One of the simplest bodies is a placozoan, a live blob first found on aquarium glass in the 1800s. It has a plate-shaped body with only four types of cells, yet they have a division of labor and rivet connections, and they communicate.
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. Scientists can build a human family tree that shows the order in these features.
Our family tree looks something like this:
Fossil data also show the developmental order: the first multi-celled fossil at 600 million years old is older than the first fossil with a three-boned middle ear (200 million years old). The three-boned middle ear fossil is in turn older than the first fossil that walked on two legs (4 million years old).
Our bodies are time capsules, containing features from ancient animals that mark key moments in the history of life. From our commonalities, we have the potential to learn what makes us human and find cures for many of our ills.
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 exploration of our primitive connections started 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 had features for living on land.
Ancient fossils like Tiktaalik, plus embryos and DNA, provide clues to a story of human development stretching back 3.5 billion years.
Casting new light on the human family tree, Your Inner Fish became a popular PBS series.
Finding important clues to our human past in fossils seems improbable when you consider that:
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.
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.
Fossils are one of three major types of evidence for how human bodies developed and how they work; the others are embryos and genes.
While fossils are sometimes stumbled upon accidentally by scientists and laypeople, their locations often can be predicted with surprising accuracy. Finding them is a matter of both planning and luck: scientists use sophisticated mapping, plus rock-dating and radiographic scanning technology, to identify promising sites. Then they depend on luck and the physical yet delicate work of digging in the right spots.
Though fossil sites are rare, scientists can predict where important fossils lie. They look for locations meeting three criteria:
Rocks can be dated because they’re arranged in layers going back billions of years, with the oldest rocks in the lowest layers and more recent ones on top. Earthquakes and faults can push some older layers over younger ones, but scientists can usually figure out the correct order.
The fossils in rock layers also follow a progression from the oldest on the bottom to younger fossils in higher layers. The lowest layers show little evidence of life; then there’s a progression from jellyfish-like creatures; to animals with skeletons, limbs, and organs such as eyes; to vertebrates.
Every species fits into a complicated classification system or taxonomy that helps paleontologists identify what they’re looking at and predict what they’ll find.
A trip to the zoo illustrates how the system works. The system organizes species and organisms by grouping them according to traits they share, like a set of Russian nesting dolls with smaller groups encompassed by larger ones.
Every species in a zoo has a head and two eyes; another group or subset has a head, eyes, and limbs; additional subsets have a large brain, walk on two legs, and so on. Each subset adds a feature. The more unique a group is, the younger or more recent it is. Scientists would look for fossils with a head and eyes in rocks far below fossils with a head, eyes, and limbs.
With computers, scientists can analyze thousands of species, their characteristics, and their DNA. Fossils provide information to help to refine the groupings, as well as to build a catalog of rock layers by time period and fossil type.
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. To choose where to look, he applied the three fossil-finding criteria this way:
1) Rocks the right age: He focused on finding rocks 375 million years old. Researchers had found 365-million-year-old rocks containing amphibian fossils and 385-million-year-old rocks with standard fish, but there was a gap in the fossil record between 365 million and 385 million years. Shubin hypothesized that the missing link was the limbed fish.
2) Rocks of the right type: Shubin looked for sedimentary rocks because they’re the best type for preserving fossils. These rocks are formed when lakes, rivers, and oceans—habitats in which the first fish with limbs could live—deposit layers of sediment.
3) Rocks that are exposed: The last step is finding rock of the right age and type that’s also exposed, with little soil, vegetation, or human disturbance. Shubin found conditions meeting these three criteria in the Arctic.
After four expeditions over six years, Shubin and his team found what they were looking for—a complete 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, allowing it to propel itself on land. Researchers called the land-fish Tiktaalik, meaning large freshwater fish in the Inuit language.
Tiktaalik, with its rudimentary joints, including a head free of the shoulder, shares this structure with amphibians, reptiles, birds, mammals—and us.
It’s as important to our 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.”
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?
How might awareness of our connection to other creatures affect how we view them?
How might it affect how we view ourselves?
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 structure, one bone connects with two, which attach to an array of small knucklebone-type “blobs,” which connect to digits. Owen attributed this pattern to a Creator’s design. However, Charles Darwin argued the skeletal pattern meant that animals share a common ancestor.
In fact, Owen’s one bone-two bones-blobs-digits pattern can be traced back to fish fins.
At first glance, fish fins don’t have much in common with human appendages. The base of a fin has four or more bones (not one) and fins consist mostly of webbing.
However, primitive living fish—called lungfish—found in Africa and Australia in the 1800s not only had large vascular sacs (lungs), but also a single bone that connects four fin bones to the fish’s shoulder. A similar fossilized fish from the Devonian period, called Esuthenopteron, had one bone connecting to two fin bones and looked like a cross between a fish and an amphibian.
Discoveries going back further led closer to the origin of fingers and toes:
But it wasn’t until the 2004 discovery of Tiktaalik—a creature with a yet more primitive form of hands, feet, wrists, and ankles—that the sequence for limb development could be put together.
In 1995, Shubin and colleagues found a 365-million-year-old fin fossil (lacking the rest of the skeleton) in a highway construction area in Pennsylvania. The fin had webbing and scales like a fish but also the bone structure of a limb: one bone at the base, attached to two bones with eight bones extending like fingers. Without a full skeleton, however, researchers couldn’t determine whether it showed the origin of limbs.
