PALEONTOLOGY

CONCEPT

Thanks to a certain 1993 blockbuster, most people know the name of at least one period in geologic history. Jurassic Park spurred widespread interest in dinosaurs and, despite its fantastic plot, encouraged popular admiration and respect for the work of paleontologists. Paleontology is the study of life-forms from the distant past, as revealed primarily through the record of fossils left on and in the earth. It is a complex and varied subdiscipline of historical geology that is tied closely to the biological sciences. As with other types of historical geology, the work of a paleontologist is similar to that of a detective investigating a case with few and deceptive clues. Reliable fossil samples are more rare, compared with the vast number of species that have lived on Earth, than one might imagine. Furthermore, several factors pose challenges for paleontologists attempting to interpret the fossil record. Nonetheless, paleontologic research has led to a growing understanding of how life emerged, how Earth has changed, and how vast animal populations became extinct over relatively short periods of time.

HOW IT WORKS

Thinking in Terms of Geologic Time

The term geologic time refers to the great sweep of Earth's history, a timescale that dwarfs the span of human existence. The essays Historical Geology and Geologic Time offer several comparisons to emphasize the proportions involved and to illustrate the very short period during which human life has existed on this planet.

As one example shows, if all of geologic time were compressed into a single year, the first Homo sapiens would have appeared on the scene at about 8:00 p.m. on December 31. Human civilization, which dates back about 5,500 years (a millennium before the building of Egypt's great pyramids) would have emerged within the last minute of the year.

In another example, geologic time is compared to the distance from Los Angeles to New York City. On this scale, the period of time in which humans have existed on the planet would be equivalent to the distance from New York's Central Park to the Empire State Building, or less than 2 mi. (3.2 km). The history of human civilization, on the other hand, would be less than 16 ft. (4.9 m) long.

THE "ABYSS OF TIME."

Needless to say, the scope of geologic time compared with the units with which we are accustomed to measuring our lives (or even the history of our civilization) is more than a little intimidating. This fact perhaps was best expressed by the Scottish geologist John Playfair (1748-1819), friend and countryman of the "father of geology," James Hutton (1726-1797). At a time when many people were content to believe that the Earth had been around no more than 6,000 years (see Historical Geology), Hutton suggested that to undergo the complex processes that had shaped its landforms, the planet had to be much, much older. Commenting on Hutton's discoveries, Playfair said, "The mind seemed to grow giddy by looking so far into the abyss of time."

Carbon: The Meaning of "Life"

A discussion of life on Earth requires us to go deep into this "abyss," though not nearly as far back as the planet's origins. It does appear that life on Earth existed at a very early point, but in this context "life" refers merely to molecules of carbon-based matter capable of replicating themselves. Knowledge of these very early forms is extremely limited.

Carbon appears in all living things, in things that were once living, and in materials produced by living things (for example, sap, blood, and urine). Hence, the term organic, which once meant only living matter, refers to almost all types of material containing carbon. The only carbon-containing materials that are not considered organic are oxides, such as carbon dioxide and carbon monoxide, and carbonates, a class of minerals that is extremely abundant on Earth.

Precambrian Time

We will return to the subject of carbon, which plays a role in one technique for dating relatively recent items or phenomena. For the present, however, let us set our bearings for a discussion of the Phanerozoic eon, the fourth and last of the major divisions of geologic time. Though extremely primitive life-forms existed before the Phanerozoic eon, the vast majority of species have evolved since it began, and consequently paleontological work is concerned primarily with the Phanerozoic eon.

The divisions of geologic time are not arranged in terms of strict mathematical relationships of the type to which we are accustomed, for example, ten years in a decade, ten decades in a century, and so on. Instead, each era consists of two or more periods, each period consists of two or more epochs, and so on. The first 4,000 million years or so of Earth's existence (abbreviated as 4,000 Ma, or 4 Ga) are known as Precambrian time. In discussing this period of time, the vast majority of the planet's history, it is seldom necessary to speak of geologic time divisions smaller than the largest unit, the eon. Precambrian time consisted of three eons, the Hadean or Priscoan, Archaean, and Proterozoic.

THE FIRST THREE EONS.

The Hadean (sometimes called the Priscoan and dating to about 4,560 Ma to 4,000 Ma ago) saw the formation of the planet and the beginnings of the oceans and an early form of atmosphere that consisted primarily of carbon dioxide. It was during this eon that the carbon-based matter referred earlier made its appearance, perhaps by means of the meteorites that bombarded the planet during that long-ago time.

In the Archaean eon (about 4,000 Ma to 2,500 Ma ago) the first clear evidence of life appeared in the form of microorganisms. These were prokaryotes, or cells without a nucleus, which eventually were followed by eukaryotes, or cells with a nucleus. Many of the prerequisites for life as we know it were established during this time, though our present oxygen-containing atmosphere still lay far in the future.

Longest of the four eons was the Proterozoic eon (about 2,500 Ma to 545 Ma). This phase saw the beginnings of very basic forms of plant life, while oxygen in the atmosphere assumed about 4% of its present levels. Animal life, meanwhile, still consisted primarily of eukaryotes.

The Phanerozoic Eon

The majority of paleontologic history has taken place during the Phanerozoic eon. In the course of this essay, we discuss its eras and periods (the second-and third-longest spans of geologic time, respectively) as they relate to life on Earth. The three Phanerozoic eras are as follows:

Eras of the Phanerozoic Eon

• Paleozoic (about 545-248.2 Ma)
• Mesozoic (about 248.2-65 Ma)
• Cenozoic (about 65 Ma-present)

Within these eras are the following periods:

Periods of the Paleozoic Era

• Cambrian (about 545-495 Ma)
• Ordovician (about 495-443 Ma)
• Silurian (about 443-417 Ma)
• Devonian (about 417-354 Ma)
• Carboniferous (about 354-290 Ma)
• Permian (about 290-248.2 Ma)

Periods of the Mesozoic Era

• Triassic (about 248.2-205.7 Ma)
• Jurassic (about 205.7-142 Ma)
• Cretaceous (about 142-65 Ma)

Periods of the Cenozoic Era

• Palaeogene (about 65-23.8 Ma)
• Neogene (about 23.8-1.8 Ma)
• Quaternary (about 1.8 Ma to the present)

The Carboniferous period of the Paleozoic era usually is divided into two subperiods, the Mississippian (about 354 to 323 Ma) and the Pennsylvanian (about 323-290 Ma). In addition, the Palaeogene and Neogene periods of the Cenozoic era often are lumped together as a subera called the Tertiary. By substituting that name for those of the two periods, it is possible to use a time-honored mnemonic device by which geology students have memorized the names of the 11 Phanerozoic periods: "Camels Ordinarily Sit Down Carefully; Perhaps Their Joints Creak Tremendously Quietly."

EPOCHS OF THE CENOZOIC ERA.

An epoch is the fourth-largest division of geologic time and is, for the most part, the smallest one with which we will be concerned. (There are two smaller categories, the age and the chron.) Listed here are the epochs of the Cenozoic era from the most distant to the Holocene, in which we are now living. Their names are derived from Greek words whose meanings are provided:

Epochs of the Cenozoic Era

• Paleocene (about 65-54.8 Ma): "early dawn of the recent"
• Eocene (about 54.8-33.7 Ma): "dawn of the recent"
• Oligocene (about 33.7-23.8 Ma): "slightly recent"
• Miocene (about 23.8-5.3 Ma): "less recent"
• Pliocene (about 5.3-1.8 Ma): "more recent"
• Pleistocene (about 1.8-0.01 Ma): "most recent"
• Holocene (about 0.01 Ma to present): "wholly recent"

A Brief Overview of Paleontologic History

The title "A Brief Overview of Paleontologic History" is almost a contradiction in terms, since virtually nothing about the history of Earth has been brief. Moreover, the history of life on Earth is so filled with detail and complexity that it could fill many books, as indeed it has. Owing to that complexity, anything approaching an exhaustive treatment of the subject would burden the reader with so much technical terminology that it would obscure the larger overview of paleontology and the materials of the paleontologist's work. Therefore, only the most cursory of treatments is possible, or indeed warranted, in the present context. For additional detail, the reader is invited to consult other texts, including those listed in the suggested reading section at the end of this essay.

As with many another process, the evolution of organisms was exceedingly slow in the beginning (and here the comparative term slow refers even to the standards of geologic time), but it sped up considerably over the course of Earth's history. This is not to suggest that the development of life-forms has been a steady process; on the contrary, it has been punctuated by mass extinctions, discussed at the conclusion of this essay. Nonetheless, it is correct to say that during the first 80%-90% of Earth's history, the few existing life-forms underwent an extremely slow process of change.

PRECAMBRIAN AND PALEOZOIC LIFE-FORMS.

Life existed in Precambrian time, as noted, but over the course of those four billion years, it evolved only to the level of single-cell microorganisms. Samples of these organisms have been found in the fossil record, but the fossilized history of life on Earth really began in earnest only with the Cambrian period at the beginning of the Paleozoic era and the Phanerozoic eon. The early Cambrian period saw an explosion of invertebrate (without an internal skeleton) marine forms, which dominated from about 545 Ma-417 Ma ago. By about 420 Ma-410 Ma, life had appeared on land, in the form of algae and primitive insects.

The beginning of the Devonian period (approximately 417 Ma) saw the appearance of the first vertebrates (animals with an internal skeleton), which were jawless fish. Plant life on land consisted of ferns and mosses. By the late Devonian (about 360 Ma), fish had evolved jaws, and amphibians had appeared on land. Reptiles emerged between about 320 Ma and 300 Ma, in the Pennsylvanian subperiod of the Carboniferous. In the last period of the Paleozoic era, the Permian (about 290-248.2 Ma), reptiles became the dominant land creatures.

MESOZOIC AND CENOZOIC LIFE-FORMS.

The next era, the Mesozoic (about 248.2-65 Ma), belonged to a particularly impressive form of reptile, known as the terrible lizard: the dinosaur. These creatures are divided into groups based on the shape of their hips, which were either lizardlike or birdlike. Though the lizardlike Saurischia emerged first, they lived alongside the birdlike Ornithischia throughout the late Triassic, Jurassic, and Cretaceous periods. Ornithischia were all herbivores, or plant eaters, whereas Saurischia included both herbivores and carnivores, or meat eaters. Naturally, the most fierce of the dinosaurs were carnivores, a group that included the largest carnivore ever to walk the earth, Tyrannosaurus.

