by Neil Shubin
This was an age of exploration of the natural world, and by looking at rocks and fossils, people began to formulate new ideas. Ships returned monthly from the far corners of the planet loaded with minerals, plants, animals, and rocks previously unknown to science. Natural philosophers of all stripes—people we would today call anatomists, paleontologists, and geologists—were in the center of the action, attempting to decipher the menagerie of biological curiosities that were being brought home.
One of the luminaries of this period, Georges Léopold Chrétien Frédéric Dagobert Cuvier, had an ego as large as his name. Born to humble origins, he died Baron Cuvier, one of the leaders of the Natural History Museum in Paris.
One expedition returned to Paris from South America with a giant six-foot-long skeleton shaped something like a small troop transport. With massive bones, large claws, and a skull with flattened teeth, this creature departed from anything in Cuvier’s experience until he looked closely at the vertebrae and limb bones. Being an astute anatomist, he saw that nestled inside this bizarre skeleton is the body plan of a sloth. But it was unlike any alive today.
Then several different kinds of elephant-like bones were brought to Cuvier’s attention. Seeing the differences from elephants, he identified the bones as reflecting a new species: mammoths. But these discoveries, satisfying as they were for understanding the diversity of life, raised a troubling question: Where on Earth were these creatures still alive?
Cuvier made the connection: perhaps the large sloths and mammoths were no longer roaming the planet but instead revealed lost worlds of creatures. The concept of extinction, something so fundamental to the way we see the world today and alien to many thinkers for millennia, now explained the goings-on inside the rocks.
One example after another of long-lost life appeared. Spelunkers in Germany ran across the large bones of a monster or dragon lying on a cave floor. An anatomist from the local medical school saw that they were from some sort of bear, but one so large and oddly proportioned that it was unlike anything walking Europe. Years later, Thomas Jefferson found giant mammoths, sloths, and other creatures near his home in Virginia.
Cuvier was a big thinker, and not satisfied with mere description, he drew generalizations from his observations, putting his theories on the line. To Cuvier, the conclusion was obvious: extinction was not only real but common. So prevalent and important was this concept that in an early monograph he declared, “All of these facts … seem to me to prove the existence of a world previous to ours, destroyed by some kind of catastrophe.”
Cuvier’s idea, like that of Phillips before him, was that catastrophes shaped Earth. This idea was the cutting-edge science of its day, having the weight of evidence and the stamp of eminent authority. It also was virtually ignored by scientists for over a hundred years.
The notion of catastrophes lay in direct conflict with the reigning scientific approach of the day. This alternate notion was so powerful in explaining Earth that it did not allow interlopers. Its success was derived from the motto “Use the present to infer the past.” This idea is so simple and elegant that we take it completely for granted. If you see a car parked on one side of the street on Monday but by Thursday it is on the opposite side, you infer that somebody drove it and later parked it in the new place. It would be a far stretch to imagine that the car flew by its own volition or was carried by a special wind. The mechanisms at work today explain yesterday; no magic or extraordinary physics need to be involved.
The same kind of reasoning applies to the history of rocks, cliffs, and layers. The major forces at work around us today are wind, rain, and gravity—all products of the laws of physics and chemistry. If they shape today’s world, they must have acted in the past to make the rock record. The Grand Canyon clearly is a deep cut in the ground with the Colorado River at its base. The known earthly cause for the formation of the canyon is the erosive action of water cutting the rock and the relative uplift of rocks around it. But these mechanisms are very slow. Sand doesn’t compact into rock layers overnight any more than a flowing river carves a canyon thousands of feet deep in a day, or even a year. The implication for the formation of the Grand Canyon, or any geological feature, is that it took millions of years to come about.
This gradual approach provided an explanation for the formation of canyons, coral reefs, and coastlines: not only were present-day mechanisms capable of explaining Earth’s history; they implied that most changes to species or the planet would be slow. Looking at Earth today, nobody could imagine, much less see, a mechanism that could bring about a global cataclysm to life on Earth.
Theories of catastrophes, like those proposed by Phillips and Cuvier, became decidedly oddball views, relegated to a kind of lunatic fringe of scientific thought. Phillips continued to work, but by the time of his passing in 1874, from a fall down the steps of All Souls College at Oxford, the notion of catastrophes was already dead—killed by the reigning dogma of gradual change.
REVOLUTION
The town of Stafford is nestled in south-central Kansas; its population consists of about a thousand households, with a high school so small they play eight-man football. In the early twentieth century, the Newells were known to locals as the go- to people for knowledge about local natural history. When farmers hit a strange rock, the patriarch of the Newell family saw it for what it really was, a mammoth tooth. Six-year-old Norman Newell observed these encounters, and they changed the way he thought about home: the flatlands that are Kansas today were once grasslands and forests that housed large mammals. Norman’s interest in paleontology grew, and he excelled to the point that he won a spot in the prestigious graduate program in paleontology at Yale University, which by this time, the 1930s, had become one of the major centers of research in the field.