That discovery came 10 years later as scientists picked apart rocks containing additional Tiktaalik fossils brought back from the Arctic expedition a year earlier. As they gradually uncovered the fossilized skeletons, they found what looked like a wrist bone with spaces for four other bones; the rest of the appendage consisted of fin webbing containing a limb structure—it was a primitive version of a limb, confirming that Tiktaalik was a transitional creature.
Further analysis of the joint structure suggested the fish was capable of doing a pushup-type of movement the way a human would, with hands flat on the ground, elbows bent, and shoulder and chest muscles moving the body up and down. Scientists concluded its hand-like fins enabled Tiktaalik to move along the bottom of streams or ponds and flop its way across mudflats. It may have begun evolving from water to land to escape larger fish predators. Tiktaalik was about four feet long whereas other fish ranged from seven to 16 feet.
The movements humans make with their hands and wrists are far more complicated than fish pushups. But 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. The basic skeleton emerged in the following stages:
Humans’ deep origins in fish and other earlier creatures don’t detract from our uniqueness—they make our existence that much more incredible. Our unique form developed from a common blueprint. We’re part of all life, not separate from it.
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.
Our hands are three dimensional, with a top/bottom, thumb side/little finger side, and base/tip. Understanding the genes that make the thumb different from the little finger or fingers different from arm bones is a key to the genetic recipe that builds hands and the rest of our bodies.
In an embryo, the genetic switches for forming limbs activate between the third and eighth week after conception and limbs start developing. First, tiny buds protrude from the embryonic body, then the tips develop into paddles. The paddles contain millions of cells that ultimately become the limb’s skeleton, nerves, and muscles.
To learn about the process, scientists experiment with genetic mutations that make limbs malform. In the fifties and sixties, scientists experimenting with chicken embryos uncovered a key mechanism—a patch of tissue—that controls development of the bone pattern of limbs. By removing the patch, they could stop limb development at various points. They called the patch, which causes the little-finger side of a hand to differentiate from the thumb side, the ZPA, or zone of polarizing activity.
Moving the ZPA from the little-finger side of the limb bud to the thumb side caused the embryo to develop a duplicate set of digits mirroring the normal set on that side.
The ZPA controls development of fingers and toes by varying the concentration of a molecule in the cells building the limb. The cells nearest to the ZPA have a high concentration of the molecule and make a little finger. The cells farther away from the ZPA have a lower concentration of the molecule and make a thumb. The cells in between have varying concentrations and make second, third, and fourth fingers.
In the 1990s, with new molecular techniques, researchers looked at DNA in flies to identify the mystery molecule dictating limb development.
In studying fruit fly development, which occurred from front to back as genes were turned on and off, they found a gene that made body segments, the front and back ends of the fly’s torso, look different from each other. They called it the hedgehog gene because flies with a mutated version of it developed bristles.
Researchers then found the hedgehog gene in chickens, mice, and fish. In chickens, scientists called it Sonic hedgehog after a video game character. They noticed it had a function similar to that of ZPA tissue, then realized the gene existed only in the ZPA and was involved in ZPA signaling.
Scientists soon found the Sonic hedgehog gene in every limbed creature, including humans, meaning that we all share the same genetic recipe for building upper arms, forearms, wrists, and digits.
Further experiments traced the Sonic hedgehog gene and the other gene activity that builds limbs all the way back to fish.
Researchers in Shubin’s lab found the Sonic hedgehog gene in shark and skate embryos.
Since an effect of the Sonic hedgehog gene in more complex animals is to make limbs different from each other, they wondered how it would work in creatures that don't have limbs. Sharks and skates were good test subjects because while they have "paired appendages" (fins), they don't have limbs.
Skates have cartilage rods in their fins that all look alike. Inserting Sonic hedgehog protein from a mouse into a skate embryo resulted in rods that looked different from each other the way fingers do.
The experiments with sharks and skates showed that fins and limbs share the same DNA recipe. Rather than new DNA, the evolution from fins to limbs likely involved using genes of ancient origin in news ways.
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.
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 material. In addition, our upper and lower teeth fit together to chew efficiently.
In contrast, reptiles’ teeth are all the same—for instance, all of a crocodile’s teeth are shaped similarly, although some are larger; they’re replaced regularly as they wear down. They also don’t fit together as mammals’ teeth do. Mammals are distinctive in having several different kinds of teeth, which are replaced only once.
Mammals’ system of precise chewing first shows up in fossils of a mouse-sized rodent between 195 million and 225 million years old. Shubin and colleagues found fossils of the early mammal, a tritheledont, in cliffs along the coast of Nova Scotia. It previously had been found only in Africa. The tiny animal had incisors, canines, and molars and its teeth occluded, indicating it could chew like a mammal, although the incisors rubbed together like scissors rather than fitting precisely as in later mammals. The tritheledont also had other mammalian features in its lower jaw and skull.
After finding the tritheledont (an intermediate mammal), researchers found various new mammal species with new kinds of occluding teeth suited to new diets. But the tritheledont paved the way for diverse eaters, including us.
The most unique feature of teeth is that they’re harder than other body parts because they have a crystal molecule called hydroxyapatite, particularly in the enamel. The same molecule appears in lesser concentrations in other parts of the teeth, bones, and other tissues.
Our hydroxyapatite-containing tissues contribute to our ability to eat, breathe, move, and metabolize minerals. The substance—which we share with fish, reptiles, birds, amphibians, and mammals—traces back to a common ancestor.
The first teeth, 250 million to 500 million years old, were found in the 1830s and belonged to jawless fish or lamprey-like creatures. However, scientists found the teeth before they found the fish the teeth belonged to. The teeth were so ubiquitous—they showed up on every continent— that scientists initially thought they were separate creatures, which they called conodonts. However, 150 years later, researchers discovered fossils of a fish with conodonts arranged as teeth.
