"What seest thou else in the dark backward and abysm of time?"
Prospero to Miranda, in Shakespeare's The Tempest

Time of Mountains

So long ago my father led me to
The dark impounded orders of this canyon,
I have confused these rocks and waters with
My life, but not unclearly, for I know
What will be here when I am here no more.

I've moved in the terrible cries of the prisoned water,
And prodigious stillness where the water folds
Its terrible muscles over and under each other.

When you've walked a long time on the floor of a river,
And up the steps and into the different rooms,
You know where the hills are going, you can feel them,
The far blue hills dissolving in the luminous water,
The solvent mountains going home to the oceans.
Even when the river is low and clear,
And the waters are going to sleep in the upper swales,
You can feel the particles of the shining mountains
Moping against your ankles toward the sea.

Forever the mountains are coming down and I stalk
Against them, cutting the channel with my shins,
With the lurch of the stiff spray cracking over my thighs;
I feel the bones of my back bracing my body,
And I push uphill behind the vertebrate fish
That lie uphill with their bony brains uphill
Meeting and splitting the mountains coming down.

I push uphill behind the vertebrate fish
That scurry uphill, ages ahead of me.
I stop to rest but the order still keeps moving:
I mark how long it takes an aspen leaf
To float in sight, pass me, and go downstream;
I watch a willow dipping and springing back
Like something that must be a water-clock,
Measuring mine against the time of mountains.

But if I go before these mountains go,
I'm unbewildered by the time of mountains,
I, who have followed life up from the sea
Into a black incision in this planet,
Can bring an end to stone infinitives.
I have held rivers to my eyes like lenses
And rearranged the mountains at my pleasure,
As one might change the apples in a bowl,
And I have walked a dim unearthly prairie
From which these peaks have not yet blown away.

(Ferril, 1934)


Some cultures, like the Hindu and Mayan and other native cultures of the Americas, have believed that time is cyclical, with human lives and events repeated in an endless sequence. A few Greek philosophers thought that time was infinite, and many cultures have believed that time is in fact circular (Gorst, 2001). In Jewish and Islamic culture, and during the Christian era of the West, however, most people believed that time had a specific origin and direction, as in the religious tradition that the world began -- was "created" -- about six thousand years ago. Many of those who do not accept the severely limited time allotted by Scriptural analysts -- like the six thousand years meticulously calculated by the Protestant Bishop Ussher (1581-1656) -- nonetheless think that the Earth came into existence quite recently.

For numerous centuries, the Bible was the "one book" with which all other information in the world had to be reconciled. But in the later years of the eighteenth century, gentlemen scientists of Western Europe, themselves good Christians who believed in the literal truth of the Bible and who often attempted to demonstrate its truth through their study of the natural world, began to realize that strictly enumerated Biblical time was insufficient to explain the evidence they saw in the rocks around them. These gentlemen scientists and students of nature, able to engage in scientific studies because of their wealthy leisure, gradually discovered that Biblical accounts of the nature of the world and its history either had to be in error or were simply metaphorical.

Most notable among them was the Scottish naturalist James Hutton (1726­97), who, in his 1788 book, Theory of the Earth, stated that his geological investigations revealed that there had been a "succession of worlds" comprising the history of the Earth. Hutton knew that certain of the rocks in his general neighborhood were sedimentary in origin, that is, they had been formed by the slow accumulation of sediments washed from the land into ocean basins. Over enormous lengths of geologic time, these sediments had hardened into solid rock, and by a then unknown process had come to be the subsoil foundation -- the bedrock -- of parts of the Edinburgh area of Scotland.

These sedimentary rocks were flat-lying, just as when they were formed on the ocean floor. It may seem common-sensical to us today, but the recognition that sedimentary rocks formed from the accumulation of muds and sands on the bottom of the sea, and thus must have originally been horizontal, is one of the many facts of our world that only became obvious when someone had specifically pointed it out. In this case the someone was a gifted Danish naturalist, Nicolaus Steno (1638­86), whose discovery is called the principle of original horizontality.

But Hutton, an astute observer, noted an additional, remarkable fact about the rocks of the Edinburgh area. In some places the sedimentary rocks overlay other sedimentary rocks, which rose at sharp angles to the rocks above them. [See below, The unconformity at Jedburgh ] Clearly these rocks could not have been produced during the same sedimentary episodes as those above them: they often were turned upright, broken, warped and misshapen. To Hutton these upturned rocks provided confirmation that long episodes of sedimentation had preceded the one which had produced the flat-lying rocks (Montgomery, 2003); they were, in fact, relics of previous "worlds."  Indeed, when Hutton evaluated all his evidence, he saw a succession of "worlds" which stretched as far back in time as could be observed. He therefore famously declared that the geological record displayed "no vestige of a beginning, no prospect of an end."

Hutton's was a stunning conclusion, unacceptable to many. Where Ussher had found a very compressed history of Heaven and Earth recorded in the Bible, Hutton, reading in the Book of Nature, found that geologic time was effectively infinite. But Hutton's evidence was compelling, and those who followed him in examining rocks and living things, confirmed that he was correct. Hutton was the first to understand the testimony of the earth itself regarding its own age, and to peer into the abyss of deep time.

We now know that geologic time does have a beginning, in a Universe which also has a beginning, about 13.7 billion years ago. Nonetheless, geologic time is enormous, and its vastness is so far outside of our own ordinary experience of the world that it may ultimately be beyond our imagining or comprehending.

To Hutton and those who shortly followed him, however, the vastness of geologic time was only beginning to be glimpsed. Hutton recognized that the sedimentary rocks he examined were the products of long natural processes. Sediments were eroded from mountains and soils, and carried to the sea by rivers. Except in flood stage, relatively little sediment was transported by the rivers, so the deposition of sediment in the deep ocean was slow and protracted. The smaller the particle of sediment, the more likely it was to complete the journey, so mud and sand made it to the ocean floor much more often than rocks and boulders.

Once at the bottom of the ocean, the layers of mud or sand built up slowly over the centuries. The farther from the land, the source of the sediment, the longer the process took. As the layers built up, however, they gradually squeezed out the water within them. Eventually, with sufficient time and sediment accumulation, they began to harden into rock. Then, somehow -- perhaps through the agency of heat and volcanism -- they could become part of the land.

Early on in the scientific investigation of the Earth -- before Hutton, in fact -- it was recognized that the lowest sedimentary layers -- "strata" -- should be the oldest, because they were the first to be deposited. The determination that the deepest strata were the oldest and the shallowest (topmost) were the youngest (other things being equal) is known to geologists as the principle of superposition (also discovered by Nicolaus Steno).

The unconformity at Jedburgh. An unconformity is a discontinuity in the rocks.
Note how the lower sedimentary strata (from the Silurian Period, between about 438 and 408 million years ago), which were deposited on the seafloor and were therefore originally horizontal, are now vertical and twisted. Once they had been turned vertically (or perhaps during that process), these older strata were highly eroded. The overlying, younger strata (called the Old Red Sandstone, from the Devonian Period, about 408 to 367 million years ago) retain the horizontality of their original deposition. (Drawing by John Clerk, Lord Eldin. Hutton, 1795.)

But the fact of superposition merely provided a guide to the relative age of sediments in particular rocks. Rates of sedimentation were carefully studied, along with rates of erosion of the sediment source material, but estimates of absolute geologic age were at best only educated guesses. The difficulties involved in obtaining good estimates of geologic time were to plague Darwin, for whom extended periods were necessary to accomplish evolution by natural selection.

