Your Inner Fish

The Zoo in You. In college, Shubin volunteered at the American Museum of Natural History, where he would listen in on weekly seminars that would often devolve into shouting sessions between biologists about the smallest details of a presentation. At the time, Shubin could not understand why these scholars were so passionate about the names or biological classifications of species, but he now sees how species classification and the description of different animals has huge effects on how scientists compare different species and use that genetic data for purposes as varied as family ancestry, forensic crime scene analysis, and the tracking of familial or inherited diseases.

There is one simple law at the heart of all biology: every living thing on the planet had parents (or at least parental genetic information, in the case of cloning). This means that organisms are modified versions of the DNA of their parents. Using this knowledge, Shubin suggests that it is possible to build a family tree of how closely related a room full of individuals might be.

Shubin illustrates this idea of descent with modification using the analogy of a family of clowns. The first generation of clowns has a mutation that gives them a red nose. The second generation has the red nose and a new mutation that gives them huge feet. The third generation has the red nose, floppy feet, and adds orange curly hair. Looking back on this lineage, it is easy to see who is more closely related based on who has more shared features. The first generation and the third generation share only a red nose, while the second and third generations share red noses and huge feet.

Now replace the “clown features” with actual human traits, and Shubin has a simple model of human genetic descent with modification. The only problem is that humans (and other animals) tend to change more than one trait with each generation. Yet through careful analysis, it is possible to trace this lineage of shared traits all the way through humans back to 3.8-million-year-old pond scum. To do this, Shubin returns to the zoo.

A (Longer) Walk Through the Zoo. Many human features are shared with other animals, but some animals share more features than others. For example, polar bears and humans have more in common than turtles and humans. Additionally, turtles and humans have more in common than fish and humans. As with the clown family, different subsets of animals seem to add on features just like the generations of clowns added clown traits.

Using the lineage of shared traits to create a biological family tree predicts that fish and amphibians would be the “grandparents,” followed by the “parent” reptiles, then the more recent generation of mammals, and finally the most recent generation of human species. There are many branches of sisters and cousins within that framework, such that it is very hard to trace one clear line back to “The Ancestor” of humans. This family tree also allows biologists to make predictions about which animals should share the most features and have the most similar DNA. If the predictions are backed up by the actual features and genes that biologists observe in animals, then biologists know that their tree is correct.

Shubin begins to walk through the family tree of the human species, noting the modifications that are made with each generation. At the top is multicellular life: organisms with a body made of many cells. Then comes Bilateria: the group of organisms with body plans that include front/back, top/bottom, and left/right symmetry. Next are vertebrates: all animals with a backbone. The next level is vertebrate tetrapods: animals with four limbs. After that are mammals: tetrapods who have a three-boned middle ear. Finally comes humans: mammals who walk on two legs and have enormous brains.

The family tree of the human species is supported by the fossil data, as the first multi-celled fossil is older that the first fossil with a three-boned middle ear. The three-boned middle ear fossil is in turn older than the first fossil that walked on two legs. In some ways, the human body acts like a time capsule, preserving features from ancient animals that reflect how life has changed over time.

Why History Makes Us Sick. Shubin says that he once hurt his knee badly, finding out that he had torn his meniscus (one of the ligaments in the knee). There are three ligaments in the knee that are particularly likely to get hurt, due to the fact that knees were not originally developed to support walking on two legs. Shubin compares the human knee to a VW Beetle that has been jury-rigged to accelerate to 150 mph.

Another place that shows the changes humans made to a body plan originally meant for fish is the paths of arteries, nerves, and veins. Some veins loop over organs or switch direction almost randomly within the body. Furthermore, the modern human’s sedentary lifestyle exacerbates blood flow problems in a body meant for a short life span full of active movement. Almost every illness humans suffer has a historical component about the past functions of different human body systems.

Our Hunter-Gatherer Past: Obesity, Heart Disease, and Hemorrhoids. Almost every level on the human family tree was an active predator in a different environment. Fast forward to modern human life, and most people have very little physical activity over the course of the day. Four of the top ten modern causes of death—heart diseases, diabetes, obesity, and stroke—arise from the conflict between humanity’s genetic wiring for an active lifestyle and the sedate lives we actually lead.

An anthropologist named James Neel looked at the conflict between active wiring and sedate lives through the lens of diet. Neel hypothesized that early humans would have likely had “thrifty genotypes” that could save food as fat when food was plentiful so that the individual could survive long stretches when food was scarce. Now that (many) humans do not have to deal with periods of famine, our genes constantly tell our bodies to save fat that is never used – leading to high rates of obesity when high-fat food is always readily available.