The primitive fish probably developed teeth so it could consume other creatures with hard exoskeletons. As more creatures developed teeth with hydroxyapatite, other fish used the development mechanism to make bone-like material to serve as armor against predators.
Fish called ostracoderms had a shield of bone protecting their head, which was composed of the pulp-enamel tissue that human teeth are made of.
While teeth provided new ways of eating and new diets, they’re also important because as animals evolved, the process for making teeth was adapted to create other body features. To make teeth, two layers of tissue interact—the layers fold and the outer layer becomes enamel and the inner layer becomes dentine and pulp.
The same two-layer process is also the mechanism for developing other structures within skin, such as hair, fur, scales, feathers, and mammary glands. The key is that in all cases, two layers of tissue fold and both layers secrete proteins that build the organ. It’s like a factory assembly-line process that can be repurposed to produce different things.
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.
The head’s components are difficult to see because they’re contained by the skull, which consists of plates, blocks, and rods.
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 trigeminal nerve affects muscles for chewing, muscles in the ear, and sensations in the face and the teeth. The facial nerve affects facial expressions and muscles in the ear. These two nerves seem to duplicate functions by going to some of the same places, which doesn’t seem very efficient. But, like the wiring of an old house, the reason for the setup can be found in the history of how the system started and was subsequently updated. Our head’s history starts in the embryo.
The human head begins forming at the base of the embryo at about three weeks. Four swellings called arches develop in the area that will be the throat. Specific cells in each arch form bone, muscle, and blood vessels:
The arches dictate the routes of the key cranial nerves. The trigeminal nerve serves the body systems (jaw and ear) formed by the first arch. The facial nerve serves structures developed by the second arch. The nerves corresponding with the third and fourth arches follow the same pattern.
The human body is segmented. Each vertebra in the body represents a segment. The nerves corresponding with each body system exit the spinal cord at a specific point, according to their segment. For example, our leg muscles are served by nerves exiting the spinal cord at a point below the nerves serving our arm muscles. The head is also segmented, as determined by the arches. This is why, for instance, both the trigeminal nerve and the facial nerve travel to the ear. They’re not redundant—each evolved with a different purpose. But this pattern can only be seen in the embryo, when the head is less complicated.
Besides mapping the way our head is put together, the arches also trace back to sharks.
While the human head is complicated, it follows a pattern common to the skulls of sharks, fish, and salamanders.
The arches of the human embryo look much like the gill slits in the throat area of sharks and fish, although human gill slits are sealed by the plates of the skull before birth. The arches in sharks and humans develop comparable body systems.
The first arch forms jaws in both; in humans, it also forms ear bones. The second arch forms the bones of the upper jaw in sharks; in humans, it forms inner ear and throat muscles. The third and fourth arches support the gills in sharks; in humans, they form the muscles we use to swallow and talk.
In the first three weeks after conception, many genes turn on and off in the arches, as brain tissue begins to develop and the genes direct cells to form other parts of the head.
Each arch has a different set of genes called Hox genes, which tell the arch what to become. (Hox genes are explained in detail in Chapter 6.)
Patterns seen in the human head stretch back even further than sharks—to worms that don’t really have heads.
A worm called amphioxus lacks a skull but has a notochord—a nerve cord and jelly-filled rod like a primitive version of a backbone. Human embryos also have a notochord, which breaks up to become jelly-filled disks between our vertebrae. When you rupture a disk, leaking jelly can pinch nerves and hinder movement.
Fossil impressions from 500-million-year-old rock show the same type of worm with gill arches—meaning that the basic structure of the human head traces back to worms that didn’t have heads.
Like our teeth, limbs, and genes, the basic structures of our head are modified and repurposed versions from earlier creatures. In the fossil worm, gill arches filtered food from water; in humans, they evolved into jaws, ear bones, and parts of the larynx.
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.
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 uterus. The cells rapidly divide and move, causing tissues to fold and form a tube with a swelling at each end. Three layers—the germ layers—take shape:
All animals with a backbone have gill arches and notochords and look like a double tube in the early embryo stages. Distinctive features, such as a bigger brain in humans, shells on turtles, and feathers on birds, develop later.
In an effort to learn more about how embryos develop body structures and organs, early scientists experimented with them by cutting, grafting, and treating them with chemicals.
In 1903, German embryologist Hans Spemann determined that more than one individual can come from a single egg—he pinched apart a newt embryo making two clumps of cells. Each clump formed a newt, showing that embryonic cells can build a whole body.
In the 1920s, another researcher, Hilde Mangold, grafted a bit of newt embryo tissue containing all three germ layers onto an embryo of another species. The transplanted tissue developed a full newt body on the back of the second embryo. Scientists called the bit of tissue, which directed other cells to form a complete body, the Organizer.
Another researcher figured out how to label cells so their development into body parts could be traced. This led to an embryo map showing where every organ begins.
All mammals, birds, fish, and amphibians have Organizers, which initiate the bodybuilding process, telling each clump of cells in the embryo to follow the body plan for that animal. The key is that the Organizer contains DNA with the recipe—shared by humans and all animals—telling cells how to build bodies.
Modern embryologists studying fruit flies learned that genes in the Organizer called Hox genes control the development of tissues and bodies.
Fruit flies are a good study subject because they often develop mutations—scientists have been studying them for over a hundred years. For instance, sometimes a leg develops where an antenna should, or there’s an extra set of wings or missing body segments.