Nonetheless, Darwin and others were able to make guesses about the rate of geological processes and the temporal extent of some geological episodes of geologic time that were not unreasonable. One estimate by Darwin (for the "denudation of the Weald," that is, the erosion of an area [the Weald] of Sussex, England, southeast of London) is now recognized to be many times longer than accurate, but the estimate was still within an order of magnitude (a factor of ten) of the correct one: not bad for someone who entirely lacked modern radiometric dating tools. Moreover, such estimates of tens and hundreds of millions of years to accomplish particular geological processes were far closer to the truth than the Biblical accounting of Archbishop Ussher. It was only with the discovery of radioactivity in the late nineteenth century, and the development of radiometric dating techniques in the twentieth that the true extent of geologic time became known.

Radioactivity was discovered by Henri Becquerel (1852-1908) and Marie (1867-1935) and Pierre (1859-1906) Curie in the final years of the nineteenth century. In 1904, Ernest Rutherford (1871-1937) suggested that radioactive decay might be used to determine the age of rocks. As quickly as 1905, the first crude estimates of the absolute ages of the planet's rocks using radioactive minerals had been made, independently, by John Strutt (later, Lord Rayleigh; 1842-1919) and Bertram Boltwood (1870-1927), and some of the rocks dated clocked in at the astonishing age of two billion years (Albritton, 1986).

The technique of employing radioactive decay to determine the age of rocks (and other materials) is called radiometric dating. It rapidly became an extremely important tool for geologists, who needed to find the absolute ages for the events they saw recorded in the rock and fossil records. Arthur Holmes (1890-1965) and others explored and improved the radiometric dating technique in the early-to mid-twentieth century; still others employ it widely today, both in geology and archaeology.

Contemporary researchers are engaged in their own attempts to plumb the immensity of geologic time. To do so, they use radioactive elements, which decay at steady rates. Over great lengths of time, uranium turns to lead. Over time, certain forms (isotopes is the proper term) of the gas argon change to other isotopes of argon, and to potassium. Other radioactive elements also change, each at its own rate. Most elements (and the isotopes of elements) are not radioactive, but those that are provide scientists with a means for exploring geologic time.

The principles behind radiometric dating are easy to understand. Consider this analogy: in my refrigerator stands a container of orange juice. Each day with breakfast I have an eight ounce glass of OJ. You come by, notice that the juice container, once full, is now partially empty. You wonder how long it has been since I started drinking the juice from the container.

The calculation is an easy one. You simply check the label to find out how much was originally there, and then measure how much is gone. Because you know the rate at which I have been drinking the OJ, you can determine how many days have passed since the container was full. Measuring the amount of time which has passed by using radioactive decay is a very similar process. The principle is just the same: one determines how much was there originally, learns the rate of decay (like the rate of consumption of the OJ) and figures out how much is gone. From that, it's simple math to calculate how much time has passed.

But while the principle is easy, its application for the determination of the passage of geologic time is quite difficult. First of all, radioactive decay does not occur in precisely the same way as the consumption of orange juice. In the orange juice analogy, I consumed a specific amount of juice each day. Radioactive decay works in a different way: in a specific period of time, called the half-life, half of the original radioactive material decays into another isotope. In an equal amount of time, half of the remaining radioactive material decays. In yet another equal amount of time, still another half of the remaining radioactive material decays. And so on.

Obviously, though one can keep dividing the remaining material in half, and half, and yet again in half, one can never get to the end of the process until the last radioactive atom decays. (This protracted decay is one of the reasons radioactive materials can be so dangerous: the radioactivity can last a long time.) But one can get to the point where there is so little of the original radioactive material remaining that it becomes impossible to accurately measure it. At this point, one can no longer use that particular radioactive material to measure the amount of time which has elapsed since the decay process began. This is why those scientists who measure geologic time generally employ radioactive materials which decay over very long periods of time (that is, those which have long half-lives) for their calculations.

Radioactive Decay. The "half-life" of a given radioactive substance is the amount of time it takes for half of the atoms of that substance to be transformed into another substance. A substance's half-life may be a tiny fraction of a second up to billions of years, but all radioactive substances decay in the same way. If a substance has a half-life of 10 minutes (or days, or years, or billions of years), for example, after 10 minutes (or days, or years, or billions of years), half will have decayed into something else. After another 10 minutes (or days, or years, or billions of years), another half will be gone. This graph shows how much of the original substance is gone after each unit of half-life, whatever that unit (minutes, days, or years, or billions of years) may be for the particular radioactive substance. (Abbott, 1996, p. 5)

In determining geologic ages, the exact rate of decay of the radioactive material used must be figured. Rocks containing the necessary radioactive materials must be located and processed. Tiny amounts of radioactive materials must be extracted from these rocks, and those amounts measured with enormous precision. Contamination from other possible sources must be excluded. Extremely sophisticated instruments and highly trained and experienced personnel need to be employed in such endeavors. Even so, determinations of exactly when particular geologic events took place often can only be established within about 1% accuracy, though precision has increased greatly within the past decade.

A dating accuracy to within about a percent, however, has given us extraordinary insight into the history of our planet and the history of its life. Earth's existence goes back some 4.6 billion -- that is, 4,600,000,000 -- years, back to the formation of the solar system. For the first 800 million years of its existence, Earth was periodically subjected to severe bombardment by rocky, gaseous and dusty objects of all sizes. Some were simply the debris left over from the formation of the solar system, others more consolidated meteors or comets or asteroids, and at least one other, a protoplanet about half the size of Earth itself, which early in our history (probably within 50 million years of the formation of the Earth) smashed our planet with such force that a huge mass of molten material spurted out and solidified into our Moon. About 3.8 billion years ago, the first rudimentary living things came into existence

Attempting to conceive of these stretches of time is a daunting, possibly ultimately impossible task. Geologic time is simply immense. Think, for example, of the founding of our nation, back in 1776. Back then, people used horses for transportation, used muskets for firearms, lit their homes with candles and oil lamps, cooked their food and heated their homes with wood or sometimes peat in their fireplaces. There weren't telephones, cars, computers, planes, the assembly line, atomic bombs, television, skyscrapers, indoor plumbing, or electricity brought through wires to power lamps, stoves, heaters and air conditioning.

Yet if we use an average American lifespan of about 75 years as a rough yardstick, the beginning of our independent existence as a nation in 1776 took place only three lifespans ago, laid end-to-end. The Civil War was less than two lifespans ago, laid end-to-end. Women won the right to vote barely more than one human lifespan ago (1920). Indeed, there are many women alive today who were born before American women had the right to vote in national elections.

Ten lifespans (750 years) ago, Europeans had not yet discovered the New World, and Europe itself was pretty much a backwater, despite having mounted several largely unsuccessful religious Crusades against Muslims in the Holy Land. Mongols were threatening Europe's eastern frontiers, and the bubonic plague lay more than a hundred years in the future.
The Abbasid Caliphate, which represented one of the high points of Islamic civilization, had reached its zenith of cultural and scientific achievement, and was in its last days. The destruction by the Mongols of the caliphate capital, Baghdad, home to perhaps a million people, lay just ahead.
The Ch'in empire of northern China had recently been conquered by the Mongols; the conquest of the southern Chinese Sung empire by Kublai Khan also lay only a few years in the future.

One hundred lifespans (7500) ago, most people lived in small agricultural villages. Animals were only beginning to be domesticated. Writing had not yet been invented, nor metal technology. What some geologists believe was the great flood that inspired the Biblical tale of the deluge of Noah was just inundating the ancient agricultural communities along the Black Sea coast.

Thought of in this way, human history, despite its own enormity -- with all the names and dates of presidents, wars, exploration, migration and settlement, the birth and death of empires -- seems very short. Now compare geologic time. The Earth was formed more than 60 million lifespans ago. The first living things appeared about 50 million lifespans ago. The first animals with skeletons, more than 7 million of our yardsticks ago. The extinction of the dinosaurs, almost a million. There is nothing in our experience or imagination -- despite our mere "three score years and ten" [plus five] years being packed with events: early childhood, elementary, junior high, high school and college, the family life we are born into and the family life we may create for ourselves, the various jobs and places we may live or visit -- which prepares us to deal with such enormities of time.

