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.
Though the details of scientific theories about certain animals may seem like useless distinctions that have no bearing on human anatomy, Shubin argues that the many basic similarities between these animals and humans mean that any information about might eventually have use for humans, as in efforts to cure genetic diseases or the ability to track genetic data on crime scenes. It takes many scientists over different generations working on similar issues across many animal species in order to see the full picture of how one scientist’s contribution might add to scientific discovery as a whole.
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 acknowledges the simple law at the heart of biology, though the details and differences between all the species on Earth quickly make attempts to make a family tree very complicated. By tracing parentage instead of comparing physical structures, biologists can block out a lot of similarities between animals that don’t actually have a shared developmental history.
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.
For the purposes of this example, it is not important how the clown family gains new traits. In humans, the crossing of genetic information from mother and father creates new combinations that sometimes results in new traits in the offspring. Though Shubin uses traits that can be seen visually for his example, geneticists actually depend more on genetic similarities in people’s DNA sequences to determine lineage, as those markers are less likely to be changed by differing environmental influences.
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.
The clown family isolates the thousands of genetic mutations that could happen in a generation of humans to one simple difference, making the process of descent with modification easier to see. Tracing the lineage back to pond scum then depends on using specific genetic markers that geneticists have seen go relatively unchanged through different species.
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.
Though the mechanism of descent with modification that Shubin used in his clown example is the same, the time frame for descent with modification actually creating new traits and new species is thousands (or even millions) of years, rather than one generation as in Shubin’s simple example.
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 has walked through a few of the shared traits that support the tree that flows from fish to humans, including limbs in Chapter 2, odor genes in Chapter 8, and eyes in Chapter 9. The family tree and animal observation itself form a circular feedback loop, as scientists use and observation to build the most likely tree based on available evidence, then use that tree to make predictions, check to see if those predictions hold true in nature, and tweak the tree as new observations come out of these predictions.
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.
There are obviously many more factors in the modifications of each species in the generations that Shubin traces from multicellular life to humans, but Shubin focuses on a few key modifications to keep his analysis as clear as possible. These classifications are useful in tracing the history and gradual development of a species, but they are somewhat limiting for intermediate stages such as Tiktaalik that might seem to fit into two categories based on their physical traits and genetic information.
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.
Shubin specifically mentions the ways that the fossil record upholds the tree he previously outlined, as he is a paleontologist and the fossil Tiktaalik that inspired this book is one of the fossils that helped cement this lineage. Yet the genetic record also supports this tree, as Shubin has described in previous chapters – showing how many scientific disciplines can come together for the same conclusions.
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.
As when Shubin walked through the cranial nerves by describing them as plumbing from an old building that needed to be brought up to new codes, the human skeletal system holds evidence of using old structures for new purposes.
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.
The nervous system is fairly clear and straightforward in fish, but increases in complication due to the complex history of humans developing their specific body plan from the original blueprint of fish. In an example much closer to our own time period, modern humans are even rewriting the lifestyle and body plan of the first humans by becoming less active with each generation. Shubin applies this broadly to all humans, but of course some communities continue to live in ways that make the best use of the original human body plan.
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.
Shubin argues that understanding the original plan for human lives built into our genes can help us live healthier lives by increasing activity that is in line with the ways that human bodies originally worked. This line of thinking is certainly useful for increasing human health in the short-term, yet pushing that idea further suggests that our new sedentary lifestyle is an external pressure that will simply shape the path of future human development to favor those humans who can best handle this sedentary “environment.” However, these adaptations will take significant time, and no adaptation can take place within the small number of generations alive on the planet today.
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.
Shubin explains the discovery of the “thrifty genotype,” a genetic disposition to saving fat that was useful in human history. There are now studies of medicines or gene therapy that might be able to address the fat storage techniques of individuals with thrifty genes so that their metabolism will not save so much. Understanding the historical adaptations that helped humans in the past allows scientists to see which paths will be most beneficial to human health in the future.
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.
The things that make humans unique – such as walking upright – require complex mechanisms that allow a body plan originally meant for another lifestyle to perform these functions. The adaptations that worked for humans who had a specific kind of active lifestyle are not helpful for humans who now have a sedentary lifestyle. This example provides a small glimpse into the environmental pressures that shaped how animal species adapted their body plans to external factors over the history of life on Earth.
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.
Though human embryos never use their gill arches to breathe, the developmental path that follows humans back to fish means that these gill structures are still in place in the human. Humans did not invent new structures to perform new actions that helped them survive (like talking and verbal communication), they simply repurposed old structures that other animals already had for other reasons. As mutations in genes help isolate the progression of genetic changes, the missteps in human anatomy help Shubin explain the origin of human anatomical adaptations.
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.
Hiccups have no real use in modern humans, yet they are a holdover from our evolutionary past. Shubin identifies two key areas for tracing the development of hiccups, making a potentially complicated backstory simple and easy to follow.
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 fish body and the human body each use the same nervous system, yet demand different functions that put different pressures on this common system. In order to have larger lungs meant for breathing air, the human chest cavity had to expand. The cost of this beneficial adaptation was stretching the nervous system to new lengths. Again, humans can only survive this adaptation because the larger chest cavity is helpful to surviving in our land environment, and the nerve cost is not fatally harmful.
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.
Unlike the adaptation of a larger chest cavity, Shubin does not explain any benefits that this throat mechanism could have for humans, simply using it as evidence of humans’ evolutionary lineage through amphibians like frogs.
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.
Given the assumption that bodies will be as efficient as possible, many pieces of human anatomy don’t actually follow that rule on the surface. Shubin already touched on this in his explanation of the cranial nerves in Chapter 5, and he brings it up again by assuming that sperm tubes would be better served by staying closer to the low position of the human gonads. Looking at human bodies through the lens of the evolution and adaptation of many species gives reasons for some strange parts of human anatomy such as the long path of the sperm tubes.
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.
Hernias are an injury that is much more likely in males. Before looking at the developmental path that links human anatomy to fish anatomy, there would have been no viable explanation for why male body walls were so much weaker than female body walls. Given the anatomical blueprint that human bodies work from, the descent of the gonads is the best way to use that fish body for human purposes, even if it leaves males open to injury.
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.
Scientists cannot easily do experiments on humans to treat fatal illnesses, due to the relatively small pool of human test subjects and the larger issue of ethical problems with experimenting on humans. The fundamental similarities between mitochondria and bacteria mean that scientists can perform the necessary experiments that might actually save human lives in a humane and safe way.
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.
Just as many people might not understand why Dahn devotes his life to the boring work studying shark embryos, or Shubin himself spends his time looking for ancient, outdated fish fossils when he could be doing genetic research, Shubin’s son does not see the importance of a space capsule that looks beat-up and dirty. These examples all point to the ways that large leaps in scientific discovery can be made from unglamorous beginnings.
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.
Shubin ties together the future of scientific research, both in outer space and on Earth, to the past that has built the foundation for scientists to continue their research. The developmental history of life on Earth illustrates how organisms themselves use this same process of taking existing structures and applying them to new functions or adaptations to a specific environment. The task of scientists is now to bring many fields of research on distinct species together in ways that take advantage of the basic similarities between all living things in order to benefit human health.