Comparative Anatomy: Andreas Vesalius

Home , Andreas Vesalius, Systema Naturae

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At the dawn of the sixteenth century, European scholars could gain only a crude understanding of the anatomy of humans and animals. At the handful of universities where students trained in medicine—such as Bologna or Paris—professors read from the books of the Roman physician Galen. Galen had combined the philosophical work of Aristotle and other Greeks with his own lifetime of dissections, creating a system that explained not just the structure of the human body, but how the body worked.

After the fall of Rome, Galen’s legacy lived on in Arab cities like Baghdad, where his work was translated, pored over, and encrusted with interpretations and commentaries. In the 1100s, Europeans began to translate Galen from Arabic and made his work the basis of medical training. But in the many steps of translation, much of the spirit of Galen’s work—especially his emphasis on observing for oneself rather than relying on authority—was lost. A tradition had emerged in which professors read Galen to their students, while a surgeon dissected an executed criminal to show the relevant parts of the body. There was no point in the professor looking for himself at the body, since everything worth learning could be found in Galen’s books.

Observing the human body A young Flemish anatomist changed all that when he realized that Galen was dramatically wrong. Andreas Vesalius (1514-1564) started out his career as a defender of “Galenism” at the University of Paris. But when he moved to the University of Padua, he began dissecting corpses for himself to show his students the fine details of anatomy. He drew charts for the students to study, and the exquisite quality of the charts made Vesalius famous—so famous that the criminal court judge of Padua made sure he had a steady supply of cadavers from the gallows.

As he grew more familiar with the human body, Vesalius began to notice that here and there, Galen had made mistakes. The human breastbone is made of three segments; Galen said seven. Galen claimed that the humerus (the upper arm bone) was the longest bone in the body, save only the femur; Vesalius saw that the tibia and fibula of the shin pushed the humerus to fourth. Over the centuries, anatomists sometimes had minor quibbles with Galen, but Vesalius began to suspect that there was something seriously wrong with his work. Vesalius widened his scope, dissecting animals, and reading over his Galen more carefully. The source of the mistake dawned on him. Galen had never dissected a human. The traditions of Rome did not allow such a practice, and so Galen had had to make do with dissecting animals and examining his patients during surgery. Instead of humans, Galen was often writing about oxen or Barbary macaques.

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Vesalius dissects a female Vesalius found that the cadaver in his anatomy lab. human breastbone has three segments, not seven as Galen claimed.

Challenging Galenism At age 25, Vesalius launched a full assault on Galen. Lecturing at Padua and then at Bologna, he rigged up skeletons of humans and of Barbary macaques, and showed the assembled students how wrong Galen had been. Vesalius then set out to put together a new anatomy book that included his discoveries. Over the next four years Vesalius worked with the finest block cutters of Venice and draftsmen from Titian’s workshop. He named his book De humani corporis fabrica libri septem, or “The Seven Books on the Structure of the Human Body”—commonly known as the Fabrica. In this 1543 masterwork, men and women now stood stripped of skin (right). Skeletons (left) leaned lazily against columns in the rolling Italian countryside.

Humans are not so unique Fabrica launched a new tradition in anatomy in Europe, in which anatomists trusted only their own observations and explored the body like a new continent. Vesalius’ discovery of the important differences between species also helped usher in the science of comparative anatomy, in which researchers studied animals to find their similarities and differences. In the process, they gradually began to recognize humans as being one species among many, with a few unique traits but many others shared in common with other animals. Some 300 years after Vesalius first Images from the Fabrica (click to see shook off the blind obedience to Galen, Darwin used that vast stock of anatomical knowledge to build his theory of larger versions) evolution.

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Vesalius and dissection images courtesy of Historical Collections and Services, The Claude Moore Health Sciences Library, University of Virginia; Fabrica images property of the Regents of the University of Michigan

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Observation and Natural Theology: William Harvey & William Paley

In the 1600s the study of life changed forever. After relying on the authority of ancient writers like Aristotle and Galen for centuries, European naturalists began to look at life for themselves. Anatomists discovered new organs in the human body, and also discovered that familiar organs didn’t work the way Aristotle and Galen said they did. The English physician William Harvey (above left), for example, discovered in the early 1600s that blood was pumped from the heart through the body in a closed loop. Meanwhile, Harvey and others were examining animals and plants and making equally astonishing discoveries. The English inventor, Robert Hooke, for example, looked through a microscope at a previously unimaginable complexity hidden in tiny animals as humble as a flea.