The 18th century naturalist Georges-Louis Buffon (1707-88), contemplating the enormous lengths of geologic time that he and his contemporaries were beginning to discover, wrote, "Why does the human mind seem to lose itself in the length of time...? Is it not that being accustomed to our short existence we consider one hundred years a long time, and have difficulties forming an idea of one thousand, cannot imagine ten thousand years, or even conceive of one hundred thousand years?" The mental difficulty people faced in comprehending vast lengths of time affected Buffon's own calculations. He found that it was "necessary to shorten it as much as possible to conform to the limits of human intelligence" (Roger, 1997, p. 412).

But even Darwin emphasized that "it is highly important for us to gain some notion, however imperfect, of the lapse of years" to understand Earth and life itself. To illustrate the vastness of geologic time, he provided the following exercise:

"Few of us, however, know what a million really means: Mr. Croll gives the following illustration: take a narrow strip of paper, 83 feet 4 inches in length, and stretch it along the wall of a large hall; then mark off at one end a tenth of an inch. This tenth of an inch will represent one hundred years, and the entire strip a million years. But let it be borne in mind, in relation to the subject of this work, what a hundred years implies, represented as it is by a measure utterly insignificant in a hall of the above dimensions" (Origin of Species, 1859, Chapter X).

In order to comprehend the history of our planet, and the history of life, however, we need ways to measure geologic time, and to place milestones along this temporal route. Walter Alvarez, a geologist at the University of California at Berkeley, has suggested one approach. He proposes thinking of geologic time in million year intervals, as indicated:

Millions of years ago (Ma) Event(s)

4590 Formation of the Earth

about 4580 Collision of large protoplanet with Earth; formation of the Moon

about 4400 Early continents

about 3800 First evidence of living things

about 2500 Great increase of oxygen in the atmosphere; evolution of the first cells with enclosed nuclei (eukaryotic cells)

about 560 Soft-bodied, multi-cellular organisms appear

543 Animals with hard parts appear in fossil record; almost all major groups of animals appear within next 10 million years. This marks the beginning of the Paleozoic, the age of "old life," commonly called the "Age of Trilobites."

250 Paleozoic ends with mass extinction; the Mesozoic, the age of "middle life," commonly called the "Age of Dinosaurs," begins.

65 Mesozoic ends with mass extinction; the Cenozoic, the age of "recent life," commonly called the "Age of Mammals," begins.

1/4 Homo sapiens appears

Clearly, however, reaching back through enormous quantities of time to attempt to elucidate exactly what happened millions of years ago, and why particular events happened, are formidable tasks.

Geologic Time and the History of Life

Though our own species has only been around for about a quarter of a million years, the solar system condensed out of gas and dust about 4.6 billion years ago. For about the first 800 million years of its existence, Earth was bombarded with comets, meteors, and similar cosmic debris. Consequently, though life may have evolved earlier, the first real evidence of life didn't appear until about 3.8 billion years ago. Single-celled organisms were the only living things on Earth for billions of years. The first multicellular organisms, which probably evolved between 1.5 and 1 billion years ago, were quite small and lacked hard parts, and like the earlier single-celled organisms, left few traces. Larger soft-bodied life forms evolved about 560 million years ago, but the first organisms with skeletons -- thus able to leave behind solid fossils of themselves -- only evolved just prior to some 543 million years ago.

The sudden appearance in the fossil record of creatures with hard parts, at 543 million years ago, marks a biological change so significant that geologists and paleontologists have marked that point on geological time as perhaps the most important in the history of the planet. The first geological time period containing those hard-part fossils is called the Cambrian, and the entire span of geologic time which preceded it as Precambrian (sometimes written as Pre-Cambrian). The 543 million years from the beginning of the Cambrian to the present is called the Phanerozoic Eon, the eon of "visible life."

The Cambrian Period represents the start of the Paleozoic Era (543 to 250 million years ago; millions of years ago is indicated by geologists as Ma), the era of "ancient life," frequently referred to colloquially as the "Age of Trilobites." The Paleozoic was characterized by the evolution of many kinds of marine invertebrates, the first fish, the emergence of advanced life forms from the sea onto the land, the first forests, and the rise of early mammals and dinosaur ancestors. The Paleozoic Era came to a close with the end of the Permian Period, about 250 million years ago, in the greatest mass extinction of the Phanerozoic.

The Paleozoic was followed by the Mesozoic Era (250 to 65 Ma), the era of "middle life," known colloquially as the "Age of Dinosaurs." This period was characterized by the emergence of the dinosaurs, large fish-like marine reptiles, flying reptiles (pterosaurs), social insects such as bees and ants, and the first birds and flowering plants. (It turns out the earliest (most ancient) flowering plant, Archaefructus sinensis, and the earliest placental mammal, Eomaia, both come from the same Mesozoic 125-million-year-old Yixian Formation in Liaoning, China: Sun, 2002; Ji, 2002). The Mesozoic Era also ended with a great mass extinction, about 65 million years ago, which killed off all dinosaurs, large marine reptiles, flying reptiles, and lots of other organisms.

Last of these eras is the Cenozoic Era (65 Ma to the Present), the era of "recent life," which followed the Mesozoic. It is colloquially referred to as the "Age of Mammals," and includes all time from the end of the Mesozoic to the present. Though their ancestors existed from before the end of the Paleozoic Era, mammals were not the dominant animal life form on the land, having been outcompeted by the dinosaurs. With the demise of the dinosaurs, however, mammals became dominant on the land, while some (bats) shared the skies with birds and others (whales, dolphins, seals and other marine mammals) shared the oceans with bony fish and sharks.

A Geologic Time Chart

Geologic time is divided into eons, which in turn are divided into eras, eras into periods, periods into stages. (This is similar to our dividing years into days, days into hours, hours into minutes, minutes into seconds.)

In addition, geologic periods are often divided into "early," referring to the older portion of the period, and "late," referring to the younger, or more recent part of the period. Sometimes a "middle" interval is also employed.

Because real geologists deal with real rocks, they also use another terminology that reflects the placement of rocks as they are normally found in rock formations. As the "early" rocks from a given period have been buried longer, they are found lower in their formations, while rocks from "late" in the period are found above, in the upper parts of formations. Thus "lower" = "early," and "upper" = "late." Wherever possible, I have employed the terms early and late.

Geologic Time Scale. This scale indicates the major divisions of geologic time from the formation of the Earth, 4600 million years ago (that is, 4.6 billion years ago) to the present. The major divisions are referred to as Eons. Although life began sometime in the Archean (perhaps around 3500 million years ago), it remained as single cells for perhaps 2000 million years. Only in the Mesoproterozoic did multicellular forms evolve, but even they remained microscopic, and without hard-part fossils, until just before the Phanerozoic. The Phanerozoic is the Eon of "visible life," during which life has left sufficient fossils that scientists may trace its development.

Geologic Time Scale showing the time divisions of the Phanerozoic. The Phanerozoic is divided into three Eras, and the Eras into Periods.

Geologic Time Scale showing the divisions of the Cenozoic. The Cenozoic is divided into two Periods, and they are divided into Epochs. Years before present are approximate in all of these time charts.



Mass Extinctions

When the miners and naturalists of western Europe began to carefully examine rocks and fossils over two centuries ago, they noted some important geologic characteristics. Certain rock types -- those called igneous and metamorphic -- did not seem to contain fossils. Because these rocks frequently contained crystals (as well as mineral ores), they were referred to as crystalline rocks. The crystalline rocks yielded no clues to their ages (though they seemed to have been around for a long time), because they had not come into existence by processes like sedimentation, where older sediments were covered by more recent ones. Moreover, containing no fossils, they revealed no secrets about the history of life. These rocks, often found in mountainous areas, were designated by the term "Primary."

