Nicolaus Steno  
  Portrait of Nicolaus Steno

In the mid-17th century, an anxious young man wrote in his journal, “I pray, thee, O God, take this plague from me and free my soul of all distraction, to work on one thing alone, and to make myself familiar with the tables of medicine alone.” Luckily for generations of biologists, geologists, and paleontologists, the prayer went unanswered, and the young man continued chasing his wandering interests.

The author of the prayer was Niels Stensen, also known by his latinized name of Nicolaus Steno. After making unprecedented discoveries in anatomy, he changed course and established some of the most important principles of modern geology.

Steno lived at a time when people believed that fossils grew inside rocks, lowly animals emerged spontaneously from decaying matter, motions of stars decided fortunes and personalities, witches skulked everywhere, and crystals and unicorn horns cured disease. It was a time of intense religious strife, when one of the few things Catholics and Protestants could agree on was the date of 4004 B.C. for the creation of the world, as determined by the Anglican archbishop James Ussher. Like most savants of his time, Steno was deeply religious. He never publicly disputed this estimate of Earth’s age, and he probably didn’t privately dispute it, either. Yet by the end of the 18th century, practitioners of the budding fields of geology and paleontology realized that Earth had to be much older, and they first glimpsed our planet’s immense geologic age and rich prehistory thanks to Steno’s work.

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Steno lived at a time when people believed that fossils grew inside rocks, witches skulked everywhere, and crystals and unicorn horns cured disease.

Portrait by Roger Kammerer

  Nicolaus Steno

Early Years

Steno was born in the Lutheran stronghold of Copenhagen during the Thirty Years War, a conflict between Catholics and Protestants that eventually engulfed most of northern Europe. While a toddler, he contracted an illness that confined him indoors for three years, and shortly after he recovered, his father died. Steno’s mother soon remarried, but her new husband died just a year later, and the boy went to live with an older half-sister and her husband. When Steno was 15, an age when most kids value their friends the most, he lost many of his; plague struck Copenhagen, and his buddies were recruited to cart away the contagious bodies of the dead. This kind of uncertainty characterized Steno’s entire life. He never owned a home, seldom lived in one place for long, rarely enjoyed a steady income, and agonized over the fate of his soul.

Between bouts of despair over his inability to focus on one topic, Steno managed to impress his elders. Spending so much time surrounded by adults during his childhood illness had made him a good listener, and after his father was gone, Steno was happy to listen to surrogate fathers. The first of these was Ole Borch, an alchemist. Steno didn’t care much for alchemy, but he remembered what he saw in Borch’s workshop, especially solid particles suspended in liquid. Memories of these particles of sediment gradually settling to the bottom of the mixture would help him envision geologic processes in later years.

Steno hoped to study mathematics, but a career as a doctor looked far more practical. He enrolled at the University of Copenhagen to study medicine, yet found himself outdoors sketching snowflakes instead. Even worse, his formal studies stopped altogether when the university closed its doors. Denmark was at war with Sweden, and between military defeats by the Swedes and two punishing winters in a row, the entire city ground to a halt. Steno made the best of a bad situation. Thomas Bartholin, Denmark’s leading anatomist, had retired from the University of Copenhagen about the time Steno enrolled, but the men became friends anyway. Steno might have learned good dissection techniques from Bartholin, or simply inherited exceptional manual dexterity from his goldsmith father. Either way, several years after scrounging for firewood in frozen Copenhagen, he found himself in Paris, dazzling Europe’s glitterati with his skill.

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Between bouts of despair over his inability to focus on one topic, Steno managed to impress his elders.

  Nicolaus Steno

The Head of a Shark

“He made us see everything there is to see in the construction of the eye,” a French physician wrote, “without putting the eye, the scissors, or his one other small instrument anywhere but in his one hand, which he kept constantly exposed to the gathered company.” The eye in question was that of a horse. The young man, of course, was Steno.