Though dinosaurs receive the most attention, the Mesozoic world was alive with varied forms, including flying reptiles and birds. (In fact, dinosaurs may have been related to birds, and, in the opinion of some paleontologists, they may have been warm-blooded, like birds and mammals, rather than cold-blooded, like other reptiles.) Botanical life included grasses, flowering plants, and trees of both the deciduous (leaf-shedding) and coniferous (cone-bearing) varieties.

A violent event, discussed in the context of mass extinction later in this essay, brought an end to the Mesozoic era. This cleared the way for the emergence of mammalian forms at the beginning of the Cenozoic, though it still would be a long time before anything approaching an ape, let alone a human, appeared on the scene. The earliest hominid, or humanlike creature, dates back to about four million years ago, in the Pliocene epoch of the Neogene period.

Historical Geology and Paleontology

One of the two principal divisions of geology (along with physical geology) is historical geology, the study of Earth's physical history. Other subdisciplines of historical geology are stratigraphy, the study of rock layers, or strata, beneath Earth's surface; geochronology, the study of Earth's age and the dating of specific formations in terms of geologic time; and sedimentology, the study and interpretation of sediments, including sedimentary processes and formations.

Paleontology, the investigation of life-forms from the distant past (primarily through the study of fossilized plants and animals), is another subdiscipline of historical geology. Though it is rooted in the physical sciences, it obviously crosses boundaries into the biological or life sciences as well. Related or subordinate fields include paleozoology, which focuses on the study of prehistoric animal life; paleobotany, the study of past plant life; and paleoecology, the study of the relationship between prehistoric plants and animals and their environments.

Classifying Plants and Animals

Given the close relationship between paleontology and the biological sciences, it is necessary to discuss briefly the taxonomic system applied in biology, botany, zoology, and related fields. Taxonomy is an area of biology devoted to the identification, classification, and naming of organisms. Devised in the eighteenth century by the Swedish botanist Carolus Linnaeus (1707-1778) and improved in succeeding years by many others, the taxonomic system revolutionized biology.

Linnaeus's taxonomy provided a framework for classifying known species not simply by superficial similarities but also by systemic characteristics. For example, worms and snakes have something in common on a surface level, because they are both without appendages and move by writhing on the ground. A worm is an invertebrate, however, whereas a snake is a vertebrate. The Linnaean system therefore would classify them in widely separated categories: they are not siblings or even first cousins but more like fourth cousins.

Moreover, the system created by Linnaeus gave scientists a means for classifying and thereby potentially understanding much about the history and characteristics of species as yet undiscovered. Thus, it would prove of immeasurable significance to the English naturalist Charles Darwin (1809-1882) in formulating his theory of evolution. As Darwin showed, the varieties of different organisms have increased over time, as those organisms developed characteristics that made them more adaptable to their environments. Plants and animals that failed to adapt simply became extinct, though failure to adapt is only one of several causes for extinction, as we shall see.

A BRIEF OVERVIEW.

The Linnaean system uses binomial nomenclature, or a two-part naming scheme (in Latin), to identify each separate type of organism. If a man is named John Smith, then "Smith" identifies his family, while John identifies him singularly. Likewise each variety of organism is identified by genus, equivalent to Smith, and species, analogous to John. In the Linnaean system, there are eight levels of classification, which, from most general to most specific, are kingdom, phylum, subphylum, class, order, family, genus, and species.

These levels can be illustrated by identifying a species near and dear to all of us: Homo sapiens, commonly known as humans. We belong to the animal kingdom (Animalia), the Chordata (i.e., possessing some form of central nervous system) phylum, and the Vertebrata subphylum, indicating the existence of a backbone. Within the mammal (Mammalia) class we are part of the primate (Primata) order, along with apes. Humans are distinguished further as members of the hominid (Hominoidea), or "human-like" family; the genus Homo ("man"); and the species sapiens ("wise").

Dating Materials From the Past

In studying the past, paleontologists and other earth scientists working in the field of historical geology rely on a variety of dating techniques. "Dating," in a scientific context, usually refers to any effort directed toward finding the age of a particular item or phenomenon. It may be relative, devoted to finding an item's age in relation to that of other items; or absolute, involving the determination of age in actual years or millions of years.

Among the methods of relative dating are stratigraphic dating, discussed in the essay Stratigraphy, as well as seriation, faunal dating, and pollen dating. Seriation entails analyzing the abundance of a particular item and assigning relative dates based on that abundance. Faunal dating is the use of bones from animals (fauna) to determine age, and pollen dating, or palynology, analyzes pollen deposits.

FAUNAL DATING AND PALYNOLOGY.

The concept of faunal dating emerged from early work by the English engineer and geologist William Smith (1769-1839), widely credited as the "father of stratigraphy." In particular, Smith established an important division of stratigraphy, known as biostratigraphy, that is closely tied to paleontology. While excavating land for a set of canals near London, he discovered that any given stratum, or rock layer, contains the same types of fossils, and therefore strata in two different areas can be correlated.

Smith stated this in what became known as the law of faunal succession: all samples of any given fossil species were deposited on Earth, regardless of location, at more or less the same time. As a result, if a geologist finds a stratum in one area that contains a particular fossil and another in a distant area containing the same fossil, it is possible to conclude that the strata are the same.

Pollen dating, or palynology, is based on the fact that seed-bearing plants release large numbers of pollen grains each year. As a result, pollen spreads over the surrounding area, and in many cases pollen from the distant past has been preserved. This has occurred primarily in lake beds, peat bogs, and, occasionally, in areas with cool or acidic soil. By observing the species of pollen deposited in an area, scientists are able to develop a sort of "pollen calendar," which provides information about such details as changes in climate.

DENDROCHRONOLOGY.

Scientists use relative dating when they must, but they would prefer to determine dates in an absolute sense wherever possible. Most methods of absolute dating rely on processes that are not immediately comprehensible to the average person, but there is one exception: dendrochronology, or the dating of tree rings. As almost everyone knows, trees produce one growth ring per year. There is nothing magical about this, since a year is not an abstract unit of time; rather, it is based on Earth's revolution around the Sun, during which time the planet undergoes changes in orientation that result in the four seasons, which, in turn, affect the tree's growth.

Though dendrochronology makes use of a principle familiar to most people, the work of the dendrochronologist requires detailed, often complex study. Just as the layers of rock beneath Earth's surface reveal information about past geologic events (a matter discussed in the essay Stratigraphy), tree rings can tell us much about environmental changes. Thin rings, for instance, suggest climatic anomalies and may provide clues about cataclysmic events that were understood only vaguely by the ancient humans who experienced them. (An example of this is the apparent cataclysm of a.d. 535, which is discussed Earth Systems.)

AMINO-ACID RACIMIZATION.

Dendrochronology is useful only for studying the relatively recent past, up to about 10,000 years—a span equivalent to the Holocene epoch, which began with the end of the last ice age. To investigate more distant phases of Earth's history, it is necessary to use forms of radiometric dating, which we will discuss shortly. The principles of radiometric dating, however, are illustrated by another method, amino-acid racimization.

With the exception of some microbes, living organisms incorporate only one of two forms of amino acids, known as L-forms. Once the organism dies, the L-amino acids gradually convert to D-amino acids. In the 1960s, scientists discovered that by comparing the ratios between the L-and D-forms, it was possible to date organisms that were several thousand years old. Unfortunately, it has since come to light that because of the many factors affecting the rate of amino-acid conversion, this method is less reliable than once was believed. Moisture, temperature, and pH (the relative acidity and alkalinity of a substance) all play a part, and because these factors vary so widely, amino-acid racimization no longer is used commonly.

Nonetheless, the basic principle behind amino-acid racimization plays a part in other, more reliable forms of absolute dating. Many of them are based on the fact that over time, a particular substance converts to another, mirror substance. By comparing the ratios between them, it is possible to arrive at some estimate of the amount of time that has elapsed since the organism died.

RADIOCARBON DATING.

The most significant method of absolute dating available to scientists today is radiometric dating, which is explained in detail in the essay Geologic Time. Each chemical element is distinguished by the number of protons (positively charged particles) in its atomic nucleus, but atoms of a particular element may have differing numbers of neutrons, or neutrally charged particles, in their nuclei. Such atoms are referred to as isotopes.

Certain isotopes are stable, whereas others are radioactive, meaning that they are likely to eject particles from the nucleus over time. The amount of time it takes for half the isotopes in a sample to stabilize is called its half-life. By analyzing the quantity of radioactive isotopes in a given sample that have converted to stable isotopes, it is possible to determine the age of the sample. In other situations, it is necessary to compare ratios of unstable "parent" isotopes to even more unstable "daughter" isotopes produced by the parent.

As we noted earlier, carbon is present in all living things, and thus an important means of dating available to paleontologists uses a radioactive form of carbon. All atoms of carbon have six protons, and the most stable and abundant carbon isotope is carbon-12, so designated because it has six neutrons. On the other hand, carbon-14, with eight neutrons, is unstable.

When an organism is alive, it incorporates a certain ratio of carbon-12 in proportion to the (very small) amount of carbon-14 that it receives from the atmosphere. Once the organism dies, however, it stops incorporating new carbon, and the ratio between carbon-12 and carbon-14 begins to change as the carbon-14 decays to form nitrogen-14. Therefore, a scientist can use the ratios of carbon-12, carbon-14, and nitrogen-14 to estimate the age of an organic sample. This method is known as radiocarbon dating.

Carbon-14, or radiocarbon, has a half-life of 5,730 years, meaning that it is useful for analyzing only fairly recent samples. Nonetheless, it takes much longer than 5,730 years for the other half of the radiocarbon isotopes in a given sample to stabilize, and for this reason radiocarbon dating can be used with considerable accuracy for 30,000-40,000 years. Sophisticated instrumentation can extend this range even further, up to 70,000 years.

THE LIMITS OF ABSOLUTE DATING.

While 70,000 years, or 0.07 Ma, may be a long time in human terms, from the standpoint of the earth scientist, 0.07 Ma is only yesterday—the latter part of the last epoch, the Pleistocene. Other forms of radiometric dating, such as potassium-argon dating and uranium-series dating, can be used to measure truly long spans of times, in the billions of years. These methods are discussed in Geologic Time.

Potassium-argon dating and uranium-series dating can be useful to the paleontologist, inasmuch as they aid in determining the age of the geologic samples in which the remains of life-forms are found. Nothing is simple, however, when it comes to dating specimens from the distant past. After all, geologists working in the realm of stratigraphy face numerous challenges in judging the age of samples, even with these sophisticated forms of radiometric dating. (For more on this subject, see Stratigraphy.)

