Your Inner Fish: A Journey into the 3.5-Billion-Year History of the Human Body

“Every rock sitting on the ground has a story to tell: the story of what the world looked like as that particular rock formed. Inside the rock is evidence of past climates and surroundings often vastly different from those today.”

Shubin’s work as a paleontologist and evolutionary biologist is often based on analyzing rocks for information about the Earth’s ancient history. Since rocks form in particular climates and time-periods, each rock contains its own story about life on Earth long ago.

“Like a fish, it has scales on its back and fins with fin webbing. But, like early land-living animals, it has a flat head and a neck. And, when we look inside the fin, we see bones that correspond to the upper arm, the forearm, even parts of the wrist”

This passage describes Shubin’s Tiktaalik fossil. The creature is unique as it shares characteristics with both fish and reptiles. The fossil offers important information about how ancient fish began to evolve limbs and leave water to walk on land. 

“To [Sir Charles] Bell, the structure of the hand was ‘perfect’ because it was complex and ideally arranged for the way we live. In his eye, this designed perfection could only have a divine origin.”

Sir Charles Bell is a 19th century surgeon who authored a study of the anatomy of hands, titled The Hand, Its Mechanism and Vital Endowments as Evincing Design. Bell was one of a number of scientists in the 19th century discovering that all animal bodies are structured according to precise patterns. These scientists often interpreted such structural “perfection” as evidence that God had intelligently designed the entire universe. 

“What [Sir Charles Owen] found, and later promoted in a series of lectures and volumes, were exceptional similarities among creatures as different as frogs and people. All creatures with limbs, whether those limbs are wings, flippers, or hands, have a common design.” =

Sir Charles Owens is a 19th century anatomist who studied the bone structure of numerous animals. He ultimately discovered that all animal limbs are constructed according to a similar pattern: one bone, then two bones, then a group of wrist bones with digits protruding outwards. Owens’ discovery prefigured the evolutionary theories of Charles Darwin, who explained animal similarities by arguing that all animals share a “common ancestor” (44). 

“But it also carried a real surprise: Jenny found that the limb was shaped like a flipper, almost like that of a seal. This suggested to her that the earliest limbs arose to help animals swim, not walk.”

The paleontologist Jenny Clack performed a study on the fossil of Acanthostega gunnari, a fish-like creature with limbs and digits. Clack’s analysis of the creature suggested that Acanthostega evolved limbs to help it swim better, countering theories that animals first evolved limbs to walk on land. 

“Despite these differences [between cells], there is a deep similarity among every cell inside our bodies: all of them contain exactly the same DNA. If DNA contains the information to build our bodies, tissues, and organs, how is it that cells as different as those found in muscle, nerve, and bone contain the same DNA?”

Recent biological experiments have tried to discover the mechanisms through which human bodies grow and develop different forms of organs and other bodily tissues. Though each cell contains a copy of the same genetic code, only select portions of the code are turned on in each cell, dictating how cells function. 

“It means that this great evolutionary transformation did not involve the origin of new DNA: much of the shift likely involved using ancient genes, such as those involved in shark fin development, in new ways to make limbs with fingers and toes”

Paleontologist Randy Dahn discovered that the Sonic hedgehog gene that helps control limb growth in animals also exists and behaves similarly in sharks’ fins. Shubin argues that the discovery shows that the tools for constructing limbs had existed for long before animals began to actually develop limbs, and reveals the “inner fish” that exists within human bodies (79).  

“Because teeth preserve so well in the fossil record, we have very detailed information about how major patterns of chewing—and the ability to use new diets arose over time. Much of the story of mammals is the story of new ways of processing food.”

Teeth play an important role in evolutionary development, with different classes of animals having distinct forms of teeth. Mammalian teeth are particularly defined by their occlusion—the top and bottom rows of teeth precisely locking together. Such occlusion let mammals consume a variety of foods, allowing further evolutionary development. 

“These creatures all had an exceptional trait: they had whole assemblages of conodonts in their mouths. The conclusion became abundantly clear: conodonts were teeth. And not just any teeth. Conodonts were the teeth of an ancient jawless fish.”

The earliest known creature to have teeth is an ancient lamprey-like fish, which boasted a primitive form of teeth known as conodonts. These teeth were the only bony portion of their bodies, which means hard teeth evolved before bony skeletons. 

“The arches are the road map for major chunks of the skull, from the most complicated cranial nerves to the muscles, arteries, bones, and glands inside.”

The structure of skulls appears complicated at first glance, with many twisting nerves linking disparate organs—such as the trigeminal nerve, which connects to both the jaw and the ear. However, the skull is actually organized based on the development of four different arches in the embryo. Different organs and tissues form in each of the arches, and the structure of the skull follows the segmented pattern established by these arches. 

“Every head on every animal from a shark to a human shares those four arches in development. The richness of the story lies in what happens inside each arch.”

The skull arches in human embryos echo arches in the embryos of every other creature. However, the arches behave differently in different animals. For instance, although the first arch in both sharks and humans the develops a similar bone, in sharks, the bone becomes a jaw bone, while in humans, the bone serves as a smaller ear bone. 

“As they looked at embryos, they found something fundamental: all organs in the chicken can be traced to one of three layers of tissue in the developing embryo. These three layers became known as the germ layers.”