By comparing chromosomal differences between normal flies and mutated flies, scientists identified the genes responsible for various mutations. They’re arranged in a sequence in order of the body part they affect—for example, the genes in the middle of the sequence affect the middle part of the fly.
This DNA sequence is called a homeobox and the genes comprising it are called Hox genes. All animals have Hox genes, with complex animals having more. Every Hox gene is a version of the same basic template. They direct the construction of the body from front to back in all animals. They control body proportions and development of organs, limbs, and genitalia. Changes in these genes result in deformities. For example, if a Hox gene controlling the middle section of a fly is missing, the fly lacks that segment.
Further study of Hox genes led scientists back to the Organizer, which, in addition to containing Hox genes, was found to contain several other types of genes that interact to affect body development.
A gene called the Noggin directs the embryo to make a head. It works with another gene called BMP-4 that forms the bottom or belly side of an animal. Where Noggin is active, it turns off BMP-4 in cells so they can’t develop into bottom cells—instead, the cells become top cells. Genes interact with each other this way at all stages of development.
Our body design has echoes not only in mammals, frogs, and fish, but even in a more primitive creature, the sea anemone, a jellyfish relative. These creatures show how the body’s front/back axis or head-to-anus line develops. This axis defines our direction of movement.
An anemone is shaped like a tree with a long trunk and tentacles at the top. It has a top/bottom, or oral-aboral, axis from its mouth to base. While it lacks a front/back axis, using one opening to both take in food and expel waste, it still has primitive versions of genes that establish the head-to-anus line in humans. These genes are active along the anemone’s oral-aboral axis, meaning that this primitive axis is genetically equivalent to our head-anus axis.
Further, when an anemone is cut open, it clearly has left and right sides, and thus has an axis directing its movement.
Sea anemones also have a version of the Noggin gene that forms the back or upper side of a frog. When scientists injected sea anemone Noggin into a frog embryo, the anemone Noggin created extra back structures in the frog (in other words, it worked the same way a frog’s own Noggin worked), further demonstrating the genetic similarities between vertebrates and more primitive creatures.
All animals, including humans, draw on the same basic genetic recipe—but they enhance it over eons, like improving a recipe for a cake as it’s passed to subsequent generations.
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:
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.
Not every clump of cells or bacteria working together constitutes a body. Bodies have several defining characteristics:
But our complex bodies stretch back to a point in time where single-celled organisms became primitive bodies by developing mechanisms for cells to attach together, communicate, and make things like molecules.
Bodies with advanced features like hands and sense organs haven’t been around all that long. Life consisted of single-celled organisms for much of Earth’s 4.5-billion-year history. If that history were compressed into a calendar year, single-celled organisms (like algae and bacteria) would be the only life until June. Animals with heads would show up in October and the first human would appear on December 31.
Rocks illustrate this immense time scale: those older than 600 million years show evidence only of colonies of algae, which are far from being bodies. In the 1920s and ‘30s, paleontologists found evidence of possibly the earliest bodies but didn’t realize what they were. The impressions looked like disks and plates—they may have been primitive algae or jellyfish.
Scientists found more impressions of disks, ribbons, and fronds in Australia in 1947. But they didn’t understand until the 1960s, when the rocks were accurately dated, that the impressions were of some of the earliest bodies. Some resembled today’s simple sponges and jellyfish; others were unlike anything that existed.
So multicellular creatures lived in the seas 600 million years ago. Some bodies had symmetrical patterns or specialized structures that, for instance, gave them the ability to move. The rocks showed evidence of scrapes or paths of movement.
To explain how bodies came about, we’ll look first at how cells combine to form tissues in the body. Part 2 looks at the first primitive bodies to form this way.
Although these early Precambrian creatures look nothing like humans, they were made of the same type of “glue” (collagen and proteoglycan) that allows human body cells to stick together to build materials and organs. In our bodies, this glue is a mix of molecules that differs depending on the organ it’s forming—for instance, a bone versus an eye. Without the molecule mix attaching cells to each other, bodies couldn’t be formed.
Bones are an example of how the molecular glue works. Bones, like bridges made of steel or cables, get their strength from their components, which are designed to withstand force but also be somewhat flexible. The balance of strength and flexibility, as well as proportions, of the skeleton allows the bodies of humans, frogs, rabbits, and horses to run and jump in specialized ways.
The properties and molecular structure of the materials, which hold bone tissues together, are also important.
Examining bones by microscope shows how they’re structured. Some cells fit closely together while others have space between them. The space between cells contains minerals such as hydroxyapatite (also in teeth), which acts like concrete—strong when compressed but less so when twisted. The space also contains collagen molecules, ropelike bundles of fibers that are strong like rope when pulled. (Collagen is our body’s most common protein.)
Another skeleton tissue, cartilage, cushions bones that move against each other. It’s a softer, more flexible material that bends under pressure but springs back into shape. Cartilage has space between its cells, which are filled by collagen and a molecule called a proteoglycan complex that absorbs water to help cartilage pad joints.
The amounts of the different materials make bones, cartilage, and teeth different.
Skeletons, whether in humans or animals, function as designed when these molecules are distributed in the proper proportions.
Like humans and animals, the earliest creatures with bodies—even those that lacked skeletons or were only clumps of cells—had molecules or collagen and proteoglycans between cells. This means primitive creatures had to find ways to: make these materials, further glue cells together, and enable cells to communicate.
