1.1 Introduction

Anatomy, the study of the structures of organisms’ bodies, is one of the oldest subdisciplines in biology. To understand how organisms work, it helps to know what components they are made of. This is one of the reasons why anatomy is one of the first courses taken by medical and veterinary students. However, anatomy is not just for future health professionals. We can learn a lot about evolution by studying comparative anatomy. This is exactly what it sounds like: We compare the anatomy of different organisms to understand why there are similarities and differences in structure. Comparing the anatomy of different organisms can also tell us about the evolutionary history of the organisms we’re studying.

Comparative vertebrate anatomy focuses specifically on vertebrates, which are animals that belong to the subphylum Vertebrata. These are animals that have a spinal column made of bone or cartilage, a distinct head with a brain, and an internal skeleton (we will define vertebrates more clearly in Chapter 3). This area is of specific interest because we, humans, are vertebrates. That means that comparing our anatomy to that of a fish, amphibian, or reptile can tell us something about our evolutionary history and why our bodies are the way they are. This is not to say that the other 78,000 species of vertebrates aren’t interesting in their own right. Perhaps you have wondered to yourself: Why don’t snakes have legs? How did whales and platypuses happen? Why can’t pigs fly? Why do people seem to always have back problems? Comparative vertebrate anatomy can help answer these questions.

Despite how long the field of anatomy has been around, we are far from knowing everything. Comparative vertebrate anatomy is a vibrant and active field. New structures are still being discovered. Advances in imaging technology and analytical tools have led to breakthroughs in our understanding of the basic relationships between form (shape) and function (how it works). This has led in turn to a deeper understanding of how vertebrate anatomy and morphology (shape, size, texture, etc.) have evolved through time. In some cases, these discoveries and principles have led people to design new adhesives, filters, and other practical items. This is called bioinspiration, where scientists either are inspired by or base their designs upon biological structures or processes. For example, understanding the form and function of manta ray gill structures has led to patents for new industrial filtering technology. Studying how geckos stick to vertical surfaces has led to the development of new adhesives.

We begin this chapter by taking a journey through the honest history of comparative anatomy. While there are a number of scientists who are typically celebrated for their accomplishments in summaries about history, many others are unsung—at least as this history is told in the Global North (countries with large amounts of wealth and technological development such as those in northern North America and Europe, Japan, South Korea, and Australia). History also often glosses over how scientific pursuits affect others, as science is not as objective as we may hope. The second part of this chapter will set up the organization of this book.

1.2 The History of Comparative Anatomy

Several large, heavy books have been written about the history of medicine, the history of anatomy, and the history of biology. We cannot hope to cover everyone and everything in one chapter. For example, numerous structures in our bodies are named for Western anatomists who described them (e.g., Eustachian tubes, Fallopian tubes, the bundle of His, the loop of Henle), and readers can easily search for those individual histories online or in other texts. For readers who are intrigued by the history below, we have included several sources for further reading at the end of the chapter.

Anatomy Is an Ancient, Global Discipline

Anatomical study has been around for thousands of years, often in pursuit of understanding how human bodies work. While animal dissections were not performed for study, Mesopotamian clay tablets and a Babylonian clay model of a sheep’s liver (including the segmentation marks; Figure 1.1) describe the structure of the organs of some of the sacrificial animals. These records date back to at least 2000 BCE.

Records in Egypt date back to at least 3000 BCE, when Egyptians studied and documented the anatomical knowledge gleaned from surgery and mummification practices. The Edwin Smith Papyrus (3000–2500 BCE) contains the first known description of the brain, including gyri and meninges (Figure 1.2). The Ebers Papyrus (1550 BCE) contains information about the human cardiovascular system, while the Kahun Papyrus (1850 BCE) correctly describes the role of the placenta in fetal nourishment. Hieroglyphs show that Egyptians’ knowledge of anatomy extended beyond humans, particularly to bovines (cattle).

During this time period, anatomical study was also taking place in many other parts of the world. For example, in China, Huangdi (2600 BCE) wrote the Nei Jing, where he stated that blood continuously flows and is under the control of the heart and correctly linked the pulse to the heart (Figure 1.3). This is 4,000 years before William Harvey’s “discovery.” Anatomical study also occurred in Persia (currently Iran). During the Achaemenian Dynasty (sixth to fourth centuries BCE), the bodies of condemned criminals were used for medical research and dissection.

An Indian physician named Sushruta (also written as Suśruta) wrote a surgical textbook, the Susruta Samhita, in the sixth century BCE, which described methods for dissecting human bodies in a systematic fashion and the anatomy of the eye, muscles, joints, and ligaments (Figure 1.4). However, because Hindu law requires people to be cremated if they are older than two years old, Sushruta had to rely on the corpses of small children for the study of internal anatomy beyond what he could see in surgical procedures. Animal sacrifices were also used for the purpose of understanding human anatomy prior to Sushruta’s time.