With no suitable position available in Denmark, Steno had traveled through Germany for several months, then moved to the bustling multicultural city of Amsterdam, where he stayed with a physician named Gerard Blaes. While visiting Blaes, Steno casually undertook the dissection of a sheep’s head, and accidentally discovered the parotid gland, which produces saliva. Blaes initially dismissed his guest’s find, but after the young man attracted the attention of others, Blaes claimed the discovery as his own. The dispute turned ugly, and soured Steno’s interest in taking credit for his discoveries, though that didn’t keep him from making new ones. In 1662, he published “Anatomical Observations on Glands” describing all the glands in the head. In Paris a few years later, he delved into studying muscles, realizing that they worked through contractions of muscle fibers, not—as was commonly believed—through ballooning. He went on to make key finds about the anatomy of the brain.

He moved on to Florence, and quickly found life under the Tuscan sun smelly. Francesco Redi, the man Steno hoped to meet, was busy conducting experiments in spontaneous generation. It was common knowledge that decay gave rise to new life, namely flies and worms. The influential German Jesuit Athanasius Kircher was so convinced of this that he even published recipes. Redi had his doubts, and to prove them, he assembled rotting meat, animal carcasses, and ox dung. He found that when he carefully covered the decaying lumps, no lowly new animals appeared. Steno felt strangely at home in this environment, and the powerful Medici brothers, the grand duke Ferdinando and his brother Leopoldo, soon took an interest in him. When they acquired the head of a great white shark in October 1666, they delivered it to Steno for dissection. Steno found that the shark’s upper and lower jaws each held 13 rows of teeth, and the teeth resembled glossopetrae, or tongue stones.

Illustration of shark head with teeth

Steno published his description of shark teeth shortly after dissecting a shark head. (Image from The Seashell on the Mountaintop by Alan Cutler)

Tongue stones turned up everywhere, yet no one knew just how they originated. Some people thought they fell from the sky, maybe on moonless nights when no one could actually see them falling. Many people associated them with a story about Saint Paul, that while he was shipwrecked on Malta, he had either turned serpents to stone or at least deprived them of their venom. Yet tongue stones looked like neither the tongues nor the fangs of snakes. What they did look like, Steno found, were shark teeth.

Steno wasn’t the first to correctly identify shark teeth, and he didn’t even state with certainty what they were, but his published illustrations left little doubt. Sharks regularly shed teeth and grow new ones throughout their lives, so their teeth are not difficult to find. Extinct sharks have left their teeth inside ancient rocks just as modern sharks have strewn teeth across modern beaches. Steno had found the origin of a common vertebrate fossil, but that didn’t mean all such mysteries were solved.

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Steno wasn't the first to correctly identify shark teeth, and he didn't even state with certainty what they were, but his published illustrations left little doubt.

  Nicolaus Steno

The Nature of Fossils

By the time Steno dissected the shark’s head, scholars had long debated the origins of fossils, and the debate was complicated by how 17th-century Europeans actually defined the word “fossil.” Today a fossil is understood to be evidence of ancient life preserved in rock, so its organic origin can be taken for granted. In Steno’s time, fossils could be anything dug up from the ground, including crystals or human artifacts; clearly some of these objects couldn’t be organic. Among those that were, some bore a recognizable resemblance to living organisms, such as mollusks that looked like their modern relatives. Others were harder to identify because they were poorly preserved, belonged to organisms no longer living in the region, or belonged to organisms that had become extinct. Few Renaissance or Enlightenment savants had even considered the possibility of extinction. Instead, most focused on two explanations they found more plausible: fossils had been deposited by Noah’s Flood, or fossils grew inside rocks.

If one explained fossils through the Biblical Flood, shell fossils found near a shoreline didn’t stretch the limits of credulity, but fossils of marine mollusks found on mountaintops did. A flood might have deposited freshwater mollusks, but not marine mollusks. More puzzling still, how could the Earth produce, and later dispose of, enough water to cover all the mountain peaks on the globe? The natural historians of Steno’s day hoped to better understand the mind of God. This meant discerning the natural laws by which God worked, not simply assuming a miracle, but they couldn’t find a satisfactory answer. Perhaps most puzzling of all, many fossils did something no short-lived flood could make them do: they resided deep inside mountains of rock.