Bodies use different methods of connecting cells. Bone cells stick together by using various types of molecular rivets. Some work like contact cement, gluing the outsides of two cells together. Other rivets connect only to cells with the same kind of rivet, a mechanism that enables cells to organize and ensures that bone cells stick to bone cells, skin cells stick to skin cells, and so on.
In the lab, cells put in a dish will sort and organize themselves according to the numbers and types of rivets they have.
To make the pattern of our skeleton and the rest of the body, cells must communicate so they know when to divide, make molecules, and die. If they acted randomly—for instance, making the wrong amounts or type of bone or skin, we would die.
Cells communicate by sending out molecules with messages. A cell sends a signal or molecule, which attaches to the outside of a receiving cell. This sets off a chain reaction of molecules within the cell moving the message from the outer membrane to the nucleus. As a result, the cell receiving the information changes its behavior: it divides, makes new molecules, or dies.
In summary, all animals with bodies have molecules like collagen and proteoglycans between cells; the molecules have rivets so they can stick together and their cells communicate by sending molecules back and forth. This is how cells create body structures like the skeleton.
Now that we know the materials and mechanisms that make bodies possible, the next challenge is identifying the first bodies to employ the mechanisms of cell attachment and communication. It turns out you can find connecting molecules in the most primitive bodies and even in life forms lacking bodies: microbes.
In the 1880s, employees at an aquarium found a live blob, now known as a placozoan, on the glass of a fish tank. It’s a creature even simpler than some of the fossil impressions of disks and plates found in Australia in the 1940s.
A placozoan has a plate-shaped body with only four types of cells. Though simple, it has features of a body: division of labor among cells (movement, digestion), rivet connections, and communication ability.
An even more primitive example of an early body is a sponge, which has cells that communicate and function as a whole, division of labor, and also collagen with molecular rivets.
The body of a sponge is actually a non-living structure of silica or calcium carbonate with collagen interspersed. Sponges have two kinds of collagens, compared to 21 in humans.
A sponge’s cells are clues to the origin of bodies. The inside of a sponge is a space divided into compartments. Cells shaped like goblets direct water through the sponge, using flagellum that they wave in tandem. Goblets also use tiny arms to catch food in the water, which other cells process. Other cells can contract and change the sponge’s shape in response to changing water currents.
The way a sponge functions, with different cells having different tasks, is a primitive version of the way cells divide labor in the human body.
Placozoans and sponges have an even more primitive relative, a single-celled microbe called a choanoflagellate that has features connecting it to creatures with bodies and ultimately to humans.
Choanoflagellates look similar to the goblet cells in sponges—but while they look somewhat like primitive sponges, their DNA is actually more similar to microbes, making them a link between single-celled microbes and organisms with bodies like sponges.
Further, they have molecules that could be used for cell adhesion or communication. Microbes that specialize in invading other cells also have primitive versions of collagen and proteoglycan.
As microbes were the only life forms in the first 3.5 billion years of Earth’s history, the potential to build bodies was there long before bodies actually appeared. There are several theories for why bodies appeared in the first place.
One theory is that microbes learned to band together to avoid being eaten by bigger microbes. Perhaps they transformed the molecules they used to catch and hold onto prey into molecules that make cells stick together.
Researchers did an experiment to test this predator explanation for body formation. They grew a batch of single-celled algae, then introduced a predator that ate single-celled microbes. The algae clumped together, eventually settling into eight-cell groups, large enough to avoid capture, but small enough that each cell could absorb enough light to survive. A simple multicellular organism thus formed from a single-celled organism lacking a body.
If predators explain why bodies appeared, it also raises the question of what took them so long to develop. The answer may be that it couldn't happen until the environment provided the amount of oxygen the microbes needed to produce collagen molecules. There was a spike in oxygen levels about a billion years ago that may have been the trigger for microbes to begin forming bodies.
In summary, the answers to the key questions of how bodies developed are:
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 molecules (or water molecules, in the case of fish), and then tissues where the air connects with neurons. There’s a basic pattern in the evolution of this system (nostrils, spaces, membrane) traceable from fish to humans.
Jawless fish such as lampreys and hagfish have a nostril that draws water molecules inside to a sac in the skull where smelling takes place (they have an external and an internal nostril). Other fish have a system more like ours where water instead of air travels from a nostril to a cavity connected with the mouth.
In addition to having nasal structures for pulling air molecules, we rely on genes to help identify odors.
In 1991, researchers identified a large family of genes that create our sense of smell. They made three assumptions, which proved to be correct: that human genes for smell resemble the odor genes in mice, that the genes are active only in the tissues associated with smell, and that a large number of genes are involved with smelling.
They determined that 3% of the human genome is involved in detecting odors, which is a lot for one function. Each of these genes creates a receptor for an odor molecule. Other researchers found odor receptor genes in other species, which helped trace the transition in animals from sensing smell in water molecules to sensing it in air molecules.
Lampreys and hagfish have receptors that can handle both water and air molecules, indicating they emerged before the odor-detecting genes split into the two types, for water and air. These fish also have a fairly small number of odor genes.
The number of odor genes in animals increased over time (humans and mammals have over 1,000)—apparently, as animals became more complex, so did the sense of smell. The more odor genes an animal has, the greater its ability to detect different odors. Mammals (think tracking dogs) are extremely specialized smellers.
But the additional odor genes in mammals seem to be modified copies of the small number in primitive fish. This means our odor genes are variations on a theme—arising from generations of duplication of the fish odor genes. But despite devoting a significant proportion of our genes to smelling, 300 of our odor genes have become useless due to mutations.