Despite this rich history, most accounts of comparative anatomy history start in Greece. Herophilus of Alexandria (335–255 BCE), often called the “Father of Anatomy,” is usually cited as the first person to perform dissection on human bodies for the express purpose of learning about anatomy. However, we have seen above that this is not true once we widen our view beyond the Western world. Herophilus’s nickname is also in part due to the sheer number of humans he dissected—potentially 600 corpses of condemned criminals, with some reportedly vivisected (dissected while alive).

Even within Greece, anatomical history extends farther back than Herophilus. Alcmaeon of Croton, in the sixth century BCE, observed anatomical structures in both humans and other animals, including sensory systems and blood vessels. Unlike many of the other anatomists discussed above, Alcmaeon has been recognized as the first anatomist to dissect animals for the express purpose of understanding their anatomy and not to use for understanding humans. Aristotle (384–322 BCE) is also known more for his contributions to human anatomy, but he was an outstanding comparative anatomist. He wrote several texts, including De Partibus Animalium (Parts of animals), Historia Animalium (History of animals), and De Generatione Animalium (Generation of animals). These texts cover several of our core concepts—structure, evolution, and development, respectively (Section 1.3). Within these texts, Aristotle also makes the case for comparative anatomy as a means to understand the human body; this is one of the core concepts of comparative anatomy.

Golden Ages of Anatomy

Comparative anatomy became an incredibly useful discipline as human dissection became illegal or taboo in cultures around the world, including in Greece, Rome, and China. Dissection of other animals was not necessarily forbidden. Claudius Galenus (usually referred to as Galen, 129–216 CE), a Greek physician, performed no human dissections. Instead, his anatomical knowledge was based not only on the surgeries he performed and the occasional human remains from destroyed tombs but especially on the dissections of apes and pigs (Figure 1.5). Galen’s texts influenced anatomists around the world for centuries, despite the fact that his texts contained several errors based on the differences between humans and the other mammals he dissected. However, he described a number of anatomical features correctly, such as features of the vertebral column, thoracic skeleton, several of the cranial nerves, and brain structures. He also correctly determined that ovaries and testes are homologous structures (sharing a common ancestry), although he did not correctly describe some of the functions of the ovaries. Galen also provided recommendations for dissection in his texts, pairing particular animals with particular organ systems—apes for the brain, bovines or equines (horses, donkeys) for reproductive anatomy, and pigs for the larynx.

As Galen’s works were translated into other languages and distributed, scholars outside of Europe—especially in Egypt, Syria, and Persia—built upon his knowledge. For example, medicine flourished particularly between 800 and 1100 CE, and with that came anatomical study; this is the same period known as the Dark Ages in Europe. Ibn al-Nafis (1210–1288 CE; Figure 1.6) was the first to correctly determine the separation of blood in the right and left ventricles of the heart, to determine the path of pulmonary circulation, and to predict the presence of pulmonary capillaries. Pulmonary capillaries were discovered again in Europe by Marcello Malpighi 400 years later. Other scholars of this period who built upon and corrected Galen’s work include Abū Bakr al-Razi, Ali ibn al-Abbas al-Majusi, and Ḥasan Ibn al-Haytham. Some of their works were used by Western scholars in the Middle Ages; indeed, some of the anatomical terms we use today are based on Arabic terms (e.g., retina and sesamoid). During this same golden age, Persian physician Yuhanna ibn Masawaih (777–857 CE) dissected apes to better understand human anatomy and wrote several texts on anatomy and embryology.

Meanwhile, in the Western world, Galen’s works were considered the definitive texts on anatomy for 12 centuries. However, early Renaissance artists also were anatomists, participating in dissections and studying the human body in service of their art. Leonardo Da Vinci’s (1452–1519 CE) relationship with anatomy is perhaps best known; he was an observant anatomist who was also interested in form and function and produced incredibly detailed drawings of almost every human anatomical system.

Andreas Vesalius (1514–1564 CE) was the anatomist who started to shift Europe away from Galen. Based on his own writings, Vesalius initially was interested in medieval anatomical works. This led him to study anatomy through the dissection of local mammals such as rodents, cats, and dogs, followed by earning a medical degree from the University of Padua (Italy) and staying on as a professor. In 1543, Vesalius published De Humani Corporis Fabrica (On the Structure of the Human Body; see Figure 1.7); this tome is often marked as the beginning of “modern anatomy” and cites Galen hundreds of times. The book contained an array of beautiful illustrations by an unknown artist (some evidence points to Jan van Calcar) and refuted some of the errors in Galen’s work. Several anatomists followed in Vesalius’s footsteps studying anatomy and teaching at Padua, including Hieronymus Fabricius (1533–1619), who studied the anatomy of humans and other vertebrates and embryology. He went as far as to write a comparative anatomy treatise, Totius Animalis Fabrkiae Theatrum, but it was never published.