Envisioning organisms as machines This new generation of naturalists envisioned life as machines. Like human-made machines, an animal had many Harvey showed how blood, pumped by the heart, circulated through vessels in different parts—muscles, eyes, bones, organs, and so on—that all played vital functions to help keep the animal the arm. alive. Naturalists found that they could apply the same scientific methods in physics that they used to invent machines, to life itself.

Natural theology and God’s design Some clergymen worried that this mechanistic approach of life smacked of atheism. But many of the naturalists themselves believed that they actually were on a religious mission. In fact, a number of them were both naturalists and theologians. They believed that God had created the entire world in such a way that his plan could be understood in part by rational creatures. By studying the intricate structures of a hand or a feather, a naturalist could appreciate God’s benevolent design.

Natural theology, as it became known, dominated English thinking for nearly two centuries. In the early 1800s, it was best known to Englishmen through the writings of Reverend William Paley (left). Natural theology was important scientifically because it guided researchers to the fundamental question of how life works. Even today, when scientists discover a new kind of organ or protein, they try to figure out its function. But it would be Charles Darwin, who actually occupied Paley’s rooms at Cambridge University and was an admirer of Paley’s work, who would take science beyond natural theology and move those questions from the religious sphere to the scientific.

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Harvey and blood circulation images courtesy of the National Library of Medicine; Paley image courtesy of The Book Page

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Fossils and the Birth of Paleontology: Nicholas Steno

If one day in history had to be picked as the birth of paleontology, it might be the day in 1666 when two fishermen caught a giant shark off the coast of Livorno in Italy. The local duke ordered that this curiosity be sent to Niels Stensen (better known as Steno), a Danish anatomist working at the time in Florence. As Steno dissected the shark, he was struck by how much the shark teeth resembled “tongue stones,” triangular pieces of rock that had been known since ancient times.

Today, most people would instantly wonder whether the tongue stones were giant petrified shark teeth, but in 1666 such a presumption was a tremendous leap. Few could imagine how living matter could be turned to stone, and beyond that, encased in solid rock—especially if the rock were well above sea level and contained remnants of a marine organism. Fossils were instead thought to have fallen from the sky, or to be “sports of nature”—peculiar geometrical shapes impressed on the rocks themselves.

From living tissue to stone Steno made the leap and declared that the tongue stones indeed came from the mouths of once-living sharks. He showed how precisely similar the stones and the teeth were. But he still had to account for how they could have turned to stone and become lodged in rock. Naturalists of Steno’s day were becoming convinced that matter was composed of different combinations of tiny “corpuscles”—what today we would call molecules. Steno argued that the corpuscles in the teeth were replaced bit by bit, by corpuscles of minerals. In this gradual process, the teeth didn’t lose their overall shape as they turned from tissue to stone.

Steno’s drawing of a shark head helped him see that “tongue stones” were actually fossil shark teeth (right).

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Steno’s Law of Superposition But how could fossils end up deep inside rocks? Steno studied the cliffs and hills of Italy to find the answer. He proposed that all rocks and minerals were originally fluid. Floating on the surface of the planet long ago, they gradually settled out of the ocean and created horizontal layers, with new layers forming on top of older ones. Molten rock sometimes intruded into the layers, reaching the top and spreading out into a new layer of its own. As the rocks formed, they could trap animal remains, converting them into fossils and preserving them deep within their layers. Those horizontal layers represent a time sequence with the oldest layers on the bottom and the youngest on top, unless later processes disturbed this arrangement. This ordering is now referred to as Steno’s Law of Superposition, his most famous contribution to geology.

Steno was not the only naturalist of his day to propose that fossils belonged to living creatures. Leonardo da Vinci and Robert Hooke, for example, also took up the same view. But Steno pushed the idea much further. He argued for the first time that fossils were snapshots of life at different moments in Earth’s history and that rock layers formed slowly over time. It was these two facts that served as the pillars of paleontology and geology in future centuries. And fossils ultimately became some of the key evidence for how life evolved on Earth over the past four billion years.

These exposed rock layers nicely illustrate Steno’s Law, with the youngest layers at the top and the oldest at the bottom.

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Rock layers image courtesy of David Smith, UCMP.

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Nested Hierarchies, the Order of Nature: Carolus Linnaeus

Homo sapiens, Tyrannosaurus rex, Escherichia coli—our English conversation is littered with pairs of Latin names for animals, plants, and microbes. How did a dead language find this renewed life? It is the 250-year-old legacy of a Swedish naturalist’s quest to discover God’s handiwork in nature.

Carolus Linnaeus (1707-1798) was far from the first thinker to try to classify life. Aristotle, for example, argued that each species had a unique form and could be classified by some of its key characteristics. In the process, he organized life in a ladder-like hierarchy, with plants on the bottom, animals in the middle, and humans on top (figure, right). Medieval European scholars were guided by both Aristotle and the Bible, and they believed that nature—including all of the species on Earth—reflected God’s benevolent organization of the world.