Dolphins and whales provide clues for why so many human odor genes don’t work. Because they’re mammals, dolphins and whales have the same large number of odor genes as do mammals that smell air molecules. But dolphins and whales don’t use their sense of smell—instead, they use their nasal passages for their breathing blowhole. None of their odor genes work.
Apparently, because dolphins and whales don’t use their sense of smell, their odor genes mutated over generations until they became useless. Nonetheless, the genes continue to be passed to the next generation, as can happen with mutations that aren’t harmful.
Humans have some nonworking odor genes because we traded smell for sight: we evolved to rely on vision more than smell for finding food and avoiding predators, so many of our olfactory genes lost their function.
Illustrating this, research shows that primates that developed color vision had large numbers of nonfunctioning smell genes.
Comparing olfactory systems shows how closely different animals are related because olfactory genes change the more they’re copied. The more alike two species’ odor genes are, the more closely they’re related. Our olfactory genes are most like those of primates, then mammals, reptiles, amphibians, and lastly, fish.
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:
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.
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, which focuses the image.
The retina has two types of light receptor cells that send signals to the brain: the most sensitive receptors see black and white, while less sensitive receptors see color. About 70% of our body’s sensory cells are light receptor cells, which shows how important vision is to us.
From fish to mammals, most animals with a skull have this camera-like eye. Other creatures have eyes ranging from light-detecting patches, to eyes with compound lenses, to early versions of the camera eye.
We can learn about human eye development by comparing how three structures differ between human eyes and other types of eyes:
The molecule in the eye that collects light breaks into two parts after absorbing light: Vitamin A and a protein called an opsin that starts a chain reaction sending an impulse to the brain.
Humans, like all animals, need three different opsins to see color and one to see in black and white. Every animal that can see light (including humans, insects, clams, and scallops) uses the same kind of opsin molecule to do so.
Opsins transmit messages by carrying a chemical across the membrane of a cell, then helping the chemical follow a convoluted path through the cell to the nucleus. This same tortuous path exists in certain molecules in bacteria, meaning that, in a sense, we have modified pieces of an ancient bacteria inside our retinas, helping us see.
Examining opsins and eye development in different animals—for instance, the development of color vision in primates—offers further clues to human eye development.
Primates’ (and humans’) vivid color vision comes from a change in the gene that makes light receptor molecules. Primates have three light receptors tuned to different kinds of light, while most other mammals have two.
The two kinds of receptors in most mammals are made by two kinds of genes. Primates apparently copied one of these two genes to improve their vision, just as mammals copied odor genes to develop a more acute sense of smell. The mutation likely helped them pick the most colorful (and therefore nutritious) fruit from among different kinds as plant life grew more diverse.
There are two basic eye types: those of invertebrates and vertebrates. Each type has a different way of increasing the light-gathering surface area of the eye tissue. Tissue in eyes of invertebrates such as flies and worms has many folds, while in vertebrate eyes, there are bristles projecting from the surface area.
Scientists found a connection between our eyes and invertebrate eyes in 2001, while studying the eyes of a polychaete or primitive worm. Although primitive, polychaetes have two types of light-sensing organs: an eye plus light-sensing patches under their skin.
Researchers discovered that worm’s eye is a normal invertebrate eye, but the light-sensing patches have the opsins found in vertebrate eyes. Further, the patches have primitive versions of the bristles in vertebrate eyes. The polychaete is a link between invertebrate and vertebrate eyes.
Eyes that look entirely different—for instance, those of worms, flies, and mice—are nonetheless closely related because they share a common genetic recipe for building eyes.
In the early 1900s, scientists studying a mutation that made flies eyeless learned that a similar mutation in mice and humans causes the same type of problems—individuals with the mutation lack large portions of eye tissues.
Also in the 1990s, other researchers found that flies, mice, and humans with the mutation had similar DNA sequences on a specific gene, which they were able to isolate (this meant that a normal version controlled the development of eyes). They experimented with turning the gene, called Pax 6, on and off. Inserting it in flies resulted in eyes growing all over the flies’ bodies. A mouse version of the gene created an eye on a fly’s body by triggering a chain of gene activity in fly cells.
Thus, the Pax 6 gene directs the development of eyes in all animals. Eyes of different animals may look quite different, but the genetic activity behind them is the same.
So like the parts of a car, the parts of our eyes have different histories. The molecules, tissues, and genes in human eyes are derived from microbes, worms, and flies.
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.
In simple terms, the ear works like this:
Our middle ear bones betray our connection with sharks and other bony fish.
Mammals have three middle ear bones, reptiles and amphibians have one, and fish don’t have any. Our three middle ear bones are the malleus, incus, and stapes, and they develop from the gill arches. The malleus and incus develop from the first arch, while the stapes develops from the second arch.
In 1827, a German anatomist studying mammal and reptile skulls discovered that two of the ear bones in mammals, the malleus and incus, looked like parts of the jaw in reptiles, indicating that the gill arch that formed part of the jaw in reptiles also formed ear bones in mammals. Thus, parts of reptiles and mammals were the same. This was pre-Darwin when they had no concept of evolution, but the implication was that mammals evolved from reptiles.
In 1913, a paleontologist connected the dots between:
Indeed, the malleus and incus in mammals evolved from reptile jawbones. So mammals ended up with three middle-ear bones (remember, reptiles and other animals had only one). Having three enabled mammals to hear higher-frequency sounds. Improved hearing was developed by repurposing bones originally used by reptiles for chewing.
While two bones in our middle ear came from reptiles, the third ear bone (the stapes) came from fish.