A sedentary lifestyle also affects human blood flow. Walking on two legs makes it harder for blood to flow “uphill” from the feet back to the heart. Our leg muscles help push the blood back up, assisted by little valves that stop the blood from rushing back down due to gravity. If a human does not use the leg muscles, the blood pools in the veins and stresses the valves. When the valves break, painful problems such as varicose veins can restrict blood flow in the legs even more. When people sit too much, blood can also pool around the rectum to form painful hemorrhoids.

Primate Past: Talk is Not Cheap. In order to be able to talk, humans have to deal with the hazards of choking and sleep apnea. Looking back to the gill arches from Chapter 5, the throat muscles that allow humans to talk are a modified version of the gill arches of a fish. When we speak, the muscles of the back of the throat contract to control how rigid or flexible the throat is, making it possible to produce a wide range of speech sounds. Yet this flexibility means that the throat can collapse so much while a human sleeps that no air can pass through, a breathing problem called sleep apnea. Another problem of a human’s modified throat is choking, as humans use the throat to swallow, breathe, and talk.

Fish and Tadpole Past: Hiccups. Many animals can hiccup, a complicated reflex triggered when the major nerves that control breathing spasm and contract the breathing muscles too fast, and then a flap over the airway closes and makes a “hic” sound. There are two issues at play in the history of hiccups: the nerve spasm and the flap closure.

The nerve spasm in hiccups comes from our fish history. Most of the time, the human brain coordinates all the breathing muscles in a well-defined rhythmic pattern where the brain stem activates specific nerves that in turn activate specific muscles. This nerve pattern is seen in fish where the major nerves and muscles involved in breathing are fairly close to the brain stem. Yet the human muscles for breathing are much farther away in our chests, meaning that the nerves must travel a long way from the brain stem to the muscles and are vulnerable to an interruption that could cause a spasm.

The flap closure of hiccups comes from our amphibian history. Tadpoles use the same pattern of sudden muscle contraction followed by throat flap closure that distinguishes hiccups in humans. Yet in tadpoles, this pattern allows the tadpoles to keep their lungs clear of water and breathe with their gills.

Shark Past: Hernias. Hernias near the groin are likely a product of repurposing a fish body for human life. In fish, the gonads that hold sperm are near the liver toward the front of the body. In humans, the gonads are much lower in the body, and separated from the main torso in the scrotum so that the temperature of the gonads can be regulated to maximize the health of the sperm. The movement from shark gonads high in the chest to human gonads low in the groin area means that the tubes that carry sperm from the gonads to the penis actually go up toward the human waist, loop back over the pelvis and then travel out through the penis.

As pre-pubescent children, human male gonads are housed near the human liver, then descend as the male matures. This descent creates a weak spot in the body wall of the torso, as the gonads push down on the body wall like a hand pushing through a sheet of rubber. This weakness in the body wall means that the guts can sometimes escape the body cavity and lie next to the spermatic cord when the guts are pushed by the abdominal muscles. The escape of the guts creates a painful injury called a hernia.

Microbial Past: Mitochondrial Diseases. Mitochondria are important in every single cell of the human body, turning oxygen and sugar into energy for the cell and performing other regulatory functions. Yet tons of things can go wrong inside mitochondria, leading to illness or death. The chemical reactions that mitochondria use to create energy are an ancient process still used by bacteria, meaning that scientists can use bacteria to study mitochondrial diseases that kill humans. By changing the bacterial energy process to match the mutation that is killing humans with faulty mitochondria, scientists can run experiments to keep the bacteria healthy in other ways. This is just one example of how knowing our evolutionary past can lead to scientific insights that help humans live healthier and longer lives.

Epilogue. Shubin says that he often takes his children to zoos, museums, and aquariums, a far different perspective than the time he spends in those buildings as a scientist and a professor. Being a visitor there reawakens his wonder at the complex workings of life on earth. At the Museum of Science and Industry in Chicago, Shubin was struck by a display of a battered space capsule. Shubin realized this display was not a replica, but the actual space craft Apollo 8 that took humans to the moon. Shubin tried to explain the momentous significance of this trip to Nathaniel, his son, but Nathaniel was too young to understand what made this space craft so special.

For Shubin, Apollo 8 represents the power of science to explain our universe and the essential human optimism that keeps humans asking questions and seeking answers. Just as Apollo 8 made space and the moon accessible to humans, paleontology and genetics make the distant past and the history of life available for human study. So far, this research has revealed that all life is a constant cycle of recombining and repurposing old materials for new functions. Shubin imagines a future where genetic research and new discoveries in the fossil record can help humans understand the fundamental building blocks of the human body and cure diseases.