Searching for a system of classification With the advent of the Renaissance, naturalists tried to understand this divine plan by searching for a rational pattern in the bewildering array of species. They grouped species with an overall similarity with one another in a larger group called a genus. Lions, tigers, and leopards, for example, all belonged to a “big cat” genus.

But did big cats and other animals fit into a larger scheme? This was difficult to know for many reasons. One problem was that European explorers and colonists began encountering many previously unknown species in the New World, Africa, and Asia. On top of that was the problem of method. Some argued that naturalists looking for a system of classification should try to take into account as many characteristics of a species as possible. That would ensure that their classification system was truly natural. Others Linnaeus’ system argued that we do not find systems in nature but construct them in our minds. Therefore naturalists should invent artificial systems diverged from Aristotle’s vision of based on a few convenient traits of their own choosing, such as the shape of a plant’s reproductive organs. a Great Chain of Being, above. Organizing life into nested hierarchies Carolus Linnaeus joined the quest for classification after having trained as a physician at the University of Uppsala. Botany was part of every medical student’s preparation, since most medicines were derived from plants. After making botanical expeditions through Lapland and central Sweden, Linnaeus became convinced that he could organize all of life into a single artificial system, one that would be his first step towards comprehending God’s design in nature.

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In 1735 he published the first edition of his landmark work, Systema Naturae. In it, he identified every species he knew of according to a standard nomenclature, a genus name followed by a species name. Before Linnaeus, naturalists used unwieldy, irregular names that sowed confusion. But he went further. He classified genera together in groups he called families, which he then placed in larger groups called orders, and then kingdoms, like boxes within boxes.

Humans as primates Linnaeus’ classification was important in many ways, not the least of which was how he classified humans. He named humans Homo sapiens, and placed us in the genus Homo. He also placed orangutans and chimpanzees, the two apes known at the time, in the genus Homo. And he placed Homo in a family, which he dubbed Primates. Primates also included two other genera, simians and lemurs. Although Linnaeus believed that humans were special beings in God’s creation, he slotted our species into his system as if it were any other. Title page of Linnaeus’ Systema Linnaeus organized life with an almost geometrical precision, and was so impressed by his own Naturae. system that he used it to organize rocks and other non-living matter. Although his classification of minerals may now be long forgotten, within the biological world, at any rate, Linnaeus’ system proved to be useful. It was clear and straightforward, making the challenge of classifying new species far easier than previous systems. It became the standard way to organize life’s diversity.

Biologists still use Linnaeus’ conventions today when they name a new species. But Darwin rendered the ideas behind those conventions obsolete. Darwin recognized that evolution could produce the hierarchy of similarities that so impressed Linnaeus, as old species gave rise to new species. Biologists still place pigs, porcupines, and people in Mammalia, but they do so because all the evidence—comparisons of fossils, anatomy, and genes— confirms that they descend from a common ancestor.

The human species in a modern Linnaean system of classification.

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Linnaeus image courtesy of the Swedish Museum of Natural History; Systema Naturae image courtesy of Systema Naturae.

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Old Earth, Ancient Life: Georges-Louis Leclerc, Comte de Buffon

No single naturalist of the 1700s epitomizes the revolutionary changes that the Enlightenment brought to the study of nature more than Georges-Louis Leclerc, Comte de Buffon (1707-1788). In the 1600s most naturalists believed the world was a few thousand years old and that species were created separately and organized into an unchanging hierarchy, with humans positioned just below the angels. In the 1800s, Darwin described a world that was inconceivably old, one in which life gradually changed from one form to another without any need for direct supernatural intervention. Roughly midway between those two views—both chronologically and intellectually—was the remarkable Georges-Louis Leclerc Buffon.

Buffon’s career centered on a single enormous project: an encyclopedia he called Histoire Naturelle, which he planned to contain everything known in his day about the natural world. (Buffon only managed to publish 36 out of his projected 50 volumes before he died.) To create it, he was able to draw on his own astonishing expertise, which ranged from astronomy to botany, as well as the knowledge of experts he consulted. But in writing his encyclopedia he did not merely parrot the opinions of others. Instead, he tried to explain all of the facts he amassed with overarching theories about the planet and its inhabitants.