As noted earlier, the second arch produces the stapes in mammals; the second arch also produces a large bone (hyomandibula) connecting the upper jaw to the brain area of fish and sharks. In both species, these two bones—in the mammal ear and shark jaw—are served by the same second-arch nerve, the facial nerve.
Thus, two very different bones in different animals share a common developmental origin and nerve pattern.
Tracing the hyomandibula from sharks to fish like Tiktaalik to amphibians shows the bone getting progressively smaller and shifting from the upper jaw to the ear (becoming the stapes).
It evolved this way as the first creatures began to live on land and needed to pick up sound vibrations in the air as opposed to in water.
For the mechanisms of our inner ear, we are also indebted to sharks and fish.
To understand the connection, it’s first necessary to understand how our hearing works. The inner ear is inside the skull (beyond the eardrum and three middle ear bones) and consists of a snail-like coil of tubes as well as gel-filled sacs. It has different parts for three functions:
Here’s how the inner ear works:
The inner ear is somewhat like a snow globe with a flexible case that moves or flops when the snow globe is tipped, sloshing the inside gel around. When we bend our heads, like tipping the globe, the gel membrane in our inner ear flops around and moves the gel.
Registering our position involves rock-like structures on top of the gel membrane (the floppy case) that move when the head is tilted. The rocks accentuate the movement or flopping of the membrane, allowing us to sense small differences in position.
We perceive acceleration through three gel-filled tubes inside the ear that move when our body accelerates or stops, causing the nerve cell projections to bend in a simulated wave.
Our position and acceleration mechanisms are both connected to eye muscles that help keep us looking in the same direction even when our head moves. (For instance, if you look at a point on this page while moving your head back and forth, your eyes stay focused on the same point.)
When you drink too much alcohol, you throw off the inner ear balancing system, which makes you tipsy or feel like you’re moving when you aren’t. When alcohol moves from the bloodstream to our inner ear gel, it sort of floats in the gel because it’s lighter, which swishes the gel around and makes the brain think you’re moving when you aren’t. The brain passes this information to your eye muscles, making your eyes twitch (a sign police look for when pulling over suspected drink drivers).
The alcohol in the inner ear eventually dissipates by moving back into the bloodstream, which gets the gel moving again—so you continue to feel off-balance or have a hangover the next day (although your eyes twitch in the opposite direction).
Our ability to balance and sense motion likely traces back to fish like trout, which have a primitive version of the human inner ear that enables them to sense the water’s current.
Trout have lines along their bodies of small organs with sensory receptors under their skin. The receptors poke hairline projections into gel-filled sacs called neuromasts. As water moves around the fish, the gel sacs or neuromasts change shape and the hairs send a signal to the brain telling the fish how fast the water is moving. Thus, trout can hold still in fast-moving water.
The similarity between neuromasts and inner ears again shows that organs developed for one function can be repurposed.
Mammals expanded and evolved the inner ear to hear better than amphibians and reptiles. Also, modern fish and other vertebrates improved the sense of acceleration by developing three ear tubes compared to one in primitive fish.
While our inner ear can be traced to fish, the specialized neurons in the gel go back even further. These neurons have a long hairlike projection along with smaller ones, and they have a fixed orientation in our inner ear and a fish’s neuromasts. These neurons have been found in animals lacking sense organs and even heads, like the worm amphioxus described in Chapter 5.
Genes also reveal the ear’s long history. A gene known as Pax 2 directs inner ear formation in mice and humans. (Mice that have a mutated version have malformed inner ears.) It’s also active in fish neuromasts—pointing to a common gene shared by fish and humans.
Further, genetic evidence suggests our eyes and ears have a common history.
There is a link between the Pax 2 gene for ear formation and the Pax 6 gene described in Chapter 9 for eye formation. The gene that forms eyes in box jellyfish (they can have more than 20 eyes all over their body) appears to incorporate aspects of both Pax 2 and Pax 6.
Thus, the major genes dictating the formation of our eyes and ears correspond to a single gene in a primitive creature. This may help explain why many human birth defects affect both the eyes and the inner ear.
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.
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 samples or by a signature—a distinct biological feature in a family. Understanding descent with modifications, it’s theoretically possible to create a family tree showing how closely related people are in any given group.
Here’s an example of how descent with modifications works in a family of clowns (assuming in the example that clown features are heritable):
Actual human traits can be traced the same way, although it’s more complicated because humans and animals typically change more than one trait with each generation. It’s even possible to trace a lineage of shared traits through humans, animals, and back to the earliest life forms.
A trip to the zoo helps show how scientists trace relationships among humans and other creatures.
Humans share many features with other animals, but we have more in common with some animals than others. For example, we have more in common with polar bears (two eyes, neck, four limbs, hair, mammary glands) than we have with turtles (two eyes, neck, four limbs). But we have more in common with turtles than we have with fish.
In the same way that generations of the clown family passed down additional traits, different subsets of animals add features. Individuals with the most shared features are the most closely related. Humans and polar bears share more features, so they should share a more recent ancestor than humans and turtles do.
While it can’t zero in on one specific ancestor, a family tree for the human species looks something like this (incorporating modifications made with each generation):
Fossil data also show the order: the first multicellular fossil at 600 million years old is older than the first fossil with a three-boned middle ear (200 million years old). The three-boned middle ear fossil is in turn older than the first fossil that walked on two legs (4 million years old).
The human body is something like a time capsule, containing features from ancient animals that mark key moments in the history of life.
Sometimes our family tree makes us sick. Many ills and ailments we suffer can be traced to the way humans adapted and modified features or body systems of our animal ancestors that were designed for other purposes.