Anatomical study was also starting to flourish in other areas of Europe at this time. French anatomist Pierre Belon (1517–1564) was interested in not only humans but other vertebrates as well. He dissected and studied cetaceans, fishes, and 200 different species of bird. He published a number of works, including L’histoire naturelle des étranges poissons marins (The natural history of strange marine fish), La nature et diversité des poissons (The nature and diversity of fish), and Histoire des oiseaux (History of birds). His 1555 publication on the history of birds contained an illustration of a human skeleton and a bird skeleton, side by side, demonstrating the homology (shared evolutionary origin) of the skeletons (although he called them archetypes rather than homologous structures; Figure 1.8).

British anatomist and physician William Harvey (1578–1657) earned his doctoral degree from the University of Padua in 1602 and was thus familiar with Vesalius’s work. Through dissection, experiments, and observing his patients, Harvey determined how blood circulates around the body and the heart’s role in circulation. While Harvey is perhaps as famous as Galen and Vesalius in the history of medicine because of that work, Harvey was a comparative anatomist at heart. De Generatione Animalium summarized many of his embryological studies, and he bonded with King Charles I over his scientific studies. Harvey, however, did not figure out how veins and arteries connected with each other. Marcello Malpighi (1628–1689), one of the first Europeans to study microscopic anatomy, used dissections of frog, hedgehog, and dog lungs to observe how blood flowed through capillary networks, which was the piece missing in Harvey’s work. Neither the findings of Harvey nor those of Malpighi were accepted by their European colleagues during their lifetimes, as the works of Galen were still heavily influential. However, we would be remiss if we did not direct our readers back to Ibn al-Nafis and remind you that he figured it out 400 years before Harvey and Malpighi!

British physician Thomas Willis (1621–1675) is known as the “Father of Clinical Neuroscience,” as he was one of the first Europeans to accurately describe the nervous system, including its blood supply, cranial nerves, and the autonomic nervous system. His Cerebri Anatome (1664) focused on humans but contains some illustrations of sheep brains, a staple of the comparative anatomy teaching lab. Willis is also known for De Anima Brutorum (1672), a comparative anatomy text that he considered to be his most important work.

Nicholas Steno (1638–1686), also known as Niels Stensen, was a Danish physician and naturalist who made a number of observations of human anatomy that had not been recorded in the Western world. He described the duct of the parotid gland, how muscles contract, and ovary function. However, Steno is better known in comparative anatomy circles for his work on paleontology and geology, which would later help inform evolutionary theory. Steno, through his work on extant (not extinct) sharks, figured out that “glossopetra” (tongue stones; Figure 1.9) looked like shark teeth because they were indeed the teeth of sharks, although those species of sharks were no longer living (extinct). This and other observations eventually led him to the principles of original horizontality and superposition—sedimentary rock layers form horizontally, and higher layers are younger than lower layers, respectively. These principles then led to a better understanding of geologic time and the fossil record.

Cadavers and Crime

In Europe and elsewhere, cadavers in the late 1600s and 1700s were still scarce due to laws that limited their availability, which provided an obstacle to those wishing to learn anatomy. Anatomists continued to use animal dissections to further their knowledge as well as teach. In Scotland, Alexander Monro Primus (the first; 1697–1767) served as the chair of anatomy at the University of Edinburgh. Known as an outstanding teacher, he taught with mammal, bird, and fish dissections alongside the human cadavers. Mostly known for his teaching, Alexander Monro Secundus (1733–1817) followed in his dad’s footsteps as a professor and anatomist and published on the neuroanatomy of humans, the anatomy of fishes, and the lymphatic system. Just as we rely on models in the teaching laboratory today, artists Anna Morandi Manzolini (1716–1774, Italy), Marie Marguerite Bihéron (1719–1795, France), and many others made extremely detailed wax models of human anatomy (Figure 1.10). Anna Manzolini is particularly interesting—upon the death of her husband, University of Bologna anatomy professor Giovanni Manzolini, she became a celebrated anatomy professor herself while continuing to do finely detailed wax sculptures of various anatomical structures. Anna Manzolini even received commissions from Catherine the Great and the king of Poland.

To meet their needs, some physicians—and anatomists in training—turned to anatomical schools, which were often privately owned. One of the first anatomical schools in London, the Windmill Street School, was established by the Hunter brothers. William Hunter (1718–1783) was a surgeon, obstetrician, and anatomist; he also was a student of Alexander Monro Primus. His younger brother, John Hunter (1728–1793), was also a surgeon as well as a comparative anatomist. William began the Windmill Street School in 1746, which functioned as both a school and a museum. The Hunters together amassed a diverse collection of specimens for teaching, including books, fossils, pathological specimens, and a variety of animal specimens. William eventually published The Anatomy of the Human Gravid Uterus in 1774 and acknowledged John as a major contributor. However, John wrote a paper on the structure of the placenta in 1780 and claimed he was the one who really did the work on the placenta in the 1774 book. The two brothers had a falling out and never spoke to each other again. However, their scientific work continued. John was a true comparative anatomist. He dissected and described more than 500 species of animals, including invertebrates and vertebrates. John was particularly interested in the link between form and function. In addition to a 1782 publication on fish auditory (hearing) organs, he wrote essays that classified various anatomical structures by function and linked these across animals (anatomical series). While the Windmill Street School is no more, John may have left another legacy. It is said his house in Leicester Square may have been the inspiration for Dr. Jekyll’s home in Robert Louis Stevenson’s famous tale of Dr. Jekyll and Mr. Hyde (Figure 1.11).