The more lithified (hardened) sedimentary rocks, seemingly later in origin, were assigned to a "Secondary" period. These rocks, often found in the hillier regions, frequently contained fossils. Although 200 years ago it was not yet clear that these fossils formed an evolutionary progression, it was nonetheless becoming obvious that certain fossil organisms were often found in related sedimentary rocks but were absent on others, where an entirely different fauna could be found.

Less lithified but nonetheless fossiliferous sedimentary deposits often including muds and gravels, were found in flat, low-lying areas. These were referred to as "Tertiary" deposits. (These terms seem to have first been used by Giovanni Arduino in 1760: Cutler, 2003, p. 196.) But even these relatively young deposits were cut by still more recent river valleys, which earned the designation "Quaternary." Thus, early on in the more systematic examination of its geology, Earth's history was divided into four different ages. Later, some sedimentary rocks were recognized to be of Primary age, and a long "Transition" period was introduced between the Primary and Secondary ages (Phillips, 1840).

Today we do not today employ this same terminology, though the terms Tertiary and Quaternary -- for the third and fourth ages -- live on. What is noteworthy, however, is that some of the basic divisions and successions of geologic time that we recognize today were also noted even during the first pioneering attempts to understand earth history, more than a century and a half ago. The divisions noted early on remain the fundamental divisions of geologic time, largely on the basis of the absence or presence of particular fossilized organisms.

The rocks that once were assigned to the Primary age are now largely considered to be of Precambrian origin (before 543 million years ago), while those of the Transition period are largely assigned to the Paleozoic Era. Two of the main divisions recognized by early nineteenth century geologists, between the Transition period and the Secondary, and between the Secondary and the Tertiary, are roughly reflected in our own divisions of the Phanerozoic between Paleozoic and Mesozoic, and Mesozoic and Cenozoic. As the paleontologists of the nineteenth century discovered and classified the unfamiliar fossils of the creatures of the Phanerozoic, the differences in characteristics between Transition, Secondary, and Tertiary (roughly, the Paleozoic, Mesozoic, and Cenozoic) faunas stood out ever more clearly.

There was a certain amount of continuity between the biotas of different time periods, to be sure, but there were also striking differences. Dinosaurs, for example, first identified in the 1840s, were found in Secondary (Mesozoic) strata, but were absent from later rocks. Mammals, on the other hand, were present in the Tertiary, but were generally absent during the Secondary, and neither mammals nor dinosaurs were present in the Transition. Hence, as the modern terminology -- Paleozoic, Mesozoic, Cenozoic -- came to be adopted, it reflected the growing understanding that significant biotic changes characterized the history of life, and that there were major, perhaps catastrophic, biological divisions between one geologic interval and the next. The French paleontologist Georges Cuvier (1769-1832), in fact, described these enormous biotic changes as "revolutions."

Charles Darwin (1809-82) and others, however, were troubled by these abrupt biotic changes. Instead of accepting the apparent abruptness of these significant biological changes which "catastrophists" like Cuvier simply recognized as part of the fossil record, Darwin and other important figures of nineteenth century geology like Charles Lyell (1797-1875) were "uniformitarians," who believed in the gradual transformation of both rock and organisms. The big biological shifts needed explanation, and Darwin (Origin of Species, 1859, Chapter X) and others suggested they were due to the fact that the fossil record was incomplete.

The incompleteness of the fossil record is an enduring problem with which paleontologists have had to wrestle. One of its causes, when it is considered carefully, is obvious: the soft parts of dead organisms decay rapidly, and usually only the hard parts -- shells, bones, teeth, plant cellulose -- remain. But even these hard parts disappear: they are gnawed upon by rodents or larger bone-crushing animals like hyenas; they dry and bleach and fall to dust under the sun; they get slowly dissolved by even slightly acidic rain. Small creatures may be consumed whole; organisms with small populations leave few bones at all.

Much has been made. of course, of the incompleteness of the geologic and fossil records by those opposed to Darwin's theory of evolution. They point to the lack of "missing links" between older and more recent organisms, for example, and especially to the small number of fossils that link man with other great apes.

But their objections are not based on common sense. Think of your own ancestors. You had two parents, and four grandparents. Each generation back, the number doubles (barring intermarriage and incest): eight great grandparents, sixteen great great grandparents, and so on. Were you to try to go back and find the remains of ancestors further and further removed in time, how successful do you think you would be, even with the best genealogical tools? Some of your ancestors died in battles and their corpses were consumed by scavengers. Some drowned in rivers, lakes or the ocean: where are their remains now? Some died in deserts and their bones were gnawed on by rats and mice for the calcium they contained. Some died in fires so hot their bones were turned to ash, just as if they were cremated. Some were buried peacefully in quiet churchyards, only to have floods bury the graveyards themselves. A few may have become part of someone else's stew.

The further back in time one would go, looking for the remains of one's ancestors, the less likely it is that one would find their undisturbed graves; and the more likely it is that they would have disappeared without a trace. Most of us would be fortunate to find any remains older than a few centuries, much less from thousands of years ago. Going back tens of thousands, or hundreds of thousands, of years, to when there were far fewer people on the planet, and they were far more dispersed, there must be far fewer remains, and our recovery of them far more likely to be a matter of luck. The lack of success in finding ever more remote ancestors, however, does not mean that they did not exist. Our being here confirms that they were indeed present.

That, simply put, is why paleontologists find so few fossils of our ancient ancestors, or any of the so-called "missing links." Those we do find, of course, are highly prized. And over time paleontologists do find more and more bits of fossil evidence which provide clues as to our evolutionary heritage.

But while some gaps in the fossil record have gotten filled, and the lineages of particular creatures have become ever more clear, significant gaps still remain. Moreover, some of these gaps involve not just a few organisms, but extremely large numbers. In addition, at the time of these particular gaps, many organisms seem to have left no descendants whatsoever, and their lineages have become extinct, as if a kind of biological holocaust had taken place.

Gaps in the fossil record -- a few major ones and a myriad minor ones -- turn out to be normal and expected in the history of any given species. So too, the coming into existence of new species, and the extinction of old ones, just like the regular appearance of birth and death notices for individual human beings found in the daily newspapers, represent the normal unfolding of the history of life. The regular demise of species is so familiar to paleontologists that the process has even been given a name: background extinction. Similarly, though without a specific name, is the equivalent background origination of species.

Species generally don't last very long, at least by geological standards: the average terrestrial species is believed to last for about a million years; marine organisms, presumably because of their more stable environments, may last an average of ten million years. Species, therefore, are constantly winking into and winking out of existence, like the flashes of fireflies over cornfields in the warm Midwestern summer dusk.

Periodically, however, there are extraordinary episodes of species mortality. They involve the demise of huge numbers of types of organisms, possibly millions at one time (the number of species on the planet has been estimated to be between a million and ten million; these episodes commonly claim more than 50%). As a consequence of this colossal mortality, they create unprecedented opportunities for surviving organisms, as decimated as the survivors themselves well may be. But despite the fact that these episodes serve as major transitions in the history of life, opening the world for a whole different suite of organisms, it is the massiveness of the biological destruction which initially captured scientific imaginations. Consequently, paleontologists refer to these biological holocausts as mass extinctions.

As the knowledge of the history of life increased during the nineteenth and twentieth centuries, the major divisions that had been noted early on, between the biotas (living things, taken as a group) of the Paleozoic Era and the Mesozoic, and between the Mesozoic Era and the Cenozoic, became ever more clear. More and more fossils of known organisms were found, as well as the fossils of an increasing number of unknown organisms. Together with greater precision in identifying the various geologic formations of which the organisms were part, and more precise dating of those formations, the patterns of the history of life emerged very distinctly.