In fact, the work of a paleontologist is much like that of the stratigrapher. The success of either type of scientist relies more on detailed and painstaking detective work than it does on sophisticated technology. Both must analyze layered samples from the past to form a picture of the chronology in which the samples evolved, and oftentimes the apparent evidence can be deceptive. The principal difference between stratigraphic and paleontologic study relates to the materials: rocks in the first instance and fossils in the second.

REAL-LIFE APPLICATIONS

Fossils and Fossilization

The term fossil refers to the remains of any prehistoric life-form, especially those preserved in rock before the end of the last ice age. The process by which a once-living thing becomes a fossil is known as fossilization. Generally, fossilization refers to changes in the hard portions, including bones, teeth, shells, and so on. This series of changes, in which minerals are replaced by different minerals, is known as mineralization. Sometimes, soft parts may experience mineralization and thus be preserved as fossils. A deceased organism in the process of becoming a fossil is known as a subfossil.

The majority of fossils come from invertebrates, such as mussels, that possess hard parts. Generally speaking, the older and smaller the organism, the more likely it is to have experienced fossilization, though other factors (which we will discuss later) also play a part. One of the most important factors involves location: for the most part, the lower the altitude, the greater the likelihood that a region will contain fossils. The best place of all is in the ocean, particularly the ocean floor. Nonetheless, fossils have been found on every continent of Earth, and the great distances that sometimes separate samples of the same species have aided earth scientists of many fields in understanding the processes that shaped our planet.

FOSSILS AND GEOLOGIC HISTORY.

The earth beneath our feet is not standing still; rather, it is constantly moving, and over the great stretches of geologic time, the positions of the continents have shifted considerably. The details of these shifts are discussed in Plate Tectonics, an area of geologic study that explains much about the earth, from earthquakes and volcanoes to continental drift.

Paleontology has contributed to the study of plate tectonics by revealing apparent anomalies, such as fossilized dinosaur parts in Antarctica. No dinosaur could have lived on that forbidding continent, so there must be some other explanation: the continental plates themselves have moved. Long ago, the present continents were united in a "supercontinent" called Pangaea. When Pangaea split apart to form the present continents, the remains of various species were separated from one another and from the latitudes to which they were accustomed in life.

Fossilized remains of single-cell organisms have been found in rock samples as old as 3.5 Ga, and animal fossils have been located in rocks that date to the latter part of Precambrian time, as old as 1 Ga. Just as paleontologists have benefited from studies in chronostratigraphy and geochronometry, realms of stratigraphy concerned with the dating of rock samples, stratigraphers and other geologists have used fossil samples to date the rock strata in which they were found. Not all fossilized life-forms are equally suited to this purpose. Certain ones, known as index fossils or indicator species, have been associated strongly with particular intervals of geologic time. An example is the ammonoid, a mollusk that proliferated for about 350 Ma from the late Devonian to the early Cretaceous before experiencing mass extinction.

MAKING THE GRADE AS A FOSSIL.

Everything that is living eventually dies, but not nearly all living things will become fossils. And even if they do, there are numerous reasons why fossils might not be preserved in such a way as to provide meaningful evidence for a paleontologist many millions of years later. In the potential pool of candidates for fossilization, as we have noted, organisms without hard structural portions are unlikely to become fossilized. Fossilization of soft-bodied creatures sometimes occurs, however, as, for instance, at Burgess Shale in British Columbia, where environmental conditions made possible the preservation of a wide range of samples.

Furthermore, location is a powerful factor. Sedimentary rock, formed by compression and deposition (i.e., formation of deposits) on the part of other rock and mineral particles, provides the setting for many fossils. Best of all is sediment, such as sand or mud, that has not yet consolidated into harder sandstone, limestone, or other rocks. Organisms that die in upland locations are more likely to be disturbed either by wind or by scavengers, creatures that feed on the remains of living things. On the other hand, an organism at the bottom of an ocean is out of reach from most scavengers. Even at lesser depths, if the organism is in a calm, relatively scavenger-free marine environment, there is a good chance that it will be preserved.

Assuming that all the conditions are right and the dead organism is capable of undergoing fossilization, it will experience mineralization of one type or another. Living things already contain minerals, which is the reason why people take mineral supplements to augment the substances nature has placed in their bodies to preserve and extend life. In the mineralization of a fossil, the minerals in the organism's body may be replaced by other ones, or other minerals may be added to existing ones. It is also possible that both the hard and soft parts will dissolve and be replaced by a mineral cement that forms a mold that preserves the shape of the organism.

FINDING AND STUDYING FOSSILS.

Only about 30% of species are ever fossilized, a fact that scientists must take into account, because it could skew their reading of the paleontologic record. If a paleontologist judges the past only from the fossils that have been found in an area, it will result in a picture of a past environment that contained only certain species, when, in fact, others were present. Furthermore, there are many factors that contribute to the loss of fossils. For instance, if the area has been subjected to violent tectonic activity, it is likely that the sample will be destroyed partially or wholly.

The removal of a fossil from its home in the rock is a painstaking process akin to restoring a valuable piece of art. Before removing it, the paleontologist photographs the fossil and surrounding strata and records details about the environment. Only when these steps have been taken is the fossil removed. This is done with a rock saw, which is used to cut out carefully a large area surrounding the fossil. The sample is then jacketed, or wrapped in muslin with an additional layer of wet plaster, and taken to a laboratory for study.

Fossil research can reveal a great deal about the history of life on Earth, including the relationships between species or between species and their habitats. Studies of dinosaur bones have brought to light proteins that existed in the bodies of these long-gone creatures, while research on certain oxygen isotopes has aided attempts to discover whether dinosaurs were warm-blooded creatures. Thanks to advances in the understanding of DNA (deoxyribonucleic acid), which provides the genetic codes for all living things, it may be possible to make even more detailed studies in the future.

Mass Extinction

The remains of dinosaurs, of course, have an importance aside from their significance to paleontology. The bodies of these giant lizards have been deposited in the earth, where over time they became coal, peat, petroleum, and other fossil fuels. The latter are discussed in Economic Geology, but the fact that the dinosaurs disappeared at all is of particular interest to paleontology. Why are there no dinosaurs roaming the earth today? The answer appears to be that they were wiped out in a dramatic event, perhaps brought about as the result of a meteorite impact.

Numerous species have become extinct, typically as a result of their inability to adapt to changes in their natural environment. More recently, some extinctions or endangerments of species have been attributed to human activities, including hunting and the disruption of natural habitats. For the most part, however, extinction is simply a part of Earth's history, a result of the fact that nature has a way of destroying organisms that do not adapt (the "survival of the fittest"). But there have been occasions in the course of the planet's past in which vast numbers of individuals and species perished at once. A natural catastrophe may destroy a large population of individuals within a locality, a phenomenon known as mass mortality. Or mass mortality may take place on a global scale, destroying many species, in which case it is known as mass extinction.

CAUSES OF MASS EXTINCTION.

The Bible depicts an example of near mass extinction, in the form of Noah's flood, and, indeed, several instances of mass extinction have resulted from sudden and dramatic changes in ocean levels. Others have been caused by tectonic events, most notably vast volcanic eruptions that filled the atmosphere with so much dust that they caused a violent change in temperature. Scientific speculation concerning other such extinctions has pointed to events in or from space—either the explosion of a star or the impact of a meteorite on Earth—as the cause of atmospheric changes and hence mass extinction.

Even though scientists have a reasonable idea of the immediate causes of mass extinction in some cases, their understanding of the ultimate or root causes is still limited. This fact was expressed by the University of Chicago paleobiologist David M. Raup, who wrote: "The disturbing reality is that for none of the thousands of well-documented extinctions in the geologic past do we have a solid explanation of why the extinction occurred."

EXAMPLES OF MASS EXTINCTION.

The five largest known mass extinctions occurred at intervals of 50 Ma to 100 Ma over a span of time from about 435 to 65 million years ago. Most occurred at the end of a period, which is no accident, since geologists have used mass extinction as a factor in determining the parameters of a specific period.

In the late Ordovician period, about 435 Ma ago, a drop in the ocean level wiped out one-fourth of all marine families. Similarly, changes in sea level, along with climate changes, appear to have caused the destruction of one-fifth of existing marine families during the late Devonian period (about 357 Ma ago). Worst of all was the "great dying," as the extinction at the end of the Permian period (about 250 Ma) is known. Perhaps caused by a volcanic eruption in Siberia, it eliminated a staggering 96% of all species over a period of about a million years.

During the late Triassic period, about 198 million years ago, another catastrophe eliminated a quarter of marine families. Paleontologists know this, as they know about other mass extinctions, by the inordinate numbers of fossilized samples found in rock strata dating to that period. This reliance on the fossil record is also reflected in the fact that the scope of early mass extinctions usually is expressed in terms of marine life. As we have seen, the ocean environment provides the most reliable fossil record. Creatures died on land as well, but the terrestrial record is simply less reliable or less complete.

Scientific disagreement over the late-Triassic mass extinction exemplifies the fact that our knowledge of these distant events is not firmly established, but rather is subject to much scientific conjecture and dispute. (This does not mean that just any old idea can compete on an equal footing: we are talking here about differences of opinion among highly trained specialists.) At any rate, some scientists refer to the late-Triassic mass extinction as being one of the less exciting or eventful mass extinctions. Of course, it is hard to see how a mass extinction could be unexciting or uneventful, but they mean this in comparative terms; on the other hand, some paleontologists maintain that the late-Triassic was among the most devastating.

As to the cause, some theorists point to a group of impact sites spread across Canada, the northern United States, and Ukraine, places that would have been more or less contiguous at the time of the mass extinction. Difficulties in analyzing the "signatures" left by the projectiles that made these impressions have prevented theorists from saying with any degree of certainty whether it was a comet or an asteroid that caused the impact. Others, in particular a team from the University of California at Berkeley led by geologist Paul R. Renne, cite a volcanic eruption as either the cause of the mass extinction, or at least a major abetting factor to an extinction already in progress. According to Renne and his team, basalt outcroppings scattered from New Jersey to Brazil to west Africa (again, areas that would have been contiguous then) suggest that a volcanic eruption of almost inconceivable magnitude occurred about 200 million years ago. Such an eruption would surely have destroyed vast quantities of living things.

The last and best known mass extinction occurred about 65 million years ago, marking the end of the Cretaceous period—and the end of the dinosaurs. As to what happened, paleontologists and other scientists have proposed a number of theories: a rapid climate change; the emergence of new poisonous botanical species, eaten by herbivorous dinosaurs, that resulted in the passing of toxins along the food web (see Ecosystems); an inability to compete successfully with the rapidly evolving mammals; and even an epidemic disease to which the dinosaurs possessed no immunity.

