A trilobite from the Cambrian Period. The last trilobites died out in the Permian.
And then you notice that the different fossil forms do not succeed each other at a uniform pace. Instead, the rocks proclaim long periods in which the kinds of life change very little, punctuated by short intervals of cataclysm, turnover, the wholesale loss of many kinds of creature, sometimes all over the Earth at once—followed astonishingly quickly by many new forms, clearly evolved from the survivors of the catastrophe. The clearest boundaries in the sedimentary column are the ready markers of fearsome, planet-wide disasters. You have a planet where for tens of millions of years everything is fine. It’s enough to give even inveterate pessimists a sense of security. But then, maybe when you’d least expect it, tumultuous changes occur—changes so striking that they are instantly apparent to the eye and mind of observers like us, who have come into being hundreds of millions of years later.
There is a steady background rate of extinction, of course. In our time, because of human activities, species become extinct every year—from land reclamation or hunting or industrial pollution or cutting down the tropical forests. The passenger pigeon, whose migratory flights once darkened the skies of North America, became extinct in the twentieth century, the great auk in the nineteenth, the dodo in the seventeenth. The last mastodon died perhaps as recently as 2000 B.C., the giant armadillo around 5800 B.C., and the giant ground sloth (it ate treetops), around 6500 B.C. We humans were probably responsible for the demise of most of these creatures, but such extinctions are part of the natural ebb and flow of life, and are caused by minor changes in the physical or biological environment—in this case a newly capable predator. What we are concerned with here, however, is something quite different: massive extinctions of dozens or hundreds of families of life simultaneously, all over the Earth.
The extinction of what biologists call a “family” of plants or animals represents a major loss. Think of the variety of dogs in the world, from Chihuahuas to Great Danes. They are all one species, because they are all interfertile. Biologists distinguish a broader category called a genus (plural, genera) that here embraces not only dogs but wolves and jackals as well. This genus, Canis, is in turn part of a much larger category, a family, that includes foxes; together they all are the Canidae. A family is a major group of beings. The human family, the family of the hominids, includes most of those primates who walked around on two feet and tried to puzzle things out over the last few million years, even though we might not recognize many of them as human were we to encounter them on the street. The loss of a family is the hacking off of a limb of the tree of life.
In the table above, six of the most spectacular known interruptions in the history of the Earth are listed, along with an indication of how massive these extinctions were. It is with a sense of shock that we look at the last columns of this table. It shows, for each of the six catastrophes, the percent of all families and genera of known marine organisms that were lost. At the Ordovician and Devonian boundaries, fully half of the genera became extinct, less for the Eocene event, almost as much for the Triassic. In the Permian catastrophe more than half of all the families of life on Earth were lost, more than three-quarters of the genera, and more than 90 percent of all species. One of the most recent of these planetary disasters was at the end of the Cretaceous Period, some 65 million years ago, when almost one out of five families, half the genera and three-quarters of the species disappeared.
Some of the creatures that were lost are pictured on these pages—a fish in armor, extinct at the end of the Devonian; a trilobite—they hunted in herds on the ocean bottoms and had existed for almost 300 million years—extinct in the Permian; and an ammonite, a relative of the octopus, extinct in the Cretaceous.
Scientists look closely at the Cretaceous extinctions because they concern us particularly. A critical turning point in the evolutionary path to humans can be glimpsed among all those reptile bones. The Cretaceous catastrophe wiped out every family, every genus, and every species of dinosaur, and they were as varied and successful as the mammals are today. It is as if all the mammals were to roll over, their appendages stiffly in the air—every shrew and whale of us, and every person. All the flying and swimming reptiles died as well, and more than a hundred families of beings that live in the oceans. It was a catastrophe enormously beyond anything humans have ever known, at least so far.
The first mammals appeared at about the same time as the first dinosaurs. The dinosaurs were the lords of the Earth, the largest creatures, the most powerful, the ones who would capture your attention on any Cretaceous landscape. The mammals, our ancestors, were then tiny, furtive, scampering creatures, cautious, mouse-like, spending much of their time keeping out of the way of the thundering reptiles. A few dozen little mammals would have made a meager lunch for a middling-sized carnivorous dinosaur. For over 100 million years our ancestors were at an apparent evolutionary cul de sac, living at the margins and in the shadows of a world dominated by dinosaurs. If you surveyed that late Cretaceous landscape—in which the trees and flowers looked pretty much as they do today, but where the dominant animals were all reptiles—you would not have bet much on the chances of our ancestors.
But the dinosaurs went and died, every last one of them, and left the planet to the mammals. Mammals were not the only ones to survive, of course: snakes and salamanders were in good shape, and fish and insects and crocodiles, and many land plants, plus legions of microbes. But the mammals soon predominated. Tentatively at first, and then with an increasing exuberance, the mammals evolved, grew, diversified, taking advantage of the demise of the competition, filling untenanted ecological niches. We owe our very existence—every one of us bigger than a mouse, anyway—to the extinction of the dinosaurs. So perhaps we humans may be excused for having a special interest in this question: Why did the dinosaurs, along with most of the other species on Earth, abruptly die some 65 million years ago?