The Parliamentary Act of 1752 enabled judges to send bodies of executed criminals from London to universities and even allowed for dissection to be a sentence that could be handed down. However, it left private anatomical schools in Scotland and England in need of a source of cadavers. Anatomical schools, including that of the Hunter brothers, instead had to obtain remains through other means and turned to resurrectionists (grave robbers; Figure 1.12). Scottish anatomist Robert Knox (1791–1862), a renowned anatomy teacher and scholar of comparative anatomy, is not remembered for his work. Instead, he is best known for falling in with resurrectionists Burke and Hare. Burke and Hare kidnapped widows, orphans, and prostitutes, murdered them via suffocation, and then sold their corpses to Knox’s anatomical school. When these and other similar murders were discovered, the public turned against anatomists. This led to the Anatomy Act of 1832, which recognized the need for human dissection to train physicians, made grave robbing illegal, and allowed unclaimed bodies in hospitals and workhouses to be given to schools for dissection. This Anatomy Act also allowed people to bequeath their remains for dissection and stated that dissected bodies must be buried.

Similar problems with resurrectionists were not limited to Britain. Thomas Sewell, who founded the George Washington University Medical School, was convicted of grave robbing for the purposes of dissection in 1818. Starting in 1831, states within the United States passed legislation similar to the Anatomy Act, as did Canada in 1843. As for Burke, his body was later dissected, and his skeleton and wallets made from his skin are in the collections of the Royal College of Surgeons of Edinburgh.

Unfortunately, human dissection for anatomical study continued to be unethically sourced through much of our recent history as well. Unethically acquired remains, particularly those of African, African American, and Indigenous peoples, are examples of colonialism and the slave trade contributing to natural history collections at universities and museums. Victims of Nazi concentration camps also were used for anatomical study and research. August Hirt (1898–1945) was the director of the Institute of Anatomy at the Reich University in Strasbourg. He had concentration camp prisoners murdered to build his collection of skeletons from Jewish people. In addition, the bodies of people executed for high treason could not be claimed. These remains were given to universities in Germany, Austria, and Czechoslovakia (now known as the Czech Republic) for dissection. Thus, anatomical work from this period and region stems from the Nazi regime and the bodies of Jewish, Polish, and Russian people, among others.

Adding in the Why Behind Comparative Anatomy: Evolution

As we have seen so far, anatomists were using other vertebrates to help understand human anatomy throughout the history of anatomical study. However, why this worked—why there are similarities in structure and function across wildly different vertebrates—was not realized until evolutionary theory became accepted.

Carolus Linnaeus (1707–1778) and Georges Louis Leclerc, Comte de Buffon (often just referred to as Buffon, 1707–1788), laid the initial groundwork. Linnaeus is best known for establishing our current system of classification for living things (taxonomy), but he also boldly placed humans in the order Primates along with apes and monkeys. This implied that not only were humans not outside of nature, but they were more closely related to primates than other animals—a very controversial statement at the time. Buffon added another piece in his 44-volume series Histoire Naturelle (1749–1804), where he wrote that living things change through time and hypothesized it was due to environmental factors or chance (Figure 1.13). He also quietly supported Linnaeus’s idea of humans and apes being closely related.

Jean-Baptiste, Chevalier de Lamarck (1744–1829), was even bolder than Linnaeus in his public support for biological change over time, which went against the widely accepted idea of divine creation and influence. Lamarck began his career as a botanist and later shifted to studying invertebrates. His focus on invertebrates led to his advocacy of a particular type of evolutionary change, the inheritance of acquired characteristics (also referred to as Lamarckian evolution due to his fierce support for this idea, although it was not his original idea). Lamarck believed that how organisms use their body parts directly causes that body to change for its purpose during the organism’s lifetime, and that new form would be inherited by offspring. His most cited example was concerning the long necks of giraffes, which he (incorrectly) believed evolved by the neck growing a little longer within each giraffe as they stretched to reach leaves, and those newly acquired neck lengths would be directly passed on to their offspring (Figure 1.14). Lamarck’s ideas were supported by some, such as Jean-Baptiste Bory de Saint-Vincent (1778–1846) and Geoffery Saint-Hilaire (1772–1844). Saint-Hilaire thought all animals shared one body plan, and the diversity in form we see is due to modifications of the common plan. This led to a big push to find homologies across organisms.