The recently deceased John Sepkoski (1948-99) of the University of Chicago developed statistical methods to evaluate the accumulating data from the fossil record. His analysis confirmed that mass extinctions had occurred both at the end of the Paleozoic and the end of the Mesozoic. These mass extinctions are generally referred to by the names of the geologic time periods which end those eras: the end-Permian (the Permian being the last geologic period of the Paleozoic Era) and the end-Cretaceous (the Cretaceous being the last geologic period of the Mesozoic). The geologic demarcation ("boundary") between the Mesozoic and the Cenozoic (the first period of which is the Tertiary) is referred to as the K-T boundary, for the Cretaceous (indicated by the geologic symbol "K") and the Tertiary ("T").

In addition, Sepkoski confirmed mass extinction events at the end of the Ordovician Period (about 443 million years ago; geologists use the abbreviation "Ma" for "millions of years ago"), in the Late Devonian Period (about 354 Ma), both in the Paleozoic Era, and at the end of the Triassic Period (about 206 Ma), during the Mesozoic. All told, some five major mass extinctions punctuate the history of life. (The middle Cambrian, end-Botoman extinction, discovered by Phil Signor in the early 1990s, also ranks as a major mass extinction, but it has not yet been incorporated by most paleontologists into their ordinary thinking about mass extinctions. This may be because the extinction came so early in the Phanerozoic, at a time when there were no fully terrestrial organisms, and few creatures that resembled modern forms.) There are several smaller but yet still massive extinctions identified in the fossil record, perhaps the most important of which is that marking the end of the Paleocene Stage (about 55 Ma).

The Greatest Mass Extinctions and Selected Lesser Extinction Events

 520 Ma  middle Cambrian (end Botoman)
 443 Ma  end Ordovician
 354 Ma  late Devonian
 250 Ma  end Permian
 206 Ma  end Triassic
 183 Ma  early Jurassic (early Toarcian)
 65 Ma  end Cretaceous
 4.8 Ma  end Paleocene
 33.7 Ma  end Eocene

(Ma = millions of years ago)
The five major mass extinctions are indicated in bold.

Mass extinctions are a prominent though rare feature of the history of life. Though we don't know about specific extinction events before the eon of visible life (the Phanerozoic), that is, prior to 543 Ma, we can assume that even then living things faced repeated crises during which large numbers of organisms perished. During the eon of visible life, however, the large numbers of fossils, and the relative completeness of the fossil record allows us to identify numerous massive die-offs in which a large percentage of species disappeared from the planet. Each of the five major mass extinctions killed off from about 60% to 90% of the then existing species, and the several lesser but still dramatic extinctions took smaller tolls.

Extinctions during the Phanerozoic, the last 543 million years of geologic time. The letters at the bottom of the chart refer to geologic periods: C = Cambrian, O = Ordovician, S = Silurian, D = Devonian, C = Carboniferous, P = Permian, T = Triassic, J = Jurassic, K = Cretaceous, T = Tertiary. Disregard the black dots. The five greatest extinctions are those at the end of the Ordovician, in the Late Devonian, at the end of the Permian (P-T), the end of the Triassic, and the end of the Cretaceous (K-T; this is the extinction which killed off the dinosaurs). (Kerr, 1995, from Sepkoski)

The notion of a mass extinction deserves some elaboration and contemplation. There are millions of species on our planet: people and domestic cats and tigers and lions and hippos and Canada geese and sugar maples and honeysuckle and string beans and roses and violets and blue whales and rainbow trout. There are lots of species of oaks and pines and mushrooms and bacteria and grasses. There are lots and lots of different species of bees, butterflies, beetles, and ants, as well as lots and lots of what are known as "true" bugs. This list, clearly, is quite inadequate to mention all but a few of the almost innumerable species -- perhaps currently about ten million in all (though fewer in the past) -- which inhabit our extremely biologically diverse globe.

During the great mass extinctions, 60 to 90% of biological species became extinct. That doesn't mean that every single individual in a species was killed off in an instant (though they could be), but it does mean that enough individuals were killed that the species population fell below the limit which the species needed to reproduce itself. Within a short time, perhaps a few generations, the species was gone. What extinction means is that every single individual which belonged to that extinct species is gone. Every one is dead.

On the other hand, those species which survived a mass extinction may not have fared much better. After all, even one pregnant female, or a single clutch of eggs, or one tiny acorn or mustard seed could live and resuscitate a decimated population. Among those single-celled organisms which reproduce asexually, a single survivor among uncountable numbers of dead could prevent extinction. In a mass extinction, the only difference between a species which survives and one which does not may be that in the surviving group there was 99.99% mortality, and in the extinct group mortality was 100%. In any case, since a species may contain anywhere from about 100,000 individuals (common for many mammals) to hundreds of millions or billions or so (as for mosquitoes or ants), a mass extinction that killed off 90% of species means that probably well over 99% of all individual organisms on the planet were destroyed. The carnage caused by mass extinctions is colossal, almost unimaginable.

Enigmas of the Mass Extinctions

As the reality of mass extinctions became evident, the question inevitably arose: what could possibly have caused such enormous carnage? It wasn't as if there were a lot of corpses lying around, as in a colossal crime scene. Besides, fossils themselves, while not exactly corpses, are the dead remains of once living things. So a mass extinction doesn't mean that there are lots of corpses, but -- counterintuitively -- that there are none. The normal evidence of death, which for paleontologists is the evidence of having lived, suddenly ceases, and no further remains of the affected organisms are found.

In the rocks, mass extinctions reveal themselves merely as major biological transitions: in the older (generally deeper) strata are assemblages of particular organisms, but in younger (generally shallower) strata many of these organisms are gone, replaced by a different set of creatures. There are few clues as to the cause of the destruction. Even the amount of time a mass extinction event took -- years or millions of years -- is obscure. Possibly the cause of each mass extinction is different. The paucity of clues is compounded by the extreme age of these mass extinction events: paleontologists must solve mysteries woven into the gossamer fabric of deep time.

One of the mass extinction events is almost mythic: that of the extinction of the dinosaurs at the end of the Cretaceous, some 65 million years ago. This mass extinction not only swept an enormously successful and highly charismatic group of animals -- the dinosaurs -- to their deaths in what was perhaps a geological eye-blink, but it also killed off numerous other creatures which had been around for many tens of millions of years: marine reptiles, flying reptiles, many birds, plants and marine organisms.

The cause of this extinction has attracted the interest and conjectures of numerous paleontologists and interested amateurs. Among the many suggestions they have put forward are that the demise of the dinosaurs was due to the rise of mammals who stole and ate dinosaur eggs, or outcompeted dinosaurs in other ways, or to disease, or to a cooling climate, or to the purported "facts" that dinosaurs were sluggish or stupid. Such suggestions run up against other (real!) facts: that the early ancestors of dinosaurs, around 250 million years ago, had outcompeted early mammals for domination of the land, that some dinosaurs could move quite rapidly (a lot faster than a human being can run), that they were quite as smart as they needed to be to survive and evolve for more than 180 million years, hundreds of times longer than our own species, Homo sapiens, has been around. But most perplexing is the fact that when the dinosaurs went, lots of other, unrelated creatures went with them.

A major breakthrough came in late 1970s, when Luis and Walter Alvarez of the University of California at Berkeley began to unravel the cause of the end-Cretaceous extinction. One of the mysteries associated with this mass extinction is that it is marked in the geologic record in some places by a thin layer of clay, sandwiched between thick layers of marine limestone. The layers below -- the older layers -- include marine fossils from the time of the dinosaurs, while those above -- the younger layers -- reveal an almost entirely different assemblage of marine organisms. Clearly the clay layer in between held at least one clue to the cause of the mass extinction. If it could be determined how long the extinction took -- a short time or millions of years -- much might be learned (Alvarez, 1997).