Interesting as many of these theories are, none has gained anything like the widespread acceptance achieved by another scenario. According to this highly credible theory, an asteroid hit Earth, hurtling vast quantities of debris into the atmosphere, blocking out the sunlight, and greatly lowering Earth's surface temperature. Around the world, geologists have found traces of iridium deposited at a layer equivalent to the boundary between the Cretaceous and Tertiary periods, the Tertiary being the beginning of the present Cenozoic era. This is significant, because iridium seldom appears on Earth's surface—but it is found in asteroids.

HUMANS AND MASS EXTINCTION.

There have been much more recent, if less dramatic, examples of mass extinction, including those caused by the most highly developed of all life-forms: humans. Among these examples are the well-documented (and very recent) mass extinctions brought on by destruction of tropical rainforests. Such activities are killing off a vast array of organisms: according to the highly respected Harvard biologist Edward O. Wilson, some 17,500 species are disappearing each year. But cases of mass extinction are not limited to modern times.

When prehistoric hunters (the ancestors of today's Native Americans) crossed the Bering land bridge from Siberia to Alaska some 12,000 years ago, they found an array of species unknown in the Americas today. These species included mammoths and mastodons; giant bears, beaver, and bison; and even saber-toothed tigers, camels, and lions. Perhaps most remarkable of all, it appears that prehistoric America was once home to a creature that would prove to be of enormous benefit to humans until the beginning of the automotive age: the horse. Horses did not reappear in the Americas until Europeans arrived to conquer those lands after a.d. 1500.

Were Dinosaurs Warm-Blooded?

One of the most significant scientific debates of the later twentieth and early twenty-first centuries, not only in paleontology but in the earth sciences or even science itself, is the question of whether or not the dinosaurs were warm-blooded. In other words, were they like modern reptiles, which must adjust their temperature by moving into the sunlight when they are cold, and into the shade when they are too hot? Or were they more like modern birds and mammals, whose bodies generate their own heat?

A warm-blooded animal always has a more or less constant body temperature, regardless of the temperature of its environment. This is due to the fact that it produces heat by the burning of food, as well as by physical activity, and stores that heat under a layer of fat just beneath the skin. Warm-blooded animals are also capable of cooling down their bodies by perspiring and panting. Birds and mammals are the only warm-blooded animals; all others are cold-blooded. A cold-blooded creature, on the other hand, lacks control over its body temperature and therefore is warm when its environment is warm, and cold when its environment is cold.

The difference between warm-and cold-blooded animals is partly one of metabolic rate, or the rate at which nutrients are broken down and converted into energy. Cold-blooded creatures have slow metabolic rates; think of a python that swallows a medium-sized mammal whole and takes several days to digest it. The dinosaur debate is therefore often framed as a question of whether the dinosaurs' bodies had a relatively high or relatively low metabolic rate.

THE DEBATE.

Until the 1960s, there was no debate: dinosaurs, whose existence had been known for about a century, were assumed to be big, dumb, slow, cold-blooded creatures. Then, in 1968, Robert T. Bakker—an undergraduate at Yale University, not a professor or a full-fledged paleontologist—revolutionized the world of paleontology with a paper called "The Superiority of Dinosaurs."

In his article, Bakker described dinosaurs as "fast, agile, energetic creatures" whose physiology was so advanced that even the biggest and heaviest of them could outrun a human. Just a year later, John H. Ostrom, a professor of paleontology who also happened to be at Yale, wrote that a recently identified species of theropod dinosaur must have been "an active and very agile predator."

Thus began the great dinosaur debate, which rages even today. Jurassic Park reinforced the Bakker-Ostrom position, portraying Velociraptor as a cunning, fast-moving predator with clear links to birds. And indeed there are many arguments for endothermy (warm-bloodedness) in dinosaurs—arguments that relate to everything from brain size to rate of growth to the latitudes at which dinosaur fossils have been located. On the other hand, there is plenty of evidence for ectothermy (cold-bloodedness), based on the dinosaurs' size, scaliness, the climate in the Mesozoic era, and so on.

To explore, compare, and judge these many arguments, the reader is encouraged to consult the "Were Dinosaurs Warm-Blooded?" Web site listed in the "Where to Learn More" section at the conclusion of this essay. However, a word of warning, as noted on that site: "The issue is a tangled, complex one. There are not just two sides to the issue; there are numerous competing hypotheses. If you're looking for a major controversy in science, look no further!"

WHERE TO LEARN MORE

Cadbury, Deborah. Terrible Lizard: The First Dinosaur Hunters and the Birth of a New Science. New York: Holt, 2001.

K-12: Paleontology: Dinos (Web site). <http://www.ceismc.gatech.edu/busyt/paleo.html>.

Morris, S. Conway. The Crucible of Creation: The Burgess Shale and the Rise of Animals. New York: Oxford University Press, 1998.

Munro, Margaret, and Karen Reczuch. The Story of Life on Earth. Toronto: Douglas and McIntyre, 2000.

Oceans of Kansas Paleontology (Web site). <http://www.oceansofkansas.com/>.

Paleontology and Fossils Resources—University of Arizona Library, Tucson (Web site). <http://www.library.arizona.edu/users/mount/paleont.html>.

Palmer, Douglas. Atlas of the Prehistoric World. Bethesda, MD: Discovery Communications, 1999.

Sanders, Robert. "New Evidence Links Mass Extinction with Massive Eruptions That Split Pangea Supercontinent and Created the Atlantic 200 Million Years Ago." University of California, Berkeley. (Web site). <http://www.berkeley.edu/news/media/releases/99legacy/4-22-1999b.html>.

Singer, Ronald. Encyclopedia of Paleontology. Chicago: Fitzroy Dearborn Publishers, 1999.

University of California, Berkeley Museum of Paleontology (Web site). <http://www.ucmp.berkeley.edu/>.

USGS (United States Geological Survey) Paleontology Home Page (Web site). <http://geology.er.usgs.gov/paleo/>.

"Were Dinosaurs Warm-Blooded?" (Web site). <http://pubs.usgs.gov/gip/dinosaurs/warmblood.html>.

KEY TERMS

ABSOLUTE AGE:

The absolute age of a geologic phenomenon is its age in Earthyears. Compare with relative age.

ATMOSPHERE:

In general, an atmosphere is a blanket of gases surrounding a planet. Unless otherwise identified, however, the term refers to the atmosphere of Earth, which consists of nitrogen (78%), oxygen (21%), argon (0.93%), and other substances that include water vapor, carbon dioxide, ozone, and noble gases such as neon, which together comprise 0.07%.

BIOSPHERE:

A combination of all living things on Earth—plants, mammals, birds, reptiles, amphibians, aquatic life, insects, viruses, single-cell organisms, and so on—as well as all formerly living things that have not yet decomposed.

BIOSTRATIGRAPHY:

An area of stratigraphy involving the study of fossilized plants and animals to establish dates for and correlations between stratigraphic layers.

CHRONOSTRATIGRAPHY:

A subdiscipline of stratigraphy devoted to studying the relative ages of rocks. Compare with geochronometry.

CONTINENTAL DRIFT:

The theory that the configuration of Earth's continents was once different than it is today; that some of the individual landmasses of today once were joined in other continental forms; and that these landmasses later separated and moved to their present locations.

CORRELATION:

A method of establishing age relationships between various rock strata. There are two basic types of correlation: physical correlation, which requires comparison of the physical characteristics of the strata, and fossil correlation, the comparison of fossil types.

DATING:

Any effort directed toward finding the age of a particular item or phenomenon. Methods of geologic dating are either relative (i.e., comparative and usually based on rock strata) or absolute. The latter, based on such methods as the study of radioactive isotopes, typically is given in terms of actual years or millions of years.

EON:

The longest phase of geologic time. Earth's history has consisted of four eons, the Hadean or Priscoan, Archaean, Proterozoic, and Phanerozoic. The next-smallest subdivision of geologic time is the era.

EPOCH:

The fourth-longest phase of geologic time, shorter than an era and longer than an age or a chron. The current epoch is the Holocene, which began about 0.01 Ma (10,000 years) ago.

ERA:

The second-longest phase of geologic time, after an eon. The current eon, the Phanerozoic, has had three eras, the Paleozoic, Mesozoic, and Cenozoic, which is the current era. The next-smallest subdivision of geologic time is the period.

FOSSIL:

The mineralized remains of any prehistoric life-form, especially those preserved in rock before the end of the lastice age.

FOSSILIZATION:

The process by which a once-living organism becomes a fossil. Generally, fossilization involves mineralization of the organism's hard portions, such as bones, teeth, and shells.

GA:

An abbreviation meaning "gigayears," or "billion years." The age of Earth is about 4.6 Ga.

GEOCHRONOMETRY:

An area of stratigraphy devoted to determining absolute dates and time intervals. Compare with chronostratigraphy.

GEOLOGIC TIME:

The vast stretch of time over which Earth's geologic development has occurred. This span (about 4.6 billion years) dwarfs the history of human existence, which is only about two million years. Much smaller still is the span of human civilization, only about 5,500 years.

HISTORICAL GEOLOGY:

The study of Earth's physical history. Historical geology is one of two principal branches of geology, the other being physical geology.

INVERTEBRATE:

An animal without an internal skeleton.

ISOTOPES:

Atoms that have an equal number of protons, and hence are of the same element, but differ in their number of neutrons. This results in a difference ofmass. An isotope may be either stable or radioactive.

LAW OF FAUNAL SUCCESSION:

The principle that all samples of any given fossil species were deposited on Earth, regardless of location, at more or less the same time. This makes it possible to correlate widely separated strata.

MA:

An abbreviation used by earth scientists, meaning "million years," or "megayears." When an event is designatedas, for instance, 160 Ma, it usually means 160 million years ago.

MASS EXTINCTION:

A phenomenon in which numerous species cease to exist at or around the same time, usually as the result of a natural calamity.

MINERALIZATION:

A series of changes experienced by a once-living organism during fossilization. In mineralization, minerals in the organism are either replaced or augmented by different minerals, or the hard portions of the organism dissolve completely.

ORGANIC:

At one time, chemists used the term organic only in reference to living things. Now the word is applied to most compounds containing carbon, with the exception of carbonates (which are minerals), and oxides, such as carbon dioxide.

PALEOBOTANY:

An area of paleontology involving the study of past plant life.