Norman Newell’s work at Yale was a family affair; his wife supported him financially, cataloging specimens for Yale’s Peabody Museum, until Newell was able to obtain a scholarship in his second year in the program. He set off studying clams, mollusks, and other animals with two shells separated by a hinge. Newell was quick to see the advantages of studying these animals. With hard shells, they readily fossilize and are very common throughout the ancient layers of the world. Newell did something few others at the time even considered important: he used living shelled animals to infer the behavior of extinct ones.
After a stint in Peru with the State Department during World War II, in 1945 Newell took a post at the American Museum of Natural History in New York City. This was a beautiful partnership: it brought Newell in contact with a renowned collection, eminent scientists on the museum’s staff, and ample resources to financially support science. At this time, the museum was heaven on earth for studies of fossils and taxonomy. The area behind the scenes at the museum consists of hallways, some of which are over a quarter of a mile long. The corridors are a buzz of scientific activity. New fossils and creatures collected from around the world are coupled with new ideas about nature. It has been, and remains, a crucible for innovative ideas.
Soon after arriving in New York, Newell was asked to produce two chapters for a major compendium, the Treatise on Invertebrate Paleontology. The volume is as intimidating as its name. The conceit behind the Treatise was to produce a running compilation of every fossil ever collected, with details on its anatomy and on the layers in which it was found. This notion sprouted to what is now a vast fifty-volume set, authored by over three hundred paleontologists, each an expert on one set of fossils. To many, this kind of work seems like nothing more than stamp collecting. In the hands of some scientists, like Newell and others to follow, it is a window into a universe of scientific discovery.
Newell dwelled in the details of his fossil shells. He knew their anatomy, diversity, and, importantly, the layers of rock they came from. Like Phillips and Smith before him, he read the layers of the world as a book. Unlike them, he was now armed with vast amounts of global data, of the kind that were going into the Treatise.
The more time Newell and others spent compiling the fossil record, the more an inescapable fact emerged. Whole worlds of animals and plants populated the globe in the past, only to disappear rapidly and nearly simultaneously around the planet. Life had experienced not one global catastrophe but several.
Newell became a voice in a small chorus of people arguing for the reality of global cataclysms of the kind Phillips and Cuvier had argued for more than a century earlier. The response was the same: the work was largely ignored. The discovery of patterns in the past record did little to change more than a century of entrenched thinking. The theory of continental drift suffered a similar fate in some quarters: the pattern of the continents was as clear as day, but lacking any mechanism to account for moving continents, many were reluctant to accept that continents moved. The same was true for catastrophes. What kinds of mechanisms could bring about such global calamities?
In the late 1970s, Walter Alvarez, a Berkeley geologist, was working on rocks about 65 million years old in Italy. This is the time that saw the demise of the dinosaurs, a period known as the Cretaceous. Walter, an acutely perceptive field geologist, was able to pinpoint the end of the Cretaceous to a single thin layer of clay. Below this were layers of dinosaurs, marine reptiles, and other kinds of life. Above, all of these creatures were missing. Walter was asking the question: How fast did creatures die out? Answers, he believed, lay inside this clay. Perhaps a chemical inside could act as a kind of clock that he could use to estimate how fast the clay was deposited?
Walter took the problem to his dad, Luis, a Nobel laureate in physics, also at Berkeley. The elder Alvarez had a restless mind; he was always looking to apply his knowledge of particle physics to solve mysteries in science. At the time Walter approached him about his clay, his dad was thinking of ideas to scan the inside of the great pyramids for treasure.
The Alvarezes hatched plans to make refined measurements of some of the elements inside the layers. One of these was the element iridium, which is rare on Earth but common on certain kinds of asteroids and meteorites. The thinking was that if meteors bombard Earth at constant rates, iridium levels should act as a kind of clock. Iridium is found in rocks in parts per billion—the equivalent of measuring a single grain of sand on an entire beach the size of Jones Beach in Long Island. Luckily for them, the elder Alvarez was associated with a team that had the expertise, and the machines at the Lawrence Berkeley National Laboratory, to make such precise measurements.
Walter and his dad were in for a huge surprise, as iridium levels in the clay defied all of their expectations. The levels of the element were by no means regular in the layers; iridium was practically absent in most layers and off-the-charts abundant in one particular place. It was clear that asteroids didn’t hit Earth at constant rates; every now and then there is the big one. And the big one they found was reflected in a huge spike in the concentration of iridium exactly at the layer that heralded one of the greatest catastrophes of all time for life on the planet.
Then Luis came up with the killing mechanism. He proposed that when an asteroid slams Earth, it vaporizes and sets off dust in the atmosphere that blocks light and kills plants. These effects cascade through the food chain, causing widespread disaster. Not only could we now imagine a mechanism for a global calamity, but we could look at the layers of rock in the world and see the effects it wrought on living things. The thrill of the scientific hunt is to have an idea whose truth is hitched to predictions that take us to new places to explore, objects to discover, and data to analyze.