It’s like trying to jury-rig an old Volkswagen Beetle to go 110 mph—whatever modifications you make will have limitations because it was originally designed for gas mileage, not speed. Similarly, when you start with the body of a fish and turn it into a human mammal that walks on two legs, you get knee problems because limbs in fish were not originally developed to support walking upright on two legs.
Another example is the convoluted paths of our arteries, veins, and nerves, which create problems that come back to bite us. Our blood flow problems are further exacerbated by living a sedentary lifestyle in a body designed for a short, active life.
This disconnect with our ancient history means we malfunction in foreseeable ways.
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.
Four of today’s top 10 causes of death—heart diseases, diabetes, obesity, and stroke—stem from a sedentary lifestyle.
In 1962, anthropologist James Neel looked at how diet factors into the equation. He theorized that early humans had “thrifty genotypes” that stored food in the body as fat, when food was plentiful, so it could be burned for energy later when food was scarce.
Although most of us don’t face periods of famine today, our genes are still telling our bodies to save fat—which leads to obesity. Neel’s theory might explain why we crave fatty foods: they’re high-energy, which would have been an advantage in hunter-gatherer days.
Our inactive lifestyle also affects our blood flow. Our leg muscles help push the blood from our legs and feet back up to the heart, while one-way valves keep blood from flowing back down due to gravity. But if we don’t use our leg muscles enough, blood pools in our leg veins and overtaxes the valves, creating varicose veins. When we sit too much, blood can pool in veins around the rectum to form hemorrhoids.
Humans also make a tradeoff for our ability to talk: the problems of choking and sleep apnea.
Throat muscles that allow us to talk are a modification of the gill arches in fish. The muscles of the back of the throat contract and relax when we speak, so we can make a range of sounds.
However, this flexibility means that the throat can collapse while we sleep to the point that air can’t pass through—a breathing problem called sleep apnea.
Another problem of our modified throat is that we’re prone to choking because we use the same passage to swallow, breathe, and talk; these functions conflict when food or liquid gets into the trachea.
Our hiccup reflex can be traced to fish and tadpoles. This annoying reflex is triggered by the nerves that control our breathing. When the nerves spasm, muscles controlling our breathing contract too fast, and a flap of tissue at the back of the throat closes over the airway. The combination of fast inhalation and the brief airway closure makes a “hic” sound.
The problem with hiccups is that they occur in bouts: if you don’t stop them after five or 10 hiccups, you may get 60 before they stop. (Breathing carbon dioxide sometimes stops them.)
Both issues—the nerve spasm and the flap closure—stem from our history:
1) The nerve spasm in hiccups stems from our fish history.
Our brain coordinates our breathing muscles: the brain stem activates nerves, which activate muscles in the chest, diaphragm, and throat in an orderly sequence.
Fish have the same nerve pattern, but their nerves and muscles for breathing are close to the brain stem. In contrast, our breathing muscles are farther from our brain stem and our nerves travel a convoluted path, leaving them vulnerable to interruptions that could cause a spasm.
2) The flap closure of hiccups is a result of our amphibian history.
Tadpoles use the same pattern of sudden muscle contraction and flap closure that causes hiccups in humans for a beneficial purpose. It’s designed to allow them to keep water out of their lungs and breathe through their gills. However, it’s a pattern that doesn’t serve any useful purpose in humans.
The human problem of developing a hernia near the groin comes from transforming a shark’s body into a mammal.
In sharks, the testes holding sperm are positioned toward the front of the body in the chest. In human males, the testes are much lower, in the scrotum, to keep the sperm at optimum temperature.
The repositioning of gonads from high in the chest to low in the groin area means the sperm cord carrying sperm from the gonads to the penis travels a complex path up and over the pelvis, then back down and out through the penis.
Like sharks’, human testes begin development near the liver, then descend as the male matures. But they have to go a lot further in humans. And as the testes push against the body wall during descent, it creates a weak spot in the wall. Guts sometimes move with the testes and get pushed against the body wall when abdominal muscles contract. Because of the weak spot, they sometimes escape the body cavity, creating a hernia.
Our cells contain mitochondria, which have many important functions, including turning oxygen and sugar into energy for the cell, metabolizing toxins, and other regulatory tasks. Their functioning can be traced to microbes that existed billions of years ago.
Many things can go wrong inside mitochondria, leading to illness or death. For example, a problem with a chemical reaction that consumes oxygen may affect specific tissues such as the eyes or may affect every body system.
Mitochondria’s bacterial past is reflected in their genetic and cellular structure. The chemical reactions they use originated in microbes. In fact, bacteria today use similar processes, which scientists study to learn about human mitochondrial diseases.
It’s just one of many ways that understanding our evolution leads to insights and advances in human genetics and medicine. Besides microbes, myriad other organisms including flies, worms, and fish can teach us how our bodies work, how they fail, and what we can do to live longer, healthier lives.
The space program of the 1960s and ‘70s changed the way we view the moon and space travel. In the same way, advances in paleontology and DNA research are changing the way we see our bodies.
As science uncovers the secrets of diverse creatures, we’re learning that seemingly unique features and advances in the development of humans and other life are recycled, adapted, or repurposed from earlier animals. Our individual organs, complex structures such as our heads, and our entire body plan connect us to billions of years of life history.
From our commonalities with some of the world’s simplest and humblest creatures, we have the potential to learn what makes us human and find cures for many of our ills.
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?
How could you be more active tomorrow? How could you build more physical activity into your day?
In what other ways could you act more in concert with your body’s design?