PALEOECOLOGY:

An area of paleontology devoted to studying the relationship between prehistoric plants and animals and their environments.

PALEONTOLOGY:

The study of life-forms from the distant past, primarily as revealed through the fossilized remains of plants and animals.

PALEOZOOLOGY:

An area of paleontology devoted to the study of prehistoric animal life.

PERIOD:

The third-longest phase of geologic time, after an era. The current eon, the Phanerozoic, has had 11 periods, and the current era, the Cenozoic, has consisted of three periods, of which the most recent is the Quaternary. The next-smallest subdivision of geologic time is the epoch.

PRECAMBRIAN TIME:

A term that refers to the first three of four eons in Earth's history, which lasted from about 4,560 to about 545 Ma ago.

RADIOACTIVITY:

A term describing a phenomenon whereby certain materials are subject to a form of decay brought about by the emission of high-energy particles or radiation. Forms of particles or energy include alpha particles (positively charged helium nuclei), beta particles (either electrons or subatomic particles called positrons, or gamma rays, which occupy the highest energy level in the electromagnetic spectrum.

RADIOMETRIC DATING:

A method of absolute dating using ratios between "parent" isotopes and "daughter" isotopes, which are formed by the radioactive decay of parent isotopes. Radiometric dating also may involve ratios between radioactive isotopes and stable isotopes.

RELATIVE AGE:

The relative age of a geologic phenomenon is its age compared with other geologic phenomena, particularly the stratigraphic record of rock layers. Compare with absolute age.

SEDIMENT:

Material deposited at or near Earth's surface from a number of sources, most notably preexisting rock.

SEDIMENTARY ROCK:

Rock formed by compression and deposition (i.e., formation of deposits) on the part of other rock and mineral particles. Sedimentary rock is one of the three major types of rock, along with igneous and metamorphic.

SEDIMENTOLOGY:

The study and interpretation of sediments, including sedimentary processes and formations.

STRATA:

Layers, or beds, of rocks beneath Earth's surface. The singular form is stratum.

STRATIGRAPHY:

The study of rock layers, or strata, beneath Earth's surface.

VERTEBRATE:

An animal with an internal skeleton.

PALEONTOLOGY

CONCEPT

Paleontology is the study of life-forms from the distant past, as revealed primarily through the record of fossils left on and in the earth. It is a subdiscipline of both the biological and the earth sciences, one that brings to bear the techniques of geologic study and several areas of biology, including botany and zoology. But the term paleontology, as used in the present context, also provides a convenient means of encompassing, in a single word, the study of the distant biological past. Such study, of course, is related intimately to the investigation of evolutionary processes and phenomena. In the present context, however, our concern is not so much with the theory and principles of evolution, discussed elsewhere in this book, but rather with a relatively short overview of biological history. This area of biological investigation calls upon such concepts as mass extinction and fossilization as well as the study of plant and animal forms that scientists know only from scientific reconstruction of the past rather than from direct experience. Chief among these life-forms are the dinosaurs, which dominated Earth for a period of more than 100 million years, ending about 65 million years ago. This span of time, impressive as it seems on the human scale, is minuscule compared with the entire history of life on Earth.

HOW IT WORKS

Paleontology Among the Sciences

Paleontology is the investigation of life-forms from the distant past, primarily through the study of fossilized plants and animals. Most people are familiar with fossils, a term that probably calls to mind an image of a flat piece of rock with a shadowy imprint of a leaf or animal on it. In fact, a fossil is the preserved remains of a once living organism that has undergone a process known as mineralization, in which the organic materials in hard parts of the organism (for example, teeth or bones) are replaced by minerals, which are inorganic.

The difference between the organic and the inorganic, which we discuss later, is more than a difference between the living and nonliving, or even the living and formerly living. For now, though, it will suffice to say that all living and formerly living things, as well as their parts and products, are organic. The stress on formerly living things is important, since paleontology by definition encompasses plants, animals, and microscopic life-forms that lived a long time ago. (As for just how long "a long time ago" really is, that subject, too, is discussed later in this essay.)

Among the fields related, or subordinate, to paleontology are paleozoology, which focuses on the study of prehistoric animal life; paleobotany, the study of past plant life; and paleoecology, the study of the relationships between prehistoric plants and animals and their environments. Despite the emphasis on past life-forms, however, paleontologists also avail themselves of evidence gleaned from observation of animals living today. In this approach, paleontology shows its relationship to geology, which, along with biology, is one of its two "parents."

GEOLOGY AND PALEONTOLOGY.

One of the key concepts in geology is the principle of uniformitarianism, which arose from the early history of the geologic sciences. At a time when the nascent science was embroiled in a debate over Earth's origins and the age of the planet, the Scottish geologist James Hutton (1726-1797) transcended this debate by focusing on the processes at work on Earth in the present day. (In terms of geologic time or even the evolution of species, the eighteenth century and twenty-first century might as well be a few seconds apart, so the expression present day applies to Hutton's time as much as to ours.) Rather than simply speculate as to how Earth had come into being, Hutton maintained that scientists could understand the processes that had formed it by studying the geologic phenomena they could see around them.

The reason this approach works is that natural laws do not change over time, nor do the processes that are at work within nature. On the other hand, particular processes may not be in operation at all times, nor can it be assumed that the rate at which the processes take place is the same. The two foregoing sentences encompass some of the observations made by the modern American paleontologist Stephen Jay Gould (1941-2002) with his formulation of the "four types of uniformity" in Ever Since Darwin: Reflections in Natural History (1977). It is certainly appropriate that paleontology and geology interface in this concept of uniform change, because both are concerned with piecing together the distant past from the materials available in the present.

In fact, from the standpoint of the earth sciences, paleontology belongs to a branch of geology known as historical geology, or the study of Earth's physical history. (This field of study is contrasted to the other principal branch of the discipline, physical geology, the study of the material components of Earth and of the forces that have shaped the planet.) Other subdisciplines of historical geology are stratigraphy, the study of rock layers, or strata, beneath Earth's surface; geochronology, the study of Earth's age and the dating of specific formations in terms of geologic time; and sedimentology, the study and interpretation of sediments, including sedimentary processes and formations.

Before Life

Later in this essay, we consider just what is meant by "geologic time," the grand sweep of several billion years during which Earth evolved from a cloud of gases to its present form. It may be surprising to discover the large proportion of that time during which life existed on Earth; it is equally amazing to learn what a small portion of the biological history of the planet involved anything that modern humans would recognize as "life."

For the moment, let us go even further back, to the very beginning—an almost inconceivably long time ago. Scientists believe that the universe began between 10 billion and 20 billion years ago, with an explosion nicknamed the "big bang," a cataclysm so powerful that it sent galaxies careening outward on a course they maintain even today. Among those galaxies moving outward from the universal point of origin was our Milky Way, which, about six billion years ago, began to develop a rotating cloud of cosmic gas somewhere between its center and its rim. That was the beginning of our solar system.

THE SUN AND EARTH.

The center of the cloud, where the greatest amount of gases gathered, was naturally the densest and most massive portion as well as the hottest. There, hydrogen—the lightest of all elements—experienced extraordinary amounts of compression owing to the density of the clouded gases around it, and underwent nuclear fusion, or the bonding of atomic nuclei.

This hot center became the Sun about five billion years ago, but there remained a vast nebula of gas surrounding it. As the fringes of this nebula began to cool, the gases condensed, forming solids around which particles began to accumulate. These were the future planets, including ours. The process of planetary formation took place over a span of about 500 million years. The planets contained various chemical elements, formed by nuclear fusion on the Sun—a fascinating aspect of life on Earth, since it means that the particles that make up the human body came from nuclear reactions on the stars.

THE EARLY ATMOSPHERE.

The brand-new Earth possessed an atmosphere that consisted primarily of elemental hydrogen, nitrogen, carbon monoxide, and carbon dioxide. This "air" would have been unbreathable to all but a very small portion of the life-forms that exist on Earth today. Yet for the development of life, it was absolutely essential that no oxygen be present in Earth's atmosphere at that very early time.

The reason is that oxygen is an extremely reactive element, which is why it is involved in an array of chemical reactions (including combustion and rusting) known collectively as oxidation-reduction reactions. If oxygen had been present at that time, it likely would have reacted with other elements immediately, rather than permitting the formation of what eventually became organic materials.

WATER.

Water appeared as the result of meteorite bombardment from space, which took place in the first half-billion years of Earth's existence. It might seem strange to learn that water, a compound essential to life on Earth, came from the void of space—where, as far as we know, there are no other life-forms. But this is not as strange as it sounds: water itself is inorganic, and even today frozen water exists on several planets in our solar system.

In any case, water eventually accumulated on Earth's surface in quantities sufficient for its condensation, with the result that clouds formed and rain fell on Earth more or less continually for many millions of years. It was then that the beginnings of life made their appearance, in the form of self-replicating molecules of carbon-based matter.

Life Begins

One famous cliché of science-fiction movies is the use of the phrase "carbon-based life-forms" to describe humans. In fact, all living things contain carbon, and if we ever do find life, intelligent or otherwise, on other planets, chances are extremely high that it, too, will be "carbon-based." Carbon, in fact, is almost synonymous with life, and hence the word organic, in its scientific meaning, refers to all substances that contain carbon. The only exceptions are the elemental carbon in diamonds or graphite, the carbonate forms that make up many of Earth's rocks, and such oxides as carbon dioxide and monoxide, all of which are considered inorganic.

It may sound as though a huge portion of carbon's possible forms already have been eliminated from the list of organic substances, but, in fact, carbon is capable of forming an almost limitless array of compounds with other elements, particularly hydrogen. A class of molecule known as hydrocarbons, which are nothing but strings of carbon and hydrogen molecules bonded together, is the basis for literally millions of organic compounds, from petroleum to polymer plastics. It may sound odd to hear plastics referred to as organic, but this only highlights the difference between the popular and scientific meanings of that term.

ORGANIC MATERIALS.

At one time, organic referred only to living things, things that were once living, and materials produced by living things (for example, sap, blood, and urine). As recently as the early nineteenth century, scientists believed that organic substances contained a supernatural "life force," but in 1828 the German chemist Friedrich Wöhler (1800-1882) made an amazing discovery.

By heating a sample of ammonium cyanate, a material from a nonliving source, Wöhler converted it to urea, a waste product in the urine of mammals. As he later observed, "without benefit of a kidney, a bladder, or a dog," he had turned an inorganic substance into an organic one. It was almost as though he had created life. Actually, what he had discovered was the distinction between organic and inorganic material, which results from the way in which the carbon chains are arranged.