The influence of the asteroid theory goes beyond rocks falling from space; it extends to how we think about catastrophes in general. For the first time in the eons that humans have looked at rocks, bodies, and fossils, not only could we imagine a mechanism for a global cataclysm, but we could reconstruct its effects and analyze its impact on the biosphere. The asteroid impact notion put catastrophism back on the intellectual agenda. The insights of scientists like Phillips, Cuvier, and Newell were no longer at the lunatic fringe of scientific thought. The question had changed from “Could catastrophes ever happen?” to “What are the consequences of global cataclysms?”
A NUMBERS GAME
In the late 1960s, Tom Schopf was a young man with a plan to transform the way we think about the past, and he didn’t care if he was going to ruffle a lot of feathers in the process. As he saw it, most paleontologists worked on their own little group of animals, on their own little sliver of time. It was a field of special cases. The way we did paleontology had to change if we were to answer the really big questions. As Stephen Jay Gould once said, Schopf wanted to “rescue paleontology” by bringing numerical rigor to the discipline.
And how was Schopf going to bring this all about? Whether he knew it or not, he was trying to bring the field back to its roots—to John Phillips.
“What can we do that’s different?” With that, Schopf laid the challenge to an unusual gathering. He brought some of the leading lights in paleontology together in a conference room at Woods Hole, on Cape Cod. When they arrived, they found boxes of the Treatise on Invertebrate Paleontology on the table waiting for them. They were going to pick up where people like Newell left off and find new general patterns in the history of life. With some of the best brains in the field, and virtually all the known fossil discoveries yet compiled, locked for three days in a room on the shores of Massachusetts, something fantastic would happen. At the very least, this setting had the makings of an Agatha Christie mystery.
What was the result of Schopf’s three-day collision between all the fossil data then compiled and some of the best brains in the field? One of Schopf’s Chicago colleagues who attended the meeting summed it up: “We got nowhere. Dead zero.”
Fortunately, Stephen Jay Gould brought one of his new hotshot graduate students to the last day of the meeting. Named Jack Sepkoski, he was a computer whiz who had just graduated from Notre Dame.
There is no record of what young Sepkoski said or did at the Woods Hole meeting. After the conference, though, Gould assigned him the task of compiling the Treatise and other databases into a form that could be computerized by digitizing every occurrence of a fossil group on a geological timescale. This was in 1972. Sepkoski set off on tabulating things, quietly assembling the data. The job grew and grew. Sepkoski continued to crank away, even after he himself became a professor, at the University of Chicago. Ten years after the Woods Hole meeting, he unveiled the first usable database in paleontology.
By the time I was a graduate student in the 1980s, Sepkoski’s database was the center of almost every debate in the field. With all the numbers crunched, it became clear that the patterns of life are most definitely not random. During the early history of animals, their diversity increases rapidly to a kind of plateau. Diversity wiggles up and down a bit, but there are five intervals where the numbers of species just crash. The most famous of these was the one that killed the dinosaurs, the so-called end-Cretaceous event at about 65 million years ago. Forever gone with the dinosaurs were the reptiles that lived in the seas, flying reptiles, ammonites, and hundreds of less-famous shelled creatures. Other extinctions happened at 375 and 200 million years ago. The general pattern looked the same for each event: species from around the world simply vanished at the same time. One of the events was nearly the end for life on Earth: 250 million years ago over 90 percent of the species living in the oceans disappeared forever.
Catastrophes were no longer pipe dreams conjured by offbeat scientists; the shape of our world was sculpted by them. And, as we’ve come to appreciate since the work of the Alvarezes, asteroids aren’t the only likely killing mechanism. Massive volcanoes and chemical changes to the oceans have been shown to also be candidates for a number of global extinctions in which asteroids do not appear to be involved. Knowing these facts, we can now ask powerful new questions.
Who survives a global catastrophe? Are there rules that determine how life responds? Sadly, neither Sepkoski nor Schopf would live to
see the progress on these big questions. Schopf was a hard-charging scientist who simply didn’t have an off switch. He attacked intellectual problems and worked them around the clock. Tragically, his heart gave out during a geology field trip in 1984, stopping his work forever at age forty-four. Sepkoski died at his home in Chicago in 1999.
After Schopf’s death, another young Turk, David Jablonski, was recruited to fill his post at Chicago. Jablonski’s office sits across campus from mine, in a 1970s-era brick remake of a Moorish fort. Dave has a corner laboratory, a large open room overlooking the science library—or at least his room was open before his collection of thousands of books, papers, and journals filled the space. Getting to Dave’s desk is a bit of a challenge. The visitor needs to meander through a maze of waist-high stacks of journal articles and past chest-high stacks of books to his small desk on the far wall. You can’t see the door from this space for all of the scientific papers that block your view. But if you ask for any paper in his collection, Dave will find it in the middle of any stack. I can barely manage to find my way around his stacks, but he knows where everything is inside them. This is no clutter of a disorganized person; this is the ideal arrangement for a mind capable of finding order in chaos.
Dave crunches databases to find signals in the history of life much as the Woods Hole group tried to do forty years before. He mostly looks at shelled creatures because they are abundant and readily preserved in the fossil record. Dave is inspired by the search for large-scale patterns. Every measurable feature is fodder for his analysis, such as how big the species were, and where and when they lived.