French naturalist Georges Cuvier (1769–1832) did not agree with Lamarck in terms of inheritance, nor did he agree with Saint-Hilaire’s idea of a single body plan. Cuvier was an advocate of catastrophism, where organisms living where natural disasters such as epic storms, floods, and mountain-building events occurred were rendered extinct by these disasters. While the idea of species going extinct held true, he did not think organisms changed through time. Instead, he believed that organisms were perfectly adapted to their environments and thus had no need to change. Although this idea was incorrect, Cuvier had another important role in the history of comparative anatomy. Not only was he a renowned vertebrate paleontologist, particularly of dinosaurs and large mammals, but he published a nine-volume series (Leçons d’anatomie comparée, 1801–1805) in which he wrote that an anatomist must follow two principles: (1) the forms of one part of an organism will depend on the forms of other parts because they must work together, and (2) some physiological systems are more important than others, which in turn should determine how to classify organisms (Figure 1.15). In Cuvier’s classification scheme, he separated vertebrates out from other animals (molluscs, radials, and jointed animals) based on their unique body plan.

Three more men helped push Charles Darwin and Alfred Wallace toward natural selection (Figure 1.16). Geologists James Hutton (1726–1797) and Charles Lyell (1797–1875) moved geology away from catastrophism and recognized that the same processes of erosion, changes in sea level, and the formation of new rocks also have happened through the Earth’s entire history and happen slowly. This became known as uniformitarianism and implied that the Earth was far older than what had been accepted until that time. In a completely different field, Thomas Malthus (1766–1834) published his Essay on the Principle of Population in 1798. Although his book dealt with political economics, in it he stated that human populations grow geometrically and, if left unchecked, would quickly outgrow the available food resources. Unfortunately, Malthus used this information to argue against improving life for those who were considered poor.

Charles Darwin (1809–1882) and Alfred Russel Wallace (1823–1913) independently arrived at the idea of natural selection through similar avenues. Darwin initially studied medicine at the University of Edinburgh but eventually left behind his medical studies to pursue natural history. Darwin was invited to sail on the HMS Beagle, a 10-gun ship belonging to the British Royal Navy that was used both for scientific expeditions and for taking control of the Falklands, from 1831 to 1836. While aboard, he worked with local people to collect fossils and living specimens, studied the geology of the places they visited, and meticulously kept a journal that would later be published. Darwin also read Lyell’s Principles of Geology during the voyage and then read Malthus’s essays upon his return. Darwin continued to quietly collect evidence and ruminate on the theory of natural selection over the next 20 years. Importantly, much of Darwin’s evidence from his voyages would not have been preserved if it hadn’t been for John Edmonstone, a formerly enslaved person from Guyana who taught Darwin taxidermy prior to his voyage on the Beagle.

Meanwhile, Alfred Russel Wallace started a journey of his own. Wallace initially worked as a land surveyor but was more interested in natural history. He embarked on several trips abroad, traveling through the Amazon and Southeast Asia from 1848 to 1862. Like Darwin, Wallace worked with local people to collect specimens, read the same books, and independently arrived at the conclusion that natural selection explained much of what he saw.

Wallace and Darwin were correspondents prior to the publication of Darwin’s most famous book. Wallace sent bird specimens to Darwin, but things took off in 1858 when Wallace described his ideas about natural selection to Darwin. Much to Darwin’s surprise, Wallace described a similar idea to what Darwin had been working on for the last two decades. There were details that the two naturalists did not agree upon. For example, they disagreed on what taxonomic level competition can happen at (Darwin said that competition happens within the same species; Wallace thought it was between species or at a higher taxonomic level). They disagreed on the importance of competition for mates (sexual selection); Darwin later wrote an entire book on it in 1871 stating its importance, while Wallace thought only “male-male” competition was important. They also disagreed on the role of artificial selection (Darwin thought it was a good analogy; Wallace thought it wasn’t). Finally, Darwin thought evolution applied to humans, but Wallace didn’t. Despite all these disagreements, Wallace and Darwin copresented the theory at a meeting of the Linnean Society in 1858. Darwin then published the book On the Origin of Species by Means of Natural Selection in 1859, plus three more from 1868 to 1872; thus he is the one often credited with having developed the theory of natural selection. There did not appear to be any hard feelings between the two naturalists. Wallace published a book titled Darwinism in 1889 that supported Origin of Species and natural selection and continued his other scientific work on zoogeography, which seeks to explain the geographic distribution of animal species.

While Darwin argued in Origin of Species that the branching version of evolution described in the text explained much of anatomy and paleontology, among other fields, many scientists did not immediately accept his ideas. The arguments against natural selection and Origin of Species were many. Some scientists held on to Lamarckian evolution, citing evidence from the fossil record and embryological development. Others did not see evidence of transitionary forms they would expect to see. Still others did not think that evolution could be linked to external forces, such as access to resources, and must be an act of the divine. British anatomist Thomas Henry Huxley (1825–1895) was one of the first to be convinced by Darwin and became such a fierce public advocate for Darwin’s ideas that he was nicknamed “Darwin’s Bulldog.” Huxley was also a fine comparative anatomist. He studied the embryology of vertebrates and determined that embryology was necessary to determine homology, and he was one of the first to realize that tunicates are in the same phylum as vertebrates (see Chapter 2). He also was particularly interested in the relationship between apes and humans, publishing “Note on the Resemblances and Differences in the Structure of and the Development in Man and Apes” as part of the second edition of Darwin’s The Descent of Man and Selection in Relation to Sex, and in 1863 he published Evidence on Man’s Place in Nature, which was one of the first books to link evolution to humans (Figure 1.17).