This explanation of the difference between organic and inorganic is pivotal to understanding how the beginnings of life first formed on Earth. It is an almost inconceivably large step from the nonliving to the living but not nearly so much of a jump from the inorganic to the organic. In fact, it appears that what happened on Earth in its distant past was that organic (but not living) substances underwent chemical reactions with inorganic ones to produce the rudiments of life. The American chemist Stanley Miller (1930-) illustrated this with an experiment that involved a mixture of hydrogen, methane (CH₄), ammonia (NH₃), and water. Subjected to a discharge of electric sparks intended to simulate lightning in Earth's early atmosphere, the mixture eventually yielded amino acids, which are among the chief components of proteins.

EARLY FORMS OF LIFE.

The course that early forms of life followed during the first 800 million years of Earth's existence was a lengthy one, and if a person could have glimpsed Earth at any interval of a few million years during this time, it might have seemed as though nothing at all was happening. In fact, however, life-forms were undergoing the most profound changes imaginable.

That span of 0.8 billion years saw a transition from elemental carbon to organic compounds and from organic compounds to organelles, which are discrete components of cells, and finally to cells themselves—the building blocks of life. The processes by which this happened were exceedingly complicated, and modern scientists have little to go on in forming their suppositions as to how these transitions came about. Among many key pieces of information missing from the picture, for instance, is the matter of how and when DNA first appeared in cells. (For more on DNA, see Cells.)

The first cells to form were known as prokaryotic cells, or cells without a nucleus. (These cells, too, are discussed in the essay Cells.) Prokaryotic cells may have been little more than sacs of DNA that were capable of self-replication—much like bacteria today, which are themselves prokaryotic. These early forms of bacteria, which dominated Earth for many millions of years, were apparently anaerobic and eventually split into three branches: archaebacteria, eubacteria, and eukaryotes. Out of the last group grew all other forms of life, including fungi, plants, and animals.

Geologic Time Marches On

By about 2.5 billion years ago, bacteria had begun to undergo a form of photosynthesis, as plants do today. As a result, oxygen started to accumulate in Earth's atmosphere, and this had two interesting implications for life on Earth. One of the results of oxygen formation was that formation of "new" cells—that is, spontaneously formed cells that did not come from already living matter—ceased altogether, because they were killed off by reactions with oxygen. Second, aerobic respiration thereafter became the dominant means for releasing energy among living organisms.

The real history of life-forms on Earth—the portion of that history about which paleontologists can learn a great deal from observation of fossils and other materials—dates from the beginning of the Cambrian period, about 550 million years ago. At this point, about 88% of Earth's history already had passed, yet the remaining 12% contains virtually all the really dramatic events in the formation of life on Earth. Later in this essay, we examine a few analogies that help put these time periods into perspective as well as the concept of geologic time itself.

PHASES IN EARTH'S HISTORY.

In the course of discussing these topics, it is sometimes necessary to make reference to geologic time divisions—eons, eras, periods, and epochs. These are not units of a specific length in years, like a century or a millennium; instead, they are distinct phases in Earth's history that historical geologists (including paleontologists) have pieced together from fossils and other materials. Their names usually refer to locations where fossils relevant to that phase of geologic time were found: for example, the Jurassic period, whose name became a household word after the release of the 1993 blockbuster movie Jurassic Park, is named after the Jura Mountains of Switzerland and France.

The historical juncture mentioned in the paragraph before last—the beginning of the Cambrian period, some 550 million years ago, was also the end of the Proterozoic eon, the third of three eons in what is known as Precambrian time. The fourth and present eon is the Proterozoic, which has included three eras: Paleozoic (about 550-240 million years ago), Mesozoic (about 240-65 million years ago), and Cenozoic (about 65 million years ago to the present).

CATACLYSMS AND CONTINENTAL DRIFT.

The divisions between these phases have not been drawn arbitrarily; rather, they are based on evidence suggesting that those points in Earth's history were marked by violent, cataclysmic events. Once life had come into the picture, these cataclysms brought about death on a vast scale, which we discuss later, in the context of mass extinctions. A number of phenomena caused mass extinctions at various points; most notable among them was the impact of meteorites. Also significant was continental drift, which, as its name implies, is the movement of the continents from distant origins.

Just as evolutionary theory informs much of our modern thinking about biology, the theory of plate tectonics holds a dominant position in geology and related earth sciences. Plate tectonics involves the movement of large segments in the crust and the upper mantle of Earth, and one of the outcomes of such movement is continental drift.

Even today, the continents we know are moving slowly away from or toward one another, but that movement is so slow that it would take millions of years for any change to be perceptible. At one time, however, the continents were distributed quite differently than they are today. For example, at the end of the Permian period and the beginning of the Triassic, when the dinosaurs began to appear on the scene, all of Earth's land-masses were joined in a single continent, Pangea, that stretched between the North and South Poles and was surrounded by a vast ocean.

REAL-LIFE APPLICATIONS

Our Place in the Grand Scheme of Things

All areas of historical geology—including paleontology—are concerned with geologic time, a term that refers to the great sweep of Earth's history. This is a timescale that dwarfs the span of human existence, and to study the history of life on Earth, a mental adjustment of monumental proportions is required. One must discard all notions used in studying the history of human civilization, including such concepts as modern, medieval, and ancient, which are essentially useless when discussing geologic time.

Human civilization has existed for about 5,500 years, the blink of an eye in geologic terms. Even the span of time since the first appearance of the genus Homo (to which humans, or Homo sapiens, belong), is minuscule: 2.5 million years compared with 4.6 billion years, or about 0.04% of the planet's history. A couple of analogies, one to a shorter span of time and another to a measurement of space, will help us understand the scope of time encompassed in the evolution of species.

LIFE APPEARS IN JUNE—SO TO SPEAK.

Suppose that the entire history of Earth were likened to a single year of 365.25 days, starting with the formation of the planet from a cloud of dust and ending with the present. At what point in the year would the first aerobic life-forms appear? We are not talking about anything approaching a human or even a clam or a fern or a piece of algae—just a single-cell organism that depends on oxygen for energy.

One might guess that such life-forms appeared sometime in January, or at least by the end of winter in late March. In fact, the first aerobic single-cell life-forms arose around June 15—nearly halfway through a year. Even at about the time of Thanksgiving, the most complex organisms would be fish and a few early amphibians.

We tend to associate the dinosaurs with the early phases of Earth's history, but this only illustrates our distorted view of geologic time. In fact, the Jurassic period would be analogous to a period of about five days from December 15-20. By Christmas Day, all remaining dinosaurs would have been headed toward extinction, their dead bodies eventually forming the fossil fuels that power present-day civilization.

By this point we are within a few days of the year's end, and yet nothing remotely resembling a human has appeared. Our own genus would not come on the scene until around 8:00 p.m. on December 31. The New Year's Eve countdown would be nearing by the time human civilization began, at about 42 seconds before midnight. Christ's birth would have occurred at about 14 seconds before midnight, and the final 10-second countdown would begin about the time the Roman Empire fell. The life span of the average person would correspond to approximately half a second or less.

ANOTHER ANALOGY: LOS ANGELES TO NEW YORK.

To use another analogy, suppose we are driving fromLos Angeles to New York City, and that this distance corresponds to Earth's history. Once we reach western Nebraska, with about 46% of the distance behind us, we would come to the beginning of the Proterozoic era and the origins of aerobic life-forms. One might wonder why "nothing" happened on all those long miles from L.A. through the deserts, mountains, and plains of the western United States, but as we have seen, a great deal happened: Earth formed from a cloud of gas, was pounded by meteors, and gradually became the home to oceans.

The end of the Proterozoic era (about 545 million years ago) would be at about 88% of the distance from Los Angeles to New York—some-where around Pittsburgh, Pennsylvania. By this point, the continental plates have formed, oxygen has entered the atmosphere, and soft-bodied organisms have appeared. We are a long way from Los Angeles, and yet almost the entire history of life on Earth, at least in terms of relatively complex organisms, lies ahead of us.

If we skip ahead by about 339 million years—a huge leap not only in time but also in biological development—we come to the point when the dinosaurs appeared. We are now 95% of the way from the beginning of Earth's history to the present, and if it is measured against the distance from Los Angeles to New York, we would be somewhere around Scranton, in eastern Pennsylvania. Another 89 mi. (142 km) from the Scranton area would put us at a point about 65 million years ago, or the time when the dinosaurs became extinct. We then would have less than 40 mi. (64 km) to drive to reach the period where genus Homo appeared, by which time we would be in the middle of Manhattan.

Compared with the distance from L.A. to New York, the span of time that Homo sapiens has existed would be much shorter than the drive from Central Park to the Empire State Building. The entire sweep of human civilization and history, from about a thousand years before the building of the pyramids to the beginning of the third millennium a.d., would be smaller than a city block. It is much smaller, in fact—close to the size of a modest storefront, or 15.54 ft. (5 m).

How Did We Get Here?

On the one hand, the history of relatively complex life-forms is relatively short compared with Earth's history; on the other hand, it is unbelievably long, when one considers the diversity of forms that have evolved in the past half-billion years. What follows is an extremely brief overview, touching on a few points in the development of life on Earth.

The discussion here is far from comprehensive, the purpose being not to provide a detailed overview of evolutionary processes but to illustrate the evolution of life-forms by a few examples. The reader is strongly encouraged to consult a textbook or other reliable information source for greater illumination of these particular topics. Along with this overview, we look at some of the more dramatic aspects of paleontologic study: dinosaurs and mass extinction.

The fossilized history of life on Earth really began in earnest only with the Cambrian period, which saw an explosion of invertebrate (without an internal skeleton) marine forms. These dominated from about 550 to about 435 million years ago. At the latter point, the boundary between the Ordovician and Silurian periods of the Paleozoic era, there occurred the first of five major mass extinctions, as the "supercontinent" of Gondwana crossed the South Pole, freezing most life-forms that were then alive. As always happened with these mass extinctions, some life-forms survived, and within a few million years life again was thriving.

COMING OUT OF THE WATER.

One of the favorite subjects of modern cartoonists is the migration of creatures from the water to the land. For example, following the enactment of stiff airport security guidelines in the wake of the September 11 terrorist attacks, a New Yorker cartoon in late 2001 showed this evolution from fish to amphibian (a creature that can live both in the water and on the ground) and, ultimately, to the human being. Then, in the last evolutionary sequence, Homo sapiens is depicted in a business suit, going through an airport security scanner. Most cartoons based on this event in paleontologic history follow a common theme, which can be summed up thus: "We went through all that evolution just for this ?"