Illustration of shell and crystal

In his 1598 treatise on German fossils, Jean Bauhin grouped shells and crystals together based on their overall shape. (Image from I Have Landed by Stephen Jay Gould)

Given these circumstances, growth of fossils inside rocks made more sense to some people, but not to Steno. He observed that the tree roots were “twisted and compressed in countless ways in harder ground, so that they assume shapes different from those in softer ground.” If tree roots did this, fossils should too, but they didn’t. Likewise, shells growing inside rocks should crack the rocks, but they didn’t. Finally, if fossils had grown inside rocks in the past, they should continue growing in rocks in Steno’s time, but he could find no evidence of this. Even Steno’s religious convictions made him reluctant to accept this explanation. Fossils growing inside rocks had no purpose, and Steno was convinced that earthly objects were designed for specific functions. In fact, Steno was not alone in this conviction. The same conviction drove the English architect-microscopist-naturalist Robert Hooke to a similar conclusion: “Nature does nothing in vain.”

Steno was among the first to realize what seems obvious today, that fossils result from once-living organisms. But understanding fossil origins was only a partial victory. It wasn’t possible to understand the progression of life on Earth without somehow relating fossils to each other in time. Scientists would outline the sequence of life on Earth centuries after Steno’s death, but in doing so, they used his principles.

next: From Tuscany to the World
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  Illustration of a live and fossil crab

Sixteenth-century naturalist Conrad Gesner recognized the similaries between living and fossil crustaceans. (Image from The Meaning of Fossils by Martin J.S. Rudwick)

  Nicolaus Steno

From Tuscany to the World

Less than two years after dissecting the head of the great white shark, Steno completed a manuscript on the geology of Tuscany. The locale was a shrewd choice; it celebrated the homeland of his patrons, the Medici. Steno intended this 78-page volume, De solido, simply as an introduction to a more in-depth dissertation, a dissertation he never published. To chart a course for geology, however, De solido was enough.

Today, geologists and paleontologists typically document the rock outcrops where they work through stratigraphic sections, diagrams of rock layers shown in cross-section. This hadn’t been done before Steno’s time, and though the diagram he made was more abstract and based on more assumptions than is common today, he pioneered the practice. More importantly, he outlined principles that underlie geologic research.

Three basic kinds of rock cover the globe. Igneous rocks result from melting rock, such as lava flows. Metamorphic rocks show dramatic change due to extreme heat or pressure. Sedimentary rocks result from sediments eroded by wind or water that have since been redeposited. Paleontologists prize sedimentary rocks because they bear fossils, and paleontologists rely on “Steno’s Principles” to organize fossils in time and space.

The first and most important of Steno’s principles seems laughably apparent today, but it was far from obvious at the time. Known as the “principle of superposition,” it states that the sediment layers are deposited in sequence, with the oldest layers on the bottom and newest layers on top. “When the lowest [rock] stratum was being formed,” he wrote, “none of the upper strata existed.” Long before anyone could determine absolute ages of rock layers, geologists could, by relying on Steno’s first principle, at least figure out which rock layers were older.

When watching experiments in the alchemy workshop of his mentor Ole Borch, Steno had observed how sediment settles to the bottom of a container. His youthful observations served him well in geology. Steno knew that when water or sediment fills a basin, its upper surface levels out to be smooth and parallel to the horizon, even if it was deposited onto an irregular surface. Succeeding layers of sediment are deposited in the same fashion. This has become known as Steno’s “principle of original horizontality,” and it has helped geologists understand that layers of sediment lying at an angle to the horizon were tilted or folded after they solidified.

Steno’s final principle is the “principle of lateral continuity,” which says that sediment layers spread out until they reach an obstacle that keeps them from spreading further, the way soup spreads out in a bowl until it reaches the sides of the dish. “Wherever bared edges of strata are seen,” he wrote, “either a continuation of that same strata must be looked for or another solid substance must be found that kept the material of the strata from being dispersed.”

  Stratigraphic column

Modern geologists diagram stratigraphic columns using the general principles developed by Steno. (Image by Michon Scott, Earth Observatory)

  Nicolaus Steno's cross section of Tuscany geology

Relying on these principles, Steno tried to recount the geologic history of Tuscany. He produced a six-phase history. In the first phase, the region was flooded and layers of sediment deposited. In the second phase, the area drained. In the third phase, the rocks collapsed to form mountains and valleys. Steno envisioned this crustal collapse resulting from the rocks giving way as air pressure or water carved out caverns underground, similar to how water washes out pockets of earth under highways to create potholes. Steno believed this cycle of flooding, draining, and collapsing happened twice, and he related it to Scripture. The first flood could have been when “the spirit of God moved on the face of the waters.” The second flood might have been Noah’s Flood, though Steno didn’t say for sure.