At this point, the hows behind evolution were also unknown. Darwin alluded to inheritance in Origin of Species but did not propose how traits are actually inherited. Gregor Mendel (1822–1884) laid out fundamental principles of inheritance in 1865 based on his many experiments with breeding and crossing common pea plants; however, it is thought that Darwin did not learn of them despite their overlap in time. Mendel’s work was not brought back into the scientific community until roughly 1900, when botanists Hugo De Vries (1848–1935), Carl Correns (1864–1933), and Erich von Tschermak-Seysenegg (1871–1962) rediscovered Mendel’s work while completing similar breeding experiments of their own. Similarly, naturalist William Bateson (1861–1926) found that Mendel’s work supported his own ideas about mutation as a source for evolutionary innovation. It took until the 1920s for scientists to understand that natural selection could act on genes, thanks to the work of Ronald Fisher, J. B. S. Haldane, and Sewall Wright, among others. Later, American biologist Thomas Hunt Morgan (1866–1945) bred thousands of fruit flies and studied mutation, refining the work that Bateson started. His student Thomas Dobzhansky (1900–1975) later linked evolution, speciation, and genetics in his 1937 book Genetics and the Origin of Species. He determined that populations of the same species of fruit flies did not have identical genes and that separate species of fruit flies were reproductively incompatible. The variability in genes due to mutation could lead to variability in phenotype and the eventual loss of interbreeding between populations (thus becoming a new species).

However, it wasn’t until 1944 that genes and inheritance would be linked to DNA (deoxyribonucleic acid), finally providing the how of evolution. DNA as a molecule was first described in 1869 by Friedrich Miescher, a Swiss chemist, who found it in human white blood cells. While work continued by others to describe the chemical properties and structure, Oswald Avery, Colin Macleod, and Maclyn McCarty of the Rockefeller Hospital in New York published a paper in 1944 that showed that genes and chromosomes were composed of DNA. Alfred Hershey and Martha Chase’s 1952 experiment further supported Avery and colleagues’ work.

Modern Comparative Vertebrate Anatomy

While we have just spent a very long time discussing an abridged history of comparative anatomy through the 1950s, the field has continued to grow and move forward. As technology continues to evolve, so does our knowledge. For example, as scientists gain access to modern imaging techniques such as computed tomography (CT) scanning, magnetic resonance imaging (MRI), and X-ray reconstruction of moving morphology (XROMM), we can now see details that even fine-scale dissection did not reveal. Similarly, advances in genetic and genomic techniques have led to important discoveries about the roles of regulatory genes in body patterning. As we move forward through this book, we will spend more time highlighting some of these incredible recent discoveries.

We are undoubtedly missing the contributions of many people to the field of comparative anatomy in this account. As we continue to study the history of any field, new artifacts emerge that add to or change the story we thought we knew. This history of science in general is starting to be examined through a more global lens, adding in the contributions of those who have been left out before. Understanding the honest history of this field is meant not to retroactively paint these scientists and physicians as bad people (and yes, some of them absolutely were) but to point to the issues inherent to our field and grapple with the implications of that history.

1.3 Moving Forward: Core Concepts of Comparative Vertebrate Anatomy

If you were to open just about any anatomy textbook (such as this one) or lab manual and look at the table of contents, you would see that most chapters in these kinds of books are arranged by system. There is a chapter on the digestive system, one or two on the skeletal system, one on the respiratory system, and so forth. We have followed a similar overall organizational scheme in this book. Within one of those chapters, you would probably see some generalized illustrations or a small number of representative vertebrates—a bony fish, a dogfish shark, a frog, some sort of reptile, a bird, and a cat. You would also see numerous figures with hundreds of structures labeled. This might give you the mistaken impression that anatomy is solely an exercise in memorization of names and locations of structures.

The field of comparative vertebrate anatomy is wide and rich. It covers vertebrates from the smallest fishes to the largest mammals (including humans!) and everything in between, whether it is extant or extinct. The field covers every anatomical system in these animals—its form, its function, its development, its evolution—and how these systems work together. The field also covers all levels of organization from cells to tissues, organs, body regions, and whole organisms. Given the scope of this field, we could easily write a textbook that is thousands of pages long, and we still wouldn’t be able to cover everything that is weird, wonderful, and interesting.

Therefore, most chapters will be structured around the five core concepts that unite the field of comparative vertebrate anatomy. Core concepts are larger themes that unite the field, regardless of which anatomical system you are studying. In other words, it doesn’t matter whether you’re looking at the digestive system or the nervous system—the same core concepts apply. These core concepts were developed by a group of scientists from the Society for Integrative and Comparative Biology who study and teach comparative vertebrate anatomy.