The Alvarezes came up with an elegantly simple but brilliant solution to figuring out the length of time represented by the clay layer. When meteors hit the Earth's atmosphere, most of them burn up. This produces a gentle "rain" of very fine particles of meteoric dust that drift through the atmosphere and end up on the ground and on the ocean floor. This meteoric dust is distinguishable from ordinary terrestrially-derived sediments because of its different chemical composition: notably, it is greatly enriched in certain elements, among them the element iridium.

Reasoning that this dust fell at a fairly constant rate, similar to its rate of fall today, the Alvarezes conjectured that if they could measure the proportion of iridium in the clay layer, they could then determine how long it had taken to emplace that layer. If there was only a little iridium in the clay layer, then the layer represented a short period of sedimentation; if there was lots of iridium, then the clay layer had to have been emplaced over a considerably longer period of time.

Their results were shocking. The thin clay layer (about a centimeter, or 0.4 inches, thick) at Gubbio, Italy was quite exceptionally enriched with iridium. In fact, the proportion of iridium was many, many times greater than that found in the surrounding rocks. It appeared that the emplacement of the clay layer by sedimentation must have taken an extraordinarily long period of time: far, far greater than the Alvarezes ever expected.

But it occurred to them that there was another explanation for the stunning results. Their initial assumption had been that meteoric dust fell at a unchanging rate. Suppose that assumption was wrong. Perhaps meteoric dust did not fall at constant rate. Perhaps, great quantities of iridium could be delivered to the atmosphere in a single event, as by an impact from a very large extraterrestrial object. In fact, that began to seem to be the only plausible explanation for the huge quantity of iridium found in the clay layer.

The Alvarez proposal, published in 1980, was immediately controversial, receiving both strong support and strong objections. Many paleontologists were extremely wary of a catastrophic explanation for a mass extinction. Their skepticism had deep roots in their profession. When the science of systematic paleontology began in the late eighteenth century, it was heavily influenced by the perspectives of the Georges Cuvier, a strong Protestant supporter of the French Revolution. As a revolutionary, Cuvier saw the gaps in the geologic and fossil records as indicating similar "revolutions" in earth history. Although Cuvier believed that all species had come into existence in a single primordial Creation event, he was the first to recognize and unequivocally state that some species had become extinct in great and ancient geologic and biologic events. He saw evidence for such events in the rocks of the Paris Basin and elsewhere.

Under the influence of geologist Charles Lyell and naturalist Charles Darwin in the middle of the nineteenth century, however, those who studied the history of the earth and its former inhabitants began to understand that geologic evidence pointed to an enormous span of geologic time, during which changes in rocks and organisms could take place very gradually over extended periods. Catastrophic events were not needed to explain the evidence found in the rocks. The sizable gaps which Cuvier had noted in the geologic record and which had given rise to his catastrophist ideas could be explained by the fact that the geologic record was dreadfully incomplete.

This gradualist view of the history of the planet and its life quite supplanted the previous catastrophist perspective. As the actual duration of geologic time became known through the invention of radiometric dating at the beginning of the twentieth century, the gradualist approach triumphed. Indeed, it triumphed so completely that catastrophist ideas were relegated to the dustbin of bad science. At the time the Alvarezes's paper was published, most paleontologists were strongly -- almost reflexively -- committed to gradualist ideas.

Into this unsympathetic intellectual milieu the Alvarezes cast their catastrophist proposal. One immediate objection was, if the iridium anomaly was due to a huge impact, where was the crater?

For many years up until about the middle of the twentieth century, geologists had believed that the only objects to hit Earth were relatively small. After all, most meteors burn up in the atmosphere, sometimes brightly, occasionally spectacularly, but completely. Some do not burn up, however, but the larger identified meteorites weighed only a few tons, and the largest just 66 tons.

Looking at the moon, some astronomers had wondered whether the ubiquitous craters could be due to impacts rather than volcanoes. Volcanoes were legitimate candidates as the cause of lunar craters: there was certainly evidence of immense lava flows in the vast but dry lunar seas, and there were even occasional reports of light in craters, as if molten lava were still being erupted. But most astronomers thought that the craters were caused by impacts, impacts which the Earth somehow had been fortunate enough to have escaped. That the Earth, our home planet, with far greater mass and therefore gravitational attraction than the moon (by almost 100 times) had escaped the kind of bombardment that the moon endured, was taken for granted. After all, if Earth had undergone the same sort of bombardment as the moon, where were Earth's craters?


The 50s and 60s brought immense challenges to this thinking. Beginning in the early 50s, graduate student geologist Gene Shoemaker became interested in an almost mile-wide crater in Northern Arizona. The origins of this crater were disputed. Some, including mining engineer Daniel Barringer had, shortly after the beginning of the twentieth century, declared it to be a meteorite impact crater, though there seems to be little remaining meteoritic debris that would attest to such an impact. Others thought it must have been produced by an enormous blast of steam, superheated by underlying molten rocks. There was no evidence that it had been caused by a volcanic eruption because no lava or ash was present. In fact, the rocks of the area are sedimentary: sandstone and limestone.

Barringer Crater, also called Meteor Crater, Arizona

Shoemaker found several kinds of evidence that the rocks of the crater had been altered by extreme pressure, the sort of pressure that could only be naturally produced by an impact at cosmic speed. Extreme pressures could also be produced by man, in nuclear explosions. By comparing the particular geologic features and rock alterations he found in the Arizona crater with those he noted in craters from nuclear explosions in Nevada, Shoemaker was able to demonstrate that similar pressures must be involved. In his 1960 Princeton Ph. D. thesis and a 1963 article, he conclusively showed that the crater, now called Meteor Crater or Barringer Meteor Crater, was of impact origin. We now know that this 4000 foot wide, 550 foot deep crater is the result of a twelve kilometer (seven and a half mile) per second impact by a several hundred thousand ton nickel-iron meteorite about 50,000 years ago. (According to new research, the impactor may have exploded in mid-air as a consequence of the frictional heating of its leading surface. The resulting fragment swarm continued on the same trajectory, producing the crater: Melosh and Collins, 2005.)

Right on the heels of Shoemaker's 1963 paper came astonishing evidence of the intensity of bombardment of other planets by objects in space. The increasing power of telescopes in the later nineteenth century had drawn public attention to our neighboring planets, and the Italian astronomer Giovanni Schiaparelli declared that he could see straight lines or "channels" ("canali" in Italian) on the surface of Mars. Jules Verne's science fiction novel "From the Earth to the Moon" suggested that travel to nearby worlds might be feasible.

In 1890, American astronomer Percival Lowell declared that a dying civilization on Mars maintained itself by transporting water from polar regions to more temperate climes. (Lowell's idea was presumably based on a mistranslation of Schiaparelli's "canali," and we now know that Mars is much too cold, and its atmosphere too thin, to permit liquid water to exist on its surface, though it is possible that liquid water did exist there in the far, far distant past.) Pulp science fiction magazines, and especially Edgar Rice Burroughs' series of novels about John Carter on Mars, and later the Martian adventures of Flash Gordon stoked the popular imagination. It is no wonder that one of the first destinations of American spacecraft, when technology began to catch up with fantasy, was the planet Mars.

So when the first American spaceprobe, Mariner 4, reached Mars in August 1965 and sent back television images (stills) of its surface, there was a collective gasp of shock. Mars, it seemed, was not covered with the cities and canals of a dying civilization (though most astronomers long before had rejected Lowell's proposal), nor by dark, seasonally-changing belts of vegetation (another suggestion); instead, it was covered with craters. Imagination had so colored expectations that scientists and non-scientists alike were stunned. Only one astronomer had predicted the craters, which seemed to be everywhere. (Photos from NASA.)

Mariner Photos of Mars.