He was wrong about the vast underground caverns leading to crustal collapse, but modern geologists understand that the movement of crustal plates certainly can fracture bedrock and build mountains. In De solido, Steno ambitiously claimed that the geology he uncovered in Tuscany held true for the entire globe. He was wrong about that too, but correct in believing that studies of rock strata could eventually lead to a global timescale.

De solido needed approval from Florentine censors before it could be published, but luckily for Steno, the censors were good friends who raised no objections. By the time the manuscript rolled off the press, however, he had reluctantly left Florence.

The geologic upheavals Steno described in his manuscript might have hinted at similar upheavals in his personal life because he made a decision that would surely disappoint his Lutheran friends back home. He converted to Catholicism. In the midst of his conversion, he received an ironic summons from King Frederick III to return to Denmark. The kind of well-paid post in Copenhagen that had eluded him years before was suddenly available, but the now-Catholic Steno didn’t want to go. He left Florence, but spent the next 20 months on a journey of nearly 4,000 miles, not exactly a direct route to the Danish court. Steno had reached Amsterdam when he received word that Frederick III was dead, and he was off the hook.

Historians don’t know what Steno planned to do next. On the long road to Denmark, he had seen some of Europe’s best geological wonders, including the Alps and Mount Vesuvius, and the next logical step might have been to follow up De solido with a more in-depth work, but no manuscript has been found. Years later, after Steno died, Gottfried Leibniz—best remembered for his dispute with Isaac Newton over the invention of the calculus—was so convinced that Steno must have kept working that he searched diligently for any writings Steno might have left behind. Unfortunately, if any manuscript existed, it wasn’t preserved. In the last phase of his life, the restless Steno changed course once again.

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Steno envisioned a six-stage history for the geology of Tuscany, including cycles of flooding, draining, and crustal collapse. (Image from The Meaning of Fossils by Martin J.S. Rudwick)

  Nicolaus Steno


In 1671, two years after its publication in Italy, De solido reached England. Members of the Royal Society of London, later known as one of the world’s most prestigious scientific bodies, quickly took notice. Henry Oldenburg, the society’s secretary, had the work translated into English, and it was soon passed to the polymath Robert Hooke for review. Hooke had reached conclusions similar to Steno’s about the origins of fossils, and the two could have become intellectual allies, but that didn’t happen. Brilliant but grouchy, Hooke accused Oldenburg—a long-time thorn in his side—of slipping his hypotheses to Steno. Hooke’s accusations were soon eclipsed by a published attack from another Royal Society member, a physician named Martin Lister.

Although Steno had provided a strong argument for the organic origin of fossils, he had made no real progress in determining the nature of “tricky” fossils, those that bore no resemblance to currently living organisms. Lister offered a prime example: ammonites. Relatives of modern cephalopods, ammonites bore some resemblances to the modern nautilus, but also striking differences. There was an explanation for this, one already suspected by the contentious Hooke: the ammonites differed so much from living mollusks because ammonites were extinct. But 17th-century Europe wasn’t ready for the notion of extinction; the concept would not achieve scientific respectability until the turn of the 19th century when it was articulated by the great French naturalist Georges Cuvier.

Lister’s criticisms must have been unpleasant, but the “support” of John Woodward was worse. Woodward was a successful physician and Royal Society member. After Steno’s death, he used—plagiarized, in fact—many of Steno’s arguments to bolster his own interpretation of Noah’s Flood during which, Woodward claimed, Newtonian gravity was suspended, causing the Earth’s solid matter to “instantly shiver into millions of Atoms and relapse into its primitive Confusion.”