Below, we step through each of the five core concepts of comparative vertebrate anatomy. You may find them in a different order within the individual chapters, but we find the original order from Danos et al. (2022) to be useful for explaining each one.

1. Evolution: The diversity, variation, and unity of vertebrate anatomy are explained by descent with modification. Evolution is the unifying theory of biology and is applicable to every subfield. Comparative vertebrate anatomy is no exception. Natural selection acts on anatomical structures just like any other phenotype, so long as there is heritable variation in that structure. The versions of the phenotype that increase an animal’s evolutionary fitness (an organism’s ability to pass its genetic material on to the next generation) are more likely to be passed on to subsequent generations; these are adaptations. This is why we tend to look for connections between the form of the structure and its function. Something about the shape of the structure is making it work well. How the structure works is often correlated with ecology. Thus, as we learn about different groups of vertebrates and their anatomy, we can look for where natural selection favored variation on these heritable traits and why. This also means that vertebrates share a common ancestry, and we see descent with modification as we move through evolutionary time. Natural selection only acts on existing structures.
2. Structure and function: The structure of vertebrate anatomy is under heavy selection to match its functional demands. However, because demands change over time and evolution acts on what already exists, an anatomical component at any given time may not have an optimal structure for its current function. The first core concept indicates that we should expect to see a solid correlation between form and function, as this will increase evolutionary fitness. We see this link across many levels of biological organization. After all, all bodies must obey the laws of physics. A structure with a form that doesn’t work well may not survive to reproduce. However, we must also recognize that constraints on structures may prevent the highest level of fitness from being reached. Sometimes this is due to evolutionary history; the organisms may not have the genetic wherewithal to make it happen. Other times there are developmental or structural requirements that constrain evolution. For example, sometimes multiple structures interact and work together (integration, see Core Concept 4), so improving the function of one structure may make the integrated structure less fit. Some structures are used for multiple purposes that may be competing with each other, leading to trade-offs—a “jack-of-all-trades, master of none” type of situation. It is also important to note that there can be more than one solution to a functional demand.
3. Morphological development: Vertebrate anatomy is expressed as phenotypes that are the result of genotypes executed through a developmental program. Major shifts in phenotype can be achieved through modularity, which allows certain aspects of the phenotype to undergo major variations yet remain integrated in other ways. Development is not as simple as having a genotype that codes for a particular phenotype. The way those genes are expressed determines phenotype. Therefore, changes in that developmental pathway can have effects at the cellular level, tissue level, and structural level of a particular anatomical feature. Natural selection can then act on those modified structures. Changing a development pathway that affects many structures can have effects on all those structures. However, we also see structures, genes, and developmental pathways that are independent or modular. If a single developmental module is modified, major changes to phenotype can be brought about without affecting other parts of the body and can perhaps lead to that structure being able to work with a structure that it couldn’t before.
4. Integration: Anatomical structures develop, function, and evolve as integrated modules. These processes occur across space, time, and biological levels of organization. Even if it has modularity in development, the animal must still function as a whole. For example, skeletal muscles and bones may develop via their own pathways, but those muscles must attach to the bones to move them. Muscle cells and those of the nervous system must work together for the muscle to contract and move the bones. Movement of bones happens at joints, where two bones interact. The development of these three components (bone, muscle, nerve) must come together to build a functional organism, as does the biochemistry, physiology, and mechanics of these structures. The study of anatomy is more than just pointing to structures; it requires the integration of fields like biochemistry, physiology, mechanics, ecology, and evolution because the structures themselves are integrated.
5. Human anatomy is a result of vertebrate evolution: As humans are vertebrate animals, their form has been constrained by phylogenetic ancestry. To the general public, vertebrate evolution is often thought of as a linear progression from fish to mammals, with humans as the pinnacle. After all, humans are unique in the ways that they have modified their environment and their possession of self-awareness. However, the tree of life has many branches; evolution is not a linear process, and there is no pinnacle. Humans are vertebrates. As such, we share a common ancestor with all other vertebrates. We have features that are the same as other vertebrates, and understanding our shared evolutionary history explains why our bodies work the way they do as well as certain pathologies. This also means that the other four core concepts explained above also apply to humans. We will treat humans as a special case of comparative vertebrate anatomy because of the interest we have in how our own bodies work, not because they are separate from other vertebrates.

1.4 Summary

If you are feeling overwhelmed by the breadth of the field of comparative anatomy and the long history of the field, you are not alone! This is an interdisciplinary field of biology that incorporates concepts not only from other areas of biology but also from geology and physics. It is easy to get intimidated by everything that is encompassed by this field, and the authors of this book remember what it’s like to get started in this course. We are going to lean heavily on those core concepts to make sure that you have the tools to explore any areas that interest you without getting lost in the particulars. The authors of this book are enthusiastic educators and scientists who love comparative vertebrate anatomy. We hope that you find it as fun and interesting as we do, and we will point out some of our favorite things as we take this journey with you.