Experiments performed by biologist Karl Ernst von Baer on developing chicken embryos revealed that its organs develop in one of three embryo layers: the ectoderm, the endoderm, and the mesoderm. Von Baer subsequently discovered that all animal embryos share these three tissue layers. 

“Mangold had discovered a small patch of tissue that was able to direct other cells to form an entire body plan. The tiny, incredibly important patch of tissue containing all this information was to be known as the Organizer.”

In the 1920s, biologist Hilde Mangold grafted a small patch of tissue from one side of a salamander embryo onto the other side. The resulting embryo formed two complete bodies, showing that the transplanted tissue, later named the Organizer, contains the genetic information to control the development of the entire body. 

“Like a cake recipe passed down from generation to generation—with enhancements to the cake in each—the recipe that builds our bodies has been passed down, and modified, for eons. We may not look much like sea anemones and jellyfish, but the recipe that builds us is a more intricate version of the one that builds them.”

Though sea creatures such as jellyfish and anemones vastly differ from humans, versions of the same genes construct the oral-aboral body axis in both. Shubin’s food recipe metaphor illustrates how this is possible—the same genetic recipe, with minor tweaks in every generation, has been used by animals since primitive jellyfish. 

“So the thought experiment reveals one of the defining features of our bodies: our component parts work together to make a greater whole. Moreover, in bodies, there is a division of labor between parts; brains, hearts, and stomachs have distinct functions.”

Many characteristics distinguish bodied organisms from non-bodied ones such as bacteria—in particular, division of labor, in which discreet parts cooperate. Division of labor even exists on the cellular level, when different parts of cellular tissue perform different tasks. 

“The question then becomes not how could bodies arise, but why didn’t they arise sooner? Answers to this puzzle might lie in the ancient environment in which bodies arose: the world may not have been ready for bodies”

Experiments have shown that ancient single-celled organisms contained the necessary tools for creating bodies long before bodies emerged. Shubin hypothesizes that bodies could not evolve until the Earth’s environment became more suitable—for instance, through elevated oxygen levels in the atmosphere. 

“[DNA] is particularly important where the fossil record is silent. Large parts of bodies—soft tissues, for example—simply do not fossilize readily. In these cases, the DNA record is virtually all we have.”

Though fossils are tremendously important for understanding the history of evolution, they typically only preserve bones and skeletons. In order to understand the development of soft tissues, or organs such as noses, evolutionary biologists must instead analyze the sections of DNA that control them. By looking at our smell genes, and comparing them to the DNA of other species, Shubin can infer how our sense of smell evolved. 

“[Yoav Gilad] found that primates that develop color vision tend to have large numbers of knocked-out smell genes. The conclusion is clear. We humans are part of a lineage that has traded smell for sight.”

Though humans contain a plethora of olfactory genes, many of these genes are no longer in use. Shubin theorizes this is because human beings evolved to rely more on eyesight than scent, meaning they no longer needed to use their DNA’s many smell genes. 

“Despite the stunning variety of photoreceptor organs, every animal uses the same kind of light-capturing molecule to do this job. Insects, humans, clams, and scallops all use opsins.”

The same basic molecules—opsins—exist within all animal eyes, and play a crucial role in transforming light into molecular information that our brains interpret as vision. The ubiquity of opsins suggests that all animals evolved eyesight from the same common ancestor. 

“This shift [in color vision] may be related to changes in the flora of the earth millions of years ago. Monkeys that live in trees would benefit because color vision enabled them to discriminate better among many kinds of fruits and leaves and select the most nutritious among them.”

New evolutionary developments typically respond to changes in the environment, often providing animals with an advantageous new ability for navigating their surroundings. Humans and certain primates are unique in having the ability to perceive a wider spectrum of color than other animals. Shubin hypothesizes that such color vision may have evolved at the same time as “changes in the composition of ancient forests” (204), allowing ancient monkeys the ability to better identify fruits and other vegetation. 

“In fact, this shift [in mammalian hearing] was accomplished not by evolving new bones per se, but by repurposing existing ones. Bones originally used by reptiles to chew evolved in mammals to assist in hearing”

Mammals contain three small bones in their middle ears, which provide them with a heightened sense of hearing compared to that of other animals. However, two of these ear bones are the same as two jaw bones in reptiles. Shubin theorizes that as mammals evolved from reptiles and required more hearing capabilities, existing bones in their jaws transformed to assist in hearing. 

“Until more evidence rolls in, we are left with one of two alternatives: either inner ears arose from neuromast organs or the other way around. Both scenarios, at their core, reflect a principle we’ve seen at work in other parts of the body. Organs can come about for one function, only to be repurposed over time for any number of new uses.”

Inner ears, which allow mammals to perceive sound, are structurally similar to fish neuromast organs, which help them to identify the movement of water currents. Though it is unknown which organ evolved first, Shubin suggests the similarity is evidence of the way evolution repurposes existing organs for new functions. When mammals required the ability to hear more sounds than fish, neuromast organs could have evolved into ears. 

“To put it in a more precise form: every living thing sprang from some parental genetic information. This formulation defines parenthood in a way that gets to the actual biological mechanism of heredity and allows us to apply it to creatures like bacteria that do not reproduce the way we do.”

The study of biology hinges on the basic law that all creatures descend genetically from their parents. Though the law may seem an obvious one, without it biology as a discipline would not function. The law allows biologists to consider how creatures have evolved and how they pass down genetic information over time.