A non-Biblical explanation of Earth’s history Buffon realized that to interpret the world, he had to understand its history. And despite censures from the Church, he did not rely on the Bible as a strict guide to that history. Instead, he used the new physics of Isaac Newton to conjecture how matter in motion might have formed the Earth. He proposed that a comet striking the sun had broken off debris that became the planets of the solar system. Initially, the Earth was scorching, but gradually it cooled until molten rock turned to dry land and clouds rained down to form oceans. Buffon estimated the entire process took over 70,000 years. To most Europeans of Buffon’s time, who considered the Earth to be fewer than 7,000 years old, this was practically an eternity.

Buffon proposed that the debris flung out from a comet’s collision with the sun became the planets.

Spontaneous origins of life

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Buffon argued that life, just like Earth, had a history. Like many other Enlightenment thinkers, he thought that it could be generated spontaneously under the right conditions. In the hot oceans of the early Earth, Buffon claimed that vast amounts of life were generated from unorganized matter—even large animals sprang into existence. In time, as the world’s climate cooled, many animals migrated to the tropics. Their migration made sense of the discoveries in Buffon’s day of fossil elephants in Siberia and North America, while living elephants were only found in Africa and South Asia (see figure, right). The Siberian species gave rise to today’s elephants, while the North American forms simply became extinct.

Change through migration According to Buffon, life originated already divided into a number of distinct types—an “internal mould” that organized the organic particles that made up any individual creature. But during migrations, life changed. As a species moved to new habitats, the supply of organic particles that could create new individuals changed, and the particles could thereby change a species’ mould. Buffon was, in other words, proposing a sort of proto- evolution. While he thought that this process couldn’t produce radically new kinds of body plans, he did claim that it could account for the geographical distribution of similar species around the world. Buffon believed that modern Indian and African elephants were migratory Buffon’s theories were visionary yet doomed, because they were based on the relatively skimpy evidence that descendants of Siberian mammoths. eighteenth-century naturalists had at their disposal. His estimate of the Earth’s age turned out to be far too young, and his notions of biological change were not based on a coherent mechanism. Yet his theories foreshadowed some of the most important developments in the natural sciences in the decades that followed his death—from Cuvier’s discoveries about extinctions, to the evidence that Lyell and other geologists found for a vast age of the planet and life itself, to Darwin’s own theory of evolution. It may be true that no single idea of Buffon’s has withstood the test of time. But his work was still a milestone of science because he thought about the Earth and life in ways that few had before—both life and the Earth had a history.

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Sun/comet image adapted from images at NASA.

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The Ecology of Human Populations: Thomas Malthus

Thomas Malthus (1766-1834) has a hallowed place in the history of biology, despite the fact that he and his contemporaries thought of him not as a biologist but as a political economist. Malthus grew up during a time of revolutions and new philosophies about human nature. He chose a conservative path, taking holy orders in 1797, and began to write essays attacking the notion that humans and society could be improved without limits.

Population growth vs. the food supply Malthus’ most famous work, which he published in 1798, was An Essay on the Principle of Population as it affects the Future Improvement of Society. In it, Malthus raised doubts about whether a nation could ever reach a point where laws would no longer be required, and in which everyone lived prosperously and harmoniously. There was, he argued, a built-in agony to human existence, in that the growth of a population will always outrun its ability to feed itself. If every couple raised four children, the population could easily double in twenty-five years, and from then on, it would keep doubling. It would rise not arithmetically—by factors of three, four, five, and so on—but geometrically—by factors of four, eight, and sixteen.

If a country’s population did explode this way, Malthus warned that there was no hope that the world’s food supply could keep up. Clearing new land for farming or improving the yields of crops might produce a bigger harvest, but it could only increase arithmetically, not geometrically. Unchecked population growth inevitably brought famine and misery. The only reason that humanity wasn’t already in perpetual famine was because its growth was continually checked by forces such as plagues, infanticide, and simply putting off marriage until middle age. Malthus argued that population growth doomed any efforts to improve the lot of the poor. Extra money would allow the poor to have more children, only hastening the nation’s appointment with famine.

A new view of humans Malthus made his groundbreaking economic arguments by treating human beings in a groundbreaking way. Rather than focusing on the individual, he looked at humans as groups of individuals, all of Between 1800 and 2000 the human population increased about six-fold. Has the food supply kept whom were subject to the same basic laws of behavior. He used the same principles that an ecologist pace? Will there be enough food to support the would use studying a population of animals or plants. And indeed, Malthus pointed out that the same projected population of 9.2 billion in 2050? forces of fertility and starvation that shaped the human race were also at work on animals and plants. If flies went unchecked in their maggot-making, the world would soon be knee-deep in them. Most flies (and most members of any species you choose) must die without having any offspring. And thus when Darwin adapted Malthus’ ideas to his theory of evolution, it was clear to him that humans must evolve like any other animal.

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