1.5 Further Reading

1. Brenna, Connor T. A. “Bygone theatres of events: A history of human anatomy and dissection.” Anatomical Record 305 (2022): 788–802.
2. Dial, Kenneth P., Neil Shubin, and Elizabeth L. Brainerd, Eds. Great Transformations in Vertebrate Evolution. Chicago: University of Chicago Press, 2015.
3. Graves, Joseph L., Jr. “African Americans in evolutionary science: Where we have been, and what’s next.” Evolution: Education and Outreach 12 (2019): 1–10.
4. Persaud, T. V. N., Marios Loukas, and R. Shane Tubbs. A History of Human Anatomy. 2nd ed. Springfield: Thomas Books, 2014.
5. Roach, Mary. Stiff: The Curious Lives of Cadavers. London: W. W. Norton & Company, 2003.
6. Shubin, Neil. Your Inner Fish: A Journey into the 3.5-Billion-Year History of the Human Body. New York: Vintage Books, 2008.

1.6 References

1. Alghamdi, Malak A. Janine M. Ziermann, and Rui Diogo. “An untold story: The important contributions of Muslim scholars for the understanding of human anatomy.” Anatomical Record 300: 986–1008.
2. Bay, Noel Si-Yang, and Boon-Huat Bay. “Greek anatomist Herophilus: The father of anatomy.” Anatomy and Cell Biology 43 (2010): 280–283.
3. Berry, Andrew, and Janet Browne. “Mendel and Darwin.” Proceedings of the National Academy of Sciences U.S.A. 119 (2022): e2122144119.
4. Brenna, Connor T. A. “Bygone theatres of events: A history of human anatomy and dissection.” Anatomical Record 305 (2022): 788–802.
5. Danos, Nicole, Katie Lynn Staab, and Lisa B. Whitenack. “The core concepts, competencies, and grand challenges of comparative vertebrate anatomy and morphology.” Integrative Organismal Biology 4 (2022): obac019.
6. Debernardi, Alberto, Elena Sala, Giuseppe D’Aliberti, Guiseppe Talamonti, Antonia Francesca Franchini, and Massimo Collice. “Alcmaeon of Croton.” Neurosurgery 66 (2010): 247–252.
7. Duckworth, Wynfrid Laurence Henry. Galen on Anatomical Procedures. Cambridge: Cambridge University Press, 1962.
8. Hunter, John. “Account of the organ of hearing in fish.” Philosophical Transactions of the Royal Society of London 72 (1782): 379–383.
9. Hunter, John. Essays and Observations on Natural History, Anatomy, Physiology, Psychology, and Geology. Edited by Richard Owen. 2 vols. London: John Van Voorst, 1861.
10. Loukas, Marios, Alexis Lanteri, Julie Ferrauiola, R. Shane Tubbs, Goppi Maharaja, Mohammadali Mohajel Shoja, Abhishek Yadav, and Vishnu Chellapilla Rao. “Anatomy in ancient India: A focus on the Susruta Samhita.” Journal of Anatomy 217 (2010): 646–650.
11. McNish, James. “John Edmonstone: The man who taught Darwin taxidermy.” Natural History Museum. Accessed January 28, 2025. https://www.nhm.ac.uk/discover/john-edmonstone-the-man-who-taught-darwin-taxidermy.html.
12. Moore, Wendy. “John Hunter left a body of work behind him.” Scottish Field. October 4, 2019. https://www.scottishfield.co.uk/culture/john-hunter-left-a-body-of-work-behind-him/.
13. Moyle, Peter B., and Joseph J. Cech Jr. Fishes: An Introduction to Ichthyology. New Jersey: Prentice-Hall, 1982.
14. Persaud, T. V. N., Marios Loukas, and R. Shane Tubbs. A History of Human Anatomy. 2nd ed. Springfield: Thomas Books, 2014.
15. Shoja, Mohammadali M., and R. Shane Tubbs. “The history of anatomy in Persia.” Journal of Anatomy 210: 359–378.
16. Stenseth, Nils Chr., Lief Andersson, and Hopi E. Hoekstra. “Gregor Johann Mendel and the development of modern evolutionary biology.” Proceedings of the National Academy of Sciences U.S.A. 119 (2022): e2201327119.
17. University of California, Berkeley. “The History of Evolutionary Thought.” Accessed January 22, 2024. https://evolution.berkeley.edu/the-history-of-evolutionary-thought/.
18. University of Michigan. “John Hunter.” Accessed January 23, 2024. https://websites.umich.edu/~ece/student_projects/anatomy/people_pages/hunterjohn.html#:~:text=Thus%2C%20Hunter’s%20work%20in%20comparative,late%2018th%20and%2019th%20centuries.
19. West, John B. “Ibn al-Nafis, the pulmonary circulation, and the Islamic Golden Age.” Journal of Applied Physiology 105 (2008): 1877–1880.