Certainly, the transition from water to solid ground was a massive step—one of almost inconceivable importance. This invasion of the land, which probably took place about 400 million years ago, perhaps started when fishes of the Devonian period (named after Devonshire in England) began breathing oxygen near the surface of shallow waters. In time they made their way onto the land, and as the number of species grew, their lobed fins developed in different ways for different groups of fish, at length becoming the many limbs and appendages specific to large groups of species, such as birds or mammals. This may seem a bit far-fetched, but even today catfish are known to pull themselves out of the water and onto land if a specific need to do so arises.

Why did the first fish leave the water? Perhaps because the water levels were receding, as they have done periodically in the course of Earth's history as a result of massive freezing of global water supplies—that is, ice ages. Whatever the reason, we can be sure of the fact that the fish were not in any way attempting to reach some "higher" state of development, nor was any force pushing them to "evolve." Therein lies one of the major misconceptions about evolution, even among many of its adherents. The very name evolution is somewhat unfortunate, because it suggests that life-forms are moving gradually toward a state of heightened development. Instead, they simply are adapting to circumstances that confront them; hence, the entire phenomenon might well be called by the much less dramatic-sounding name adaptation.

Mass Extinction

Let us now pause in our narrative to discuss examples of a phenomenon that has been mentioned several times already: mass extinction. One of the truly amazing things about the history of life on Earth is the way in which forces have seemingly conspired to wipe out virtually all living things, not just once but at least five major times, along with a number of more limited instances.

Over the course of Earth's history—even its very recent history—numerous species have become extinct, usually as a result of their inability to adapt to changes in their natural environment. In the extremely recent past (recent geologically, that is—since the end of the last ice age about 10,000 years ago), some extinctions or endangerments of species have been attributed to human activities, including hunting and the disruption of natural habitats. For the most part, however, extinction is simply a part of Earth's history, a result of the fact that organisms incapable of adaptation soon die out. This is the infamous "survival of the fittest," an aspect of evolutionary theory that many a social or political ideologue has misappropriated for the purpose of insisting that one particular ethnic or social group is naturally superior to another (see Evolution).

Why does mass extinction occur? As we noted earlier, one possible cause of mass extinction is a sudden and dramatic change in ocean levels. Other causes include volcanic eruptions or the effects of events or objects from space—the explosion of a star, perhaps, or the impact of a meteorite on Earth. Although scientists have a reasonable idea of the immediate causes of mass extinction in some cases, their understanding of the root causes still is limited. This fact was expressed by the University of Chicago paleobiologist David M. Raup, who wrote in Extinction: Bad Genes or Bad Luck? : "The disturbing reality is that for none of the thousands of well-documented extinctions in the geologic past do we have a solid explanation of why the extinction occurred."

THE GREAT FIVE MASS EXTINCTIONS.

The five largest known mass extinctions occurred at intervals of between 50 million and 100 million years over a span of time from about 435 million to 65 million years ago. The first of them we have mentioned as perhaps being associated with the first migrations of organisms to land. This was at the end of the Ordovician period, about 435 million years ago, when a drop in the ocean level wiped out one-fourth of all marine families. No wonder some of the fish adapted to life on land!

Changes in sea level along with climate changes also appear to have brought about the second great mass extinction, which we also have mentioned, near the end of the Devonian period, about 370 million years ago. But no biological cataclysm in Earth's history can compare to the mass extinction that attended the close of the Permian period some 240 million years ago—an event so traumatic to life on Earth that it has been dubbed the great dying.

Imagine if 96% of all people alive today were killed—a staggering and terrifying concept. This would be far more fatalities than the combined death toll of both world wars; the Black Death of 1347-1351 and the influenza epidemic of 1918-1920; the various Nazi, Stalinist, and Maoist acts of genocide; and all major earthquakes and other natural disasters in history. Now imagine that along with all those people, 96% of all other life-forms—plants and trees, birds and fishes, single-cell organisms and the like, which vastly outnumber the relatively small human population of Earth—all ceased to exist as well. Such was the almost incomprehensible scale of the so-called great dying, an event whose cause has not been determined, though it may have resulted from a volcanic eruption in Siberia.

The Age of the Dinosaurs

Now that we have mentioned only three of the five great mass extinctions, it is appropriate to discuss the creatures that died in the fourth and fifth: the dinosaurs, or "terrible lizards." Though they were the most well-known creatures of the Mesozoic era, which lasted from about 240-65 million years ago, dinosaurs were far from the only notable species on Earth in that phase of geologic history. In fact, it was during the first period of the Mesozoic era, the Triassic, that a group of creatures destined to make a greater impact on Earth—mammals—first appeared. During this time, botanical life included grasses, flowering plants, and trees of both the deciduous (leaf-shedding) and coniferous (cone-bearing) varieties.

Backtracking just a bit, at the beginning of the Devonian period, about 410 million years ago, the first vertebrates (animals with an internal skeleton) made their appearance in the form of jawless fishes. Plant life on land consisted of ferns and mosses. Then came the late-Devonian mass extinction we have mentioned, which removed many of the newly evolved invertebrates, including most fishes. By the end of the Devonian, about 360 million years ago, fish had evolved jaws, and life was thriving on land. Among these life-forms were reptiles in the animal kingdom, and gymnosperms, or plants that reproduce sexually by spreading exposed seeds—a primary example from our own time being pine trees.

MONSTERS OF THE MESOZOIC.

Although the existence of dinosaurs has been an established fact for as long as anyone alive can remember, we have had knowledge of them only since the mid-nineteenth century. It was then that early paleontologists began to piece together evidence from fossils, which revealed the record of a time when enormous reptiles had walked the earth. Not all of these dinosaurs were so large, however. Some were as small as chickens, while others—the sauropods, the largest terrestrial animals of all time—were the equivalent of half a dozen stories tall.

Some dinosaurs were awesomely fierce predators, while others were mild-mannered plant eaters. Dinosaurs fell into two groups based on the shape of their hips, which were either lizardlike or birdlike. Though the lizardlike Saurischia emerged first, they lived alongside the birdlike Ornithischia throughout the late Triassic, Jurassic, and Cretaceous periods. Ornithischia were all herbivores, or plant eaters, whereas Saurischia included both herbivores and carnivores, or meat eaters. Naturally, the most fierce of the dinosaurs were carnivores, a group that included the largest carnivore ever to walk the Earth, Tyrannosaurus rex.

Most dinosaurs had long tails and necks and walked on four legs, though some were bipedal, meaning either that they had only two legs or that they used only their two rear legs for movement or locomotion. Again, the shape of the creature correlated to some degree with its habits: all four-legged dinosaurs were relatively gentle, slow-moving herbivores, whereas most of the bipedal ones were fast-moving predators. Their teeth also reflected their diet: not surprisingly, carnivores had much sharper cutting surfaces shaped like serrated knives, which made it easy to rip their prey into smaller pieces before swallowing the pieces whole.

As recently as the 1970s, scientists tended to believe that dinosaurs were cold-blooded creatures markedly lacking in brain power. This view is reflected in a number of popular-culture sources, such as the 1983 song by the Police, "Walking in Your Footsteps," which suggests that if humans were to destroy themselves by nuclear warfare, they would seem even more "dumb" than the dinosaurs. By contrast, the film Jurassic Park, made a decade later, reflects changes in paleontologists' attitudes toward dinosaurs, gained as a result of continued research. As it turns out, dinosaurs were far from dumb, and they may even have been warm-blooded—unlike most reptiles but like birds, to whom they may have been related.

SOME TYPES OF DINOSAUR.

Few creatures have ever captured the imagination of humans as much as Tyrannosaurus rex —a particularly impressive feat, given the fact that the last T. rex died tens of millions of years before humans ever came on the scene. With a name that means "absolute ruler lizard" in Greek, this terrifying creature reached a maximum length of 45 ft. (14 m) and may have weighed as much as 9 tons (8 metric tons).

T. rex must have made a truly awesome sight running through the forests of its Mesozoic world. Moving on its powerful hind legs with a gait that was at once swift and lumbering, it used its tail as a counterbalance and stabilizer. Once it had come abreast of its prey, T. rex probably attacked its victim with powerful head butts and then tore the animal apart with its massive jaws before ripping into it with some 60 dagger-shaped teeth as much as 6 in. (15 cm) long.

Of course, T. rex is notable precisely because it was so unusual among dinosaurs in size and, for the most part, power. Deinonychus, for example, was much smaller—only about 10 ft. (3 m) and 220 lb. (100 kg)—but these "running lizards" were still fearsome predators, noted for their sickle-like claws. The phrase "sickle-like claws" might call to mind another creature, this one made famous by Jurassic Park: Velociraptor, or "swift plunderer," a walking death machine that for all the terror it inspired in moviegoers (and, no doubt, in many a Mesozoic creature) attained a length of only about 6 ft. (2 m).

Among the many lessons about dinosaurs to be learned from Jurassic Park was the striking contrast between herbivores and carnivores, a fact emphasized by the scene in which the two children take refuge with a herd of sauropods. These gentle giants possessed four legs the size of large pillars and had extraordinarily long necks and tails. Most famous among the sauropods was the Apatosaurus, which achieved a length of 65 ft. (20 m) and a weight of 30 tons (27 metric tons). Apatosaurus 's appearance in the scene with the children was not the great creature's first time in the limelight of popular culture: known as Brontosaurus until the early 1990s, this dinosaur was a popular fixture of the Flintstones cartoon.

In addition to land-based dinosaurs, there were such creatures as the flying Triceratops, which may have been a link between dinosaurs and birds. (The name raptor, incidentally, usually refers not to a type of dinosaur, but to a bird of prey.) It should be noted that Triceratops and Velociraptor were creatures of the late Cretaceous period, during which about a hundred different dinosaur genera (plural of genus ) flourished. This is an important point, because many books picture all manner of dinosaurs coexisting, when, in fact, various ones existed at different times over a period of about 180 million years. Also, it is worth noting that the major nonhuman "star" of Jurassic Park did not even live during the Jurassic period!

Later Mass Extinctions—and New Life

Today, of course, the dinosaurs are long gone—so distant in time, in fact, that the remains of many millions have become petroleum. This has happened in situations where specific conditions prevail: for example, the remains could not be allowed to rot, and decay had to be anaerobic and had to take place within certain types of rock. Yet oil reserves (for as long as they last) are not the only reason why the dinosaurs' disappearance was beneficial from the human standpoint. At the time when dinosaurs controlled the world, mammals were small and hid from predators such as T. rex. Without the mass extinction of the dinosaurs, this class of creatures might never have come to the forefront, and human beings might not have evolved.