It does seem possible and even probable that a comet collision with the Earth destroyed the dinosaurs and initiated the Tertiary division of geologic time … It will most probably be millions of years before the next collision occurs.
—HAROLD C. UREY, “COMETARY COLLISIONS AND GEOLOGICAL PERIODS,”
NATURE, VOLUME 242, PAGE 32, 1973
Dimetrodon, a sail-finned pelycosaur from the Permian, rendered extinct long before the demise of the dinosaurs.
In Book 4, Chapter 4 of Laplace’s System of the World, the great French scientist at once criticized as superstition the worldwide human fear of comets, and advanced as sober scientific hypothesis a good and practical reason for comet dread:
In those times of ignorance, mankind were far from thinking that the only mode of questioning nature is by calculation and observation: According as phenomena succeeded with regularity or without apparent order, they were supposed to depend either on final causes or on chance; and whenever any happened which seemed out of the natural order, they were considered as so many signs of the wrath of heaven.
But these imaginary causes have successively given way to the progress of knowledge, and will totally disappear in the presence of sound philosophy, which sees nothing in them, but expressions of the ignorance of the truth.
To the terrors which the apparition of comets then inspired succeeded the fear, that of the great number which traverse the planetary system in all directions, one of them might overturn the earth.
Laplace himself, we remember, had carefully studied the orbital evolution responsible for the near impact with the Earth of Comet Lexell in 1770 (Chapter 5).
They pass so rapidly by us, that the effects of their [gravitational] attraction are not to be apprehended. It is only by striking the earth that they can produce any disastrous effect. But this circumstance, though possible, is so little probable in the course of a century, and it would require such an extraordinary combination of circumstances for two bodies, so small in comparison with the immense space they move in, to strike each o
ther, that no reasonable apprehension can be entertained of such an event.
An armored fish, none of which survived the Devonian Period.
Something like the Laplacian view of the consequences of a comet hitting the Earth. The hyperbolic caption of this picture from Pearson’s Magazine of December 1908 reads: “If a large comet approached within measurable distance of the Earth, the doom of our world would be sealed. Such tremendous heat would be engendered that everything would spring into spontaneous combustion. The hardest rocks would become molten, and no living things would remain upon the Earth’s surface. Buildings and human beings would be scorched to cinders in a second. ”
So is the fear of a cometary collision with the Earth in the same category as superstitious dread of comets? Not at all. Laplace, who was one of the inventors of modern probability theory, explains:
Nevertheless, the small probability of this circumstance may, by accumulating during a long succession of ages, become very great. It is easy to represent the effect of such a shock upon the Earth: the axis and motions of rotation changed, the waters abandoning their previous position, to precipitate themselves towards the new equator; the greater part of men and animals drowned in a universal deluge, or destroyed by the violence of the shock given to the terrestrial globe; whole species destroyed; all the monuments of human industry reversed; such are the disasters which a shock of a comet would produce.
The expectation that comets—through universal deluge or other means—are in fact harbingers of disaster has been a respectable speculation for the whole scientific history of the subject, dating back to Edmond Halley himself (who proposed that the biblical Flood was produced by “the causal Choc [shock] of a Comet”). The brand of mischief that comets are said to bring—flood, darkness, fire, rending the Earth asunder—changes with time and astronomical fashion. But the association of comets with catastrophe remains curiously steady through the generations.
Laplace connected this awful vision of a comet striking the Earth with an apparent paradox in timescales. Human history was only a few thousand years old. But Laplace knew—from arguments such as those of Halley on the salt content of sea water—that the Earth was much older. Laplace, the cosmic evolutionist, had not a glimmering of biological evolution. Charles Darwin’s The Origin of Species was sixty years in the future. Laplace could not imagine the world existing for long periods before there were humans. So why weren’t human history and human civilization much older?
We see … then why the ocean has abandoned the highest mountains, on which it has left incontestable marks of its former abode: we see why the animals and plants of the south may have existed in the climates of the north, where their relics and impressions are still to be found: lastly, it explains the short period of the existence of the moral world, whose earliest monuments do not go much farther back than 5,000 years. The human race reduced to a small number of individuals, in the most deplorable state, occupied only with the immediate care for their subsistence, must necessarily have lost the remembrance of all sciences and of every art; and when the progress of civilization has again created new wants, every thing was to be done again, as if mankind had been just placed upon the earth.