Mars does have lots and lots of craters. But it also has many other features: huge volcanoes, one of which is about the size of the state of Oregon; a chasm as long as the United States is wide, that makes the Grand Canyon look minute in comparison; and an enormous basin covering much of its northern hemisphere, that billions of years ago possibly held a great but shallow ocean. (Schiaparelli's "canali" -- Lowell's "canals" -- turned out to be optical illusions caused by the poor resolving power of the telescopes then in use.) The 22 television images sent by Mariner had covered only about 1% of the Martian surface. Mariner's television camera had been programmed to provide limited coverage during the fly-by, and the selection had accidentally overemphasized cratered landscape rather than other features.

In some ways, however, the jarring images were useful, because they caused scientists to be more realistic about the forces which had shaped the planets and moons of the solar system. The lunar landings of the late 1960s and early 1970s confirmed the role of impacts: the lunar craters were also the products of impactors, not volcanoes. Obviously, if both the moon and Mars had been heavily cratered by hits from incoming bodies, it was highly unlikely that Earth had escaped a similar bombardment.

Indeed, it turns out that the Earth had been hit by many big "rocks" -- specifically comets and asteroids -- speeding in from outer space. In the decades since Shoemaker's determination regarding Meteor Crater, many more terrestrial impact craters, lots of them much greater in size -- and therefore involving much larger impactors -- have been identified. Water and wind erosion, together with vegetative cover, have served, over millions of years, to disguise most of those found on land. Because most of the Earth's surface is covered by ocean, however, many craters lie hidden beneath the waves and ocean sediment.

More than 150 terrestrial craters have now been identified, most of them larger than Meteor Crater because evidence of the smaller craters more quickly disappears. An impactor like the one which produced Meteor Crater -- weighing about a half million tons -- hits about every 2000 years, displacing 200 million tons of terrestrial rock. On average, incoming solar system debris causes a one kiloton (= 1000 tons of TNT) explosion high in the atmosphere every month. In 1908, a huge impact over Siberia flattened many hundreds of square miles of forest.

After a decade-long search, the end-Cretaceous impact crater was identified in 1991. A team of scientists had discovered a large subterranean structure underlying the northern part of the Yucatan peninsula in Mexico. Abnormal gravity measurements had indicated a deep subsurface structure centered on the town of Puerto Chicxulub (pronounced cheek-shoe-lube). The shadowy circular outline of the structure revealed in the gravity readings was more than 180 kilometers (100 miles) across. Later labeled by Walter Alvarez the "Crater of Doom," the Chicxulub crater was invisible on the surface of the earth because it was buried about a half-mile deep in sediments and limestone.

Other lines of evidence -- some identified well before the discovery of the crater itself -- confirm that an enormous impact had occurred at the K-T boundary. Numerous iridium anomalies in K-T boundary sections worldwide have been discovered. Sites in western Haiti, northeastern Mexico, and along the Brazos River in Texas reveal evidence of a colossal tsunami, a huge wall of water, thrown up in this case by the energy of the impact. Ocean Drilling Project drill cores from the Gulf of Mexico show the seafloor was swept clean of later Cretaceous sediments by the tsunami. Rock specimens from the Haitian and Mexican sites and Gulf of Mexico drilling cores contained tiny impact-produced molten droplets of glass called microtektites.

Cores drilled in the Chicxulub area by the Mexican oil company Petróleos Mexicanos (PEMEX) also contained impact-melted rocks. Numerous K-T boundary sites in the American and Canadian West, and in the Pacific to the west of Chicxulub provided grains of quartz shocked by the intense impact. Soot from continent-sized forest fires ignited by the impact has been found at several K-T localities. Fossilized spores from the ferns which briefly took over in the wake of the forest conflagrations are abundant in the strata immediately overlying the K-T boundary.

The evidence is still coming in: minute spheres of clay, the weathered remnants of microtektites, have been found in K-T boundary sediments off the New Jersey coast (Olsson, 1997). A gigantic impact-induced earthquake -- with possibly a million times more energy than that which leveled San Francisco in 1906 -- caused the world's largest-ever submarine landslide along much of the east coast of North America from the Canadian Grand Banks to Puerto Rico, according to a recent study of seafloor deposits (Norris, 2000). Impact-generated acids dissolved the bones of dinosaurs and other creatures on and in the soils of North America down to a depth of almost six feet below the K-T boundary (Pearson, 2001).

The impact hypothesis provides an abundance of killing mechanisms to explain the biological devastation the impact caused. The initial brilliant light flash, like that of a nuclear explosion, would have burned or blinded nearby creatures. The ensuing concussive shock wave through air and water would have destroyed eardrums, internal organs, and the buoyancy systems of fish and ammonites, the distant cousins of today's Chambered Nautilus.

The impact would have hurled vast quantities of vaporized rock into ballistic orbits high above the atmosphere; on falling back to earth this rock vapor would have heated the atmosphere to such an extent that it probably broiled to death many of the dinosaurs and other animals of North America. The ensuing forest fires not only devastated the forests themselves (Paine, 1999; Kring and Durda, 2003), but their inhabitants as well, not merely by burning, but also by suffocation as the fire storms exhausted available oxygen. Enormous tsunamis would have destroyed nearby coastlines. Carbonic (H¸2CO¸3), sulfuric (H¸2SO¸4), and nitric (HNO¸3) acid produced from the vaporized limestone and anhydrite (calcium sulfate) rock layers at the impact site and by the burning of atmospheric nitrogen would have rained down on the survivors for months afterwards.

After the initial heat from the infalling vaporized rock would have come the cold. The huge quantity of dust and soot in the atmosphere would have caused intense darkness and blocked sunlight from heating and lighting the globe. The dark and cold would have shut down or curtailed the activities of photosynthetic organisms, reducing the production of both marine and atmospheric oxygen and causing global famine. (The ability of this dust to suppress photosynthesis has been challenged. Although a great deal of dust was produced, and much was injected into the stratosphere, there simply would not have been enough minute ash particles to produce an interruption of photosynthesis, a careful analysis shows. The dust that was produced was more than 100 times too little to shut down photosynthesis, and even that dust was largely confined to downwind areas at roughly the same latitude. These results do demonstrate the extreme difficulty in shutting off photosynthesis. Pope, 2002.) When the dust and soot was dissipated by rain over the ensuing years, the carbon dioxide in the atmosphere would have raised worldwide temperatures for many centuries.

What may be surprising is not how many creatures perished, but how many of them survived. In the far reaches of the planet, protected by lakes or oceans or soil or rocks, seeds and spores and eggs and even many fully-grown organisms must have persevered. We know this because their descendants -- including our own ancestors -- repopulated the world; many creatures, like the dinosaurs are gone forever, but the evolutionary offspring of the dinosaurs, the birds, are with us today.

Despite the strong initial opposition to the impact proposal, and despite the continued opposition of a few holdouts, the vast majority of paleontologists now accept the Alvarez scenario. Indeed, for a while, some paleontologists became such avid enthusiasts that they thought almost every mass extinction event was due to an extraterrestrial impactor. Some proclaimed that all big extinctions were caused by big rocks falling from the sky, while little extinction events were caused by little rocks falling from the sky; these impact events seemed to occur on a periodic basis (Raup and Sepkoski, 1984; Raup and Sepkoski, 1986).

(The alleged periodicity prompted a search for a low luminosity star or massive solid object which could periodically disrupt the Oort cloud, the cloud of trillions of comets that lies far beyond the orbit of Pluto. According to the "Nemesis" hypothesis, such a disturbance would send a storm of comets into the inner solar system, of which the Earth is part. There has been no evidence to support the Nemesis proposal, and little to support the periodicity of extinction events.)

Paleontologists looked everywhere for evidence of additional mass extinction-causing impacts. By and large, they were unsuccessful, though even the flimsiest evidence was accepted. Unquestionably, there is more substantial impact evidence for some major and minor mass extinction events, but, as believing becomes seeing all too frequently, much more purported evidence has been amassed than actually will bear scientific scrutiny. But only a very few extinction events now remain on the list of those possibly the result of extraterrestrial impacts.