Ammonite illustration

Martin Lister cited ammonites to refute Steno’s claims about fossilization. (Image from The Seashell on the Mountaintop by Alan Cutler)

The setbacks, however, proved temporary. In the 18th century, De solido was reprinted at least twice, in Latin and French, and miners and mineral surveyors began applying Steno’s principles to their work. By regarding the lowest rocks as the oldest and the highest rocks as the newest, an Italian miner named Giovanni Arduino classified the rock layers of the Alps into three categories. Primary rocks were the oldest and had no fossils. Secondary rocks consisted of tilted sedimentary layers with fossils. Tertiary rocks included still-horizontal sedimentary rocks, also containing fossils. Arduino didn’t relate these rock groups to biblical events, though they became linked in popular use, most people assuming that Secondary rocks resulted from Noah’s Flood.

As Steno’s principles gained acceptance, however, they began to undermine the biblical chronology he had believed. In 1720, chemist René Réaumur published a description of sediment layers composed mostly of broken shells. Because many of these layers were several meters thick, he argued that they could not have been deposited by a flood lasting, as the Bible described Noah’s Flood, less than a year. By the late 18th century, some geologists had also begun to question the antiquity of humankind compared to other forms of life. In deeper and therefore older sediment layers, they found “relics” of Noah’s Flood which, according to the Old Testament, happened after the creation of humans. So they should have found human remains in the older rock layers, too, but they didn’t; human remains appeared only in the youngest rocks. Even more amazing, fossil collectors found a menagerie of remains—of marine and flying reptiles, oddly-proportioned mammals, and a huge variety of invertebrates—recorded nowhere in history. An astonishing succession of life forms had apparently come and gone before humans arrived on the planet. By the close of the 18th century, the famous French naturalist Georges Buffon publicly described humans as recent arrivals on a planet roughly 75,000 years old. Privately, he estimated the planet’s age at closer to 10 million years. No one would know the Earth’s age until the 20th century, after the discovery of radioactivity enabled scientists to attach absolute ages to rocks. (In 1953, two independent studies estimated the age of the Earth at approximately 4.5 billion years.) Yet because of Steno’s principles, scientists could arrange fossils chronologically, and even locate rich seams of fossil fuel to power the 19th and 20th centuries.

The eventual discoveries of geology were all unimaginable to the 17th-century savant. Although King Frederick III died before Steno reached Copenhagen, the savant wound up there anyway. Steno missed Florence, and after two years, he petitioned the new monarch for permission to return. Permission granted, Steno returned to Tuscany and tutored a young Medici prince, but he wasn’t satisfied. In 1675, Steno took a vow of poverty and became a priest. He hoped for a simple life of pastoral duties, but the Catholic Church summoned him to Rome and made him a bishop in 1677. His new assignment was in northern Europe, converting Protestants to Catholicism in Germany, Norway, and Denmark. It was a tough assignment, and as before, he missed Florence.

Portrait of Steno as a bishop

Steno became a Catholic bishop in 1677. (Image from The Seashell on the Mountaintop by Alan Cutler)

Steno took his vow of poverty seriously, and acquaintances watched in dismay as he grew skinnier. A friend he had recently converted recounted finding Steno “without a house, without a servant, devoid of all life’s comforts, lean, pale and emaciated.” Over time, Steno’s self-denial took its toll, and he died at the age of 48. Three centuries after his birth, a group of Danish pilgrims appealed to Pope Pius XI to name him a saint. Pope John Paul II beatified Steno in 1988, putting the 17th-century savant on the road to sainthood.

In the last years of his life, Steno cared more about saving souls than studying rock strata, yet he never renounced his scientific work. Steno certainly could not have imagined where his geologic principles would lead over the next three centuries. Likewise, succeeding geologists, paleontologists, and even anatomists would have had a hard time imagining their fields of research without his pioneering insights.

  • References:
  • Cutler, Alan. The Seashell on the Mountaintop: A Story of Science, Sainthood, and the Humble Genius Who Discovered a New History of the Earth. New York: Dutton, 2003.
  • Ellis, Richard. Monsters of the Sea. New York: Alfred A. Knopf, Inc., 1994.
  • Rudwick, Martin J.S. The Meaning of Fossils: Episodes in the History of Paleontology. Chicago: University of Chicago Press, 1976.
  • Rudwick, Martin J.S. Scenes from Deep Time: Early Pictorial Representations of the Prehistoric World. Chicago: University of Chicago Press, 1992.

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Steno certainly could not have imagined where his geologic principles would lead over the next three centuries.