There is here not only the unmistakable suggestion of global cometary disasters, but, also, cometary extinctions; and even a hint that such catastrophes have occurred through the whole previous history of the Earth. These ideas are being taken up again in our time.*
After Laplace, cometary catastrophism became almost fashionable. Some authors imagined debris from the comets—the clay in Donnelly’s Ragnarok, for example—distributed over much of the Earth; others imagined the effects of the collision to be felt only in limited areas. Occasionally, consequences were imagined that were even more dire than those Laplace had depicted. In 1893 the French writer Camille Flammarion—who seemed to have had a penchant far beyond that supported by the facts for frightening people with comets—wrote a science fiction story called The End of the World:
Like a great celestial projectile the solid nucleus of the comet pierced the egg-shell crust of the Earth and buried itself in the semimolten interior. The comet tore its way on like a shot piercing the boiler of a battleship. The Earth was immediately converted into a planetary volcano. Oceans were spilled like thimbles of water … continents were twisted and torn like paper.
This is rather overdoing it. But if comets are a kilometer or more across and traveling at great speed in the same part of the solar system as the Earth, it follows that sooner or later a largish comet will indeed strike the Earth, with consequences that we can be sure would be catastrophic, even if we were unable to trace exactly what would happen. This is the basis of Laplace’s argument. It is easy to calculate—it can be worked out on the back of the proverbial envelope—roughly how long the Earth abides between successive collisions with sizable comets. In the first decade of the twentieth century W. H. Pickering of Harvard calculated that a fair-sized cometary nucleus should strike the Earth once every forty million years. But the dozens of collisions he imagined the Earth to have suffered during its history seemed to have done little damage, a conclusion he deduced from the fact that life is all around us.
The leading astronomy textbook in America after the end of World War II* included this paragraph:
It is probable that the Earth has undergone many collisions with comets during geological time. It may readily be computed that a small, rapidly moving body which approaches the Sun within one Astronomical Unit stands about one chance in four hundred million of hitting the earth. As about five comets come within this distance every year, the nucleus of a comet should hit the earth, on the average, once in about eighty million years.
But because the authors had accepted the sand-bank hypothesis of comets (Chapter 6), they did not believe in massive cometary nuclei. So they concluded that collisions of comets with the Earth “would probably fall very short of producing wholesale destruction of terrestrial life.” Throughout the twentieth century scientists have intermittently connected biological extinction with cometary impacts, although the prevailing wisdom held that a catastrophe on such a scale could not be visited by so minor a body.
About halfway from Florence to Rome on the Autostrada, you make a left turn, drive through Perugia, and continue an equal distance up into the Apennines. Eventually, you come to a little village called Gubbio; it dates back to the Middle Ages. There, at the side of the road, is the sequential banding of a wonderfully preserved sedimentary column (see plate XIX); it antedates by millions of years any of those erected by the Caesars. If you go up to it, you can see a thin layer of pink and gray sitting above a bright white bed of rock. It marks the end of the Cretaceous Period.
You cut out a little piece and bring it back to the laboratory. The white rock is limestone, and under the microscope you can see a matrix of calcite plates and shells, manufactured by the microscopic plants and animals that lived in the warm seas. The chalk of the white cliffs of Dover, in Britain, was manufactured by such calcite-secreting oceanic microorganisms. They became extinct in the Cretaceous disaster. In fact, this is where the Cretaceous gets its name: creta is Latin for chalk. At Gubbio, you can clearly see that these oceanic limestones are punctuated by the gray/pink layer; it is about a centimeter thick, and made of clay. The clay and the limestone both must have accumulated as individual grains, settling through the quiet waters of the ocean tens of millions of years ago. Just above the clay, the fossils in the limestone are very different from those just below. Similar layers are found at the Cretaceous boundary all over the world. You can tell at a glance that the clay marks a catastrophe.
The kind of microscopic pollen from flowering plants changes abruptly above the clay layer—the land equivalent of the oceanic extinctions of microorganisms. Below the Cretaceous clay you find the fossils of dinosaurs who prowled the Earth for 160 million years or more. Above it there are no dinosaur fossils at all, but the remains of mammals are abundant.* This gray boundary marks
a catastrophe that ended the Cretaceous Period—and with it much of life on Earth. What is in that clay? What caused the Great Dying?
Gold and platinum are valued in part because they are rare. But when you look at the spectrum of the Sun and the stars, or examine a meteorite newly fallen to Earth, you find that there is relatively much more of these precious metals up in the sky. Not that meteorites have lodes of gold or veins of platinum; but, compared to some plentiful element such as silicon, the Earth seems anomalous, strangely impoverished in precious metals. Now in molten rock, gold and platinum tend to move with iron. And the iron that was once uniformly mixed through the bodies that formed the Earth is now mainly concentrated in the liquid core of our planet, 3,000 kilometers beneath our feet. It is a good bet that most of the gold and platinum migrated there along with the iron, when the newly formed Earth was partly molten. This is true of other elements as well, elements less well known than gold and platinum—in particular, the elements iridium, osmium, and rhodium. So if iridium were present in sufficient quantities in a particular layer of the sedimentary column, it might constitute a telltale sign of some sort of extraterrestrial intervention in earthly affairs.