Nonetheless, I possess a geologic chart showing a major impact (in fact, the largest of the Phanerozoic) marking the mass extinction at the end of the Permian. There is absolutely no unambiguous evidence for such an end-Permian impact. No crater, no iridium layer, no undisputed shocked quartz, no glass spherules (or their clay successors), no tsunami deposits, and so on. As for most other major and minor mass extinction events, the greatest extinction of them all, that at the end of the Permian, has awaited explanation.

Past Worlds

When Hutton called what he saw in the rocks a "succession of worlds," he was describing particular sequences of strata, each tilted at a different angle, representing distinct episodes in the geologic evolution of the planet. In the late eighteenth century, there was little indication that living things might have undergone a similar kind of evolution, despite the fact that a few scientists, including Charles Darwin's grandfather, Erasmus Darwin, recognized biological change through time. As he wrote in his poem, The Temple of Nature (1802),

Organic life beneath the shoreless waves
Was born and nurs'd in ocean's pearly caves;
First forms minute, unseen by spheric glass,
Move on the mud, or pierce the watery mass;
These, as successive generations bloom,
New powers acquire and larger limbs assume;
Whence countless groups of vegetation spring,
And breathing realms of fin and feet and wing.

(from the University of California Museum of Paleontology website; "unseen by spheric glass" means that they were too small to be seen in the then rudimentary microscopes).

What we now understand of the enormous changes that have occurred during the history of life, however, makes Hutton's description particularly appropriate. Past ages provide glimpses of life so different from our own, so unfamiliar, that they may be best portrayed as different "worlds."

Think of those living things which constitute your own picture of modern Earth. Take plants: grass, for example. Before about 80 million years ago, there were no grasses. (Evidence for some of the earliest grasses comes from what is presumed to be fossilized dinosaur dung, dated to about 71 to 65 million years ago: Prasad, 2005; Piperno and Sues, 2005.) No lawn grass. No bamboo. No sugar cane. And no other members of the grass family: No corn. No wheat. No rice. No barley, oats, or rye.

Take flowers. Flowering plants have been around for perhaps 125 million years, perhaps slightly longer, but they weren't very common until about 100 million years ago. Before 150 million years ago, there were no flowering plants. Not only no roses, violets, daffodils, or tulips, but also none of our common vegetables, fruits, nuts or trees: No potatoes. No beans. No tomatoes. No broccoli, carrots, spinach, onions. No apples, pears, peaches, lemons, oranges. And no walnuts, no almonds, no peanuts, no chestnuts. No oaks, no maples, no elms, no palms, no magnolias.

The point is that as we look back in time, the inhabitants and plants of the landscape, as well as the creatures of the ocean, inevitably become less familiar. There are exceptions, of course: because dinosaurs and pterosaurs ("flying reptiles") have become so well known, through picture books, television and movies, they no longer appear strange. And in fact, reaching back through time, many things do remain familiar. Before 100 million years ago, for example, we see no oaks, maples, elms, palms, or magnolias, but the forests closely resembled some of our own. There were lots of cone-bearing trees: the ancestors of today's pines, firs, spruces, hemlocks, and redwoods.

But 150 million years ago, the forests consist not merely of conifers, but also trees that are unrecognizable today: the cycadeoids. The cycadeoids do have modern descendants, the cycads, but today these are not trees but squat plants only a few feet high with sprawling fronds and pineapple-like stems which do not look anything like tree trunks.

Still further back, at about 300 million years ago, the conifers and the cycad trees disappear, and the forests become even more exotic, with fern trees, and lycopod trees, ever more unlike the trees of our own day. Nonetheless even these strange forms have modern progeny, but they are much diminished in size, inconspicuous, or found only in out-of-the-way places. But before about 400 million years ago, the forests themselves disappear, and the tallest vegetation would have barely served to cover one's toes.

The distance that time imposes on our recognition of past, unfamiliar "worlds" is compounded by our own general unfamiliarity with the other inhabitants of our planet. Except for those who study organisms of exotic lands or the wide stretches of the ocean, or those who take a special interest in them, even many of the Earth's current inhabitants are likely to be unfamiliar and their significance unknown to many of us. For example, try to think of the animal with the greatest biomass. (Biomass is the total weight of all individual organisms; it doesn't apply just to the weight of a single individual.) Elephants? Well, elephants have the greatest individual weights (about 5 tons) of any of today's land animals, but they are vastly outweighed by blue whales at up to 120 tons each. But even blue whales don't have the greatest total biomass. Is the answer a group of animals that may weigh very little but have enormous, uncountable numbers? Perhaps mosquitoes or ants?

No. The actual answer is jellyfish.

The reason that most of us -- excepting marine biologists and those who have visited the Monterey Bay Aquarium in recent years to see the wonderful "Planet of the Jellies" exhibit -- don't know the answer to the question is that we tend to know less about our world and its inhabitants the further they are removed from our "neighborhood": those places we spend time in. Unless we travel into wilderness areas (or live in them), we probably haven't seen many bears outside the zoo. Cougars and wolverines are even less frequently seen. Few Americans (that is, those Americans who live in the United States) have seen monkeys in their native habitats; fewer still have seen in the wild our close primate relatives, the chimpanzees, and only a handful have observed gorillas in their home territories.

Plant species tend to be more familiar to us insofar as they are useful, as with the plants we eat or grow for their shade, beauty or other qualities in our parks or home gardens. Some are familiar because they are prominent, as sequoia or redwoods, or because we notice them in botanical gardens and plant nurseries and conservatories. Likewise, cats and dogs are everyday domestic animals, as are many other types of mammals, birds, reptiles, and fish.

If we don't actually possess some of these creatures as pets, we have seen them in pet stores, public aquariums, or zoos. We also tend to know many resident and migratory birds, including sparrows, pigeons, robins, ducks, and geese, and a wide array of ever-encroaching suburban wildlife, as well as those animals which are easily observed in the great outdoors. Even television has enormously expanded our natural horizons, revealing, for example, deep sea organisms which few people have directly seen.

But there are only about 4000 mammal species on the planet, and only about 40,000 to 45,000 species of vertebrate in all. About half are fish, and some 9000 or so are birds, the rest being amphibians (frogs, toads, salamanders, newts, and legless, worm-like creatures known as caecilians) and reptiles (lizards, turtles, crocodiles, snakes, and an almost extinct group of lizard look-alikes called tuataras). Plants number about 270,000 species.

There are an estimated 10 million species on Earth, however, of which possibly 2 million have been identified, described, and named by scientists. Most of the creatures currently living on the planet, therefore, are quite unfamiliar even to specialists, much less those of us who do not search out these beings. In far-off lands, in exotic environments like the tree canopies of tropical forests, within the soil, and beneath the waves are countless organisms -- many tiny or even microscopic -- that have escaped most of our attentions.

The fact that the organisms of "worlds" of the remote past may have been vastly different from our own, however, should not detract from our recognition of the magnitude of the extinction event on the then-existing organisms. The end-Permian mass extinction extinguished such unfamiliar groups as Glossopterid trees, the reptile group Dinocephalians, and rugose corals, but these were some of the most successful creatures of their time. Most importantly, as unfamiliar as they may be, the organisms that then existed -- and survived -- are the ancestors of all of the organisms that exist today.

Despite the fact that the Permian world lacked many of today's organisms, it should not be assumed that that world was in any sense empty. In fact, the Permian world was quite full of organisms, though they would have seemed unfamiliar to modern eyes. There were plenty of trees, lots of other vegetation, and a considerable number of four-legged creatures (tetrapods) -- some, like the dinocephalian Moschops, 3 meters (ten feet) long, or longer -- wandering about.