Actually, two mass extinctions rocked the world of the dinosaurs. The first (fourth among the list of five major mass extinctions) happened at the end of the Triassic, the first period of the Mesozoic era. This was about 205 million years ago, when an asteroid may have hit Earth. Whatever the cause, the result was that creatures in the seas suffered major mass extinction. So, too, did those on the land, but many species of dinosaurs and mammals managed to survive the event, which marked the transition between the Triassic and Jurassic periods.

THEORIES REGARDING THE DINOSAURS' DEMISE.

The dinosaurs continued to flourish for another 140 million years, and their descendants might still be walking the earth were it not for the last great mass extinction, which took place about 65 million years ago. What killed the dinosaurs? Paleontologists and other scientists have proposed several theories: a rapid climate change; the emergence of new poisonous botanical species, eaten by herbivorous dinosaurs, that resulted in the passing of toxins along the food web; an inability to compete successfully with the rapidly evolving mammals; and even an epidemic disease to which the dinosaurs possessed no immunity.

Interesting as many of these theories are, none has gained anything like the widespread acceptance achieved by another scenario. According to this highly credible theory, an asteroid hit Earth, hurtling vast quantities of debris into the atmosphere, blocking out the sunlight, and greatly lowering Earth's surface temperature. Around the world, geologists have found traces of iridium deposited at a layer equivalent to the boundary between the Cretaceous and Tertiary periods, the Tertiary being the beginning of the present Cenozoic era. This is significant, because iridium seldom appears on Earth's surface—but it is found in asteroids.

THE ASTEROID HITS.

It appears that the asteroid smashed into what is now the northern tip of Mexico's Yucatán peninsula. Today that area is home to a crater some 6 mi. (9.6 km) deep and 186 mi. (300 km) in diameter. Note, however, that 65 million years ago, the Yucatán was not exactly where it is now. It had begun to drift in the direction of its present location, but the map of the future continent of North America was quite different from what it is today and included large submerged areas. Among them was the region of impact, which only later rose to the surface as a result of tectonic activity.

The nearest large landmass at the time was a good distance away—equivalent to northern Louisiana. But it hardly mattered that there were no dinosaurs at the point of impact. Traveling at more than 100,000 MPH (160,000 km/h), which is almost four times as fast as the fastest spacecraft built by human beings, the asteroid probably brought about an explosion as intense as many thousands of hydrogen bombs. It may not have raised a mushroom cloud per se, but it probably produced an even more dramatic formation as it sent more than 48,800 cu. mi. (200,000 km³) of debris and gases into the atmosphere. This would be enough dust to cover the state of Mississippi to a depth of 1 mi. (1.6 km)

The sound of the impact must have been ear-shattering, even far away on what would one day become North America. Then came the tidal waves, some as tall as 394 ft. (120 m), with earthquakes soon to follow. But while these tidal waves undoubtedly caused massive localized death, or mass mortality, what really brought about the mass extinction of the dinosaurs was the aftermath of impact. All that dust in the atmosphere effectively blocked out the Sun's light, bringing about a dramatic cooling on Earth's surface. As a result, plants died, thus depriving herbivorous dinosaurs of an energy source. The herbivores died and then, in a domino effect brought about by the interdependence of components in the food web, the carnivores soon followed.

MAMMALS, HUMANS, AND MASS EXTINCTION.

When that asteroid hit, it brought an end to the dinosaurs and their Mesozoic world, ushering in the Cenozoic era, which saw the rise of mammals. Some examples of the creatures that proliferated in the course of the past 65 million years are tiny prehistoric horses with four toes as well as a giant rhinoceros-like herbivore, both of which lived in the Eocene epoch about 40 million years ago.

As time went on, the range of species—not only animals and plants but also the much less complex organisms of the other three kingdoms (Protista, including protozoa and algae; Monera such as bacteria; and Fungi)—grew larger and larger. An ice age struck the planet about 1.65 million years ago, bringing about much smaller cases of mass extinction than the ones we have discussed. But other instances of mass extinction would occur at the hands of a creature that appeared about 2.5 million years ago: Homo sapiens. The ice age—which was only one of many and thus is referred to often as the "last ice age"—would prove a major turning point in the history of the human species. At the beginning, we were just embarking on the beginnings of the Stone Age, whereas at the end, just 10,000 years ago (or a few seconds ago, in geologic terms), we emerged from the cold, ready to take over the world.

Near the end of the last ice age, peoples from Siberia crossed a land bridge spanning what is today the Bering Strait between Russia and Alaska. Their descendants, of course, are the Native Americans, and native is a fitting term, since they arrived in the Americas about 12,000 years ago, or about 7,000 years before the Europeans' ancestors arrived in Europe. What they found, as they poured into the Americas, was a range of species quite different from those known today. There were mammoth and mastodons; giant bears, beaver, and bison; and even saber-toothed "tigers" (which were not directly related to modern-day tigers), camels, and lions. Prehistoric America was also home to horses, but these creatures and many others were wiped out by hunting. Horses did not reappear in the New World until Europeans brought them after a.d. 1500, when they would prove an indispensable aid in European efforts to conquer the Native Americans' lands.

WHERE TO LEARN MORE

Cadbury, Deborah. Terrible Lizard: The First Dinosaur Hunters and the Birth of a New Science. New York: Holt, 2001.

Gould, Stephen Jay. Ever Since Darwin: Reflections in Natural History. New York: W. W. Norton, 1977.

K-12: Paleontology—Dinos (Web site). <http://www.ceismc.gatech.edu/busyt/paleo.html>.

Morris, S. Conway. The Crucible of Creation: The Burgess Shale and the Rise of Animals. New York: Oxford University Press, 1998.

Munro, Margaret, and Karen Reczuch. The Story of Life on Earth. Toronto: Douglas and McIntyre, 2000.

Oceans of Kansas Paleontology (Web site). <http://www.oceansofkansas.com/>.

Paleontology and Fossils Resources. University of Arizona Library, Tucson (Web site). <http://www.library.arizona.edu/users/mount/paleont.html>.

Palmer, Douglas. Atlas of the Prehistoric World. Bethesda, MD: Discovery Communications, 1999.

Raup, David M. Extinction: Bad Genes or Bad Luck? New York: W. W. Norton, 1991.

Singer, Ronald. Encyclopedia of Paleontology. Chicago: Fitzroy Dearborn Publishers, 1999.

Starr, Cecie, and Ralph Taggart. Biology: The Unity and Diversity of Life. 7th ed. Belmont, CA: Wadsworth, 1995.

University of California, Berkeley Museum of Paleontology (Web site). <http://www.ucmp.berkeley.edu/>.

USGS (United States Geological Survey) Paleontology Home Page (Web site). <http://geology.er.usgs.gov/paleo/>.

KEY TERMS

AEROBIC:

Oxygen-breathing.

AMPHIBIAN:

A creature capable of living either in the water or on solid ground.

ANAEROBIC:

Non-oxygen-breathing.

CARNIVORE:

A meat-eating organism.

EON:

The longest phase of geologic time. Earth's history has consisted of four eons, the Hadean, Archaean, Proterozoic, and Phanerozoic. The next-smallest subdivision of geologic time is the era.

EPOCH:

The fourth-longest phase of geologic time, shorter than an era. The current epoch is the Holocene, which began about 10,000 years ago.

ERA:

The second-longest phase of geologic time, after an eon. The current eon, the Phanerozoic, has had three eras, the Paleozoic, Mesozoic, and Cenozoic, which is the current era. The next-smallest subdivision of geologic time is the period.

EUKARYOTE:

A cell that has a nucleus and organelles (sections of the cell that perform specific functions) bound bymembranes.

FOOD CHAIN:

A series of singular organisms in which each plant or animal depends on the organism that precedes it. Food chains rarely exist in nature; therefore, scientists prefer the term food web.

FOOD WEB:

A term describing the interaction of plants, herbivores, carnivores, omnivores, decomposers, and detritivores in an ecosystem. Each of these organisms consumes nutrients and passes them along to other organisms (or, in the case of the decomposer food web, to the soil and environment). The food web may be thought of as a bundle or network of food chains, but since the latter rarely existseparately, scientists prefer the concept of a food web to that of a food chain.

FOSSIL:

The mineralized remains of any prehistoric life-form, especially those preserved in rock before the end of the lastice age.

FOSSILIZATION:

The process by which a once living organism becomes a fossil. Generally, fossilization involves mineralization of the organism's hard portions, such as bones, teeth, and shells.

GEOLOGIC TIME:

The vast stretch of time over which Earth's geologic development has occurred. This span (about 4.6 billion years) dwarfs the history of human existence, which is only about 2.5 million years. Much smaller still is the span of human civilization, only about 5,500 years.

GYMNOSPERM:

A type of plant that reproduces sexually through the use of seeds that are exposed, not hidden in an ovary as with an angiosperm.

HERBIVORE:

A plant-eating organism.

ICE AGE:

A period of massive and widespread glaciation (the spread of glaciers). Ice ages usually occur in series overstretches of several million or even several hundred million years and have taken place periodically—often in conjunction with mass extinctions—throughout Earth's history.

INVERTEBRATE:

An animal without an internal skeleton.

MASS EXTINCTION:

A phenomenon in which numerous species cease to exist at or about the same time, usually as the result of a natural calamity.

MINERAL:

A naturally occurring, typically inorganic substance with a specific chemical composition and a crystalline structure (that is, a structure in which the constituent parts have a simple and definite geometric arrangement that is repeated in all directions).

MINERALIZATION:

A series of changes experienced by a once living organism during fossilization. In mineralization, minerals in the organism are replaced or augmented by different minerals, or the hard portionsof the organism dissolve completely.

ORGANIC:

At one time chemists used the term organic only in reference to living things. Now the word is applied to most compounds containing carbon, with the exception of carbonates (which are minerals) and oxides, such as carbon dioxide.

PALEOBOTANY:

An area of paleontology involving the study of past plant life.

PALEOECOLOGY:

An area of paleontology devoted to studying the relationships between prehistoric plants and animals and their environments.

PALEONTOLOGY:

The study of life-forms from the distant past, primarily as revealed through the fossilized remains of plants and animals.

PALEOZOOLOGY:

An area of paleontology devoted to the study of prehistoric animal life.

PERIOD:

The third-longest phase of geologic time, after an era. The current eon, the Phanerozoic, has had 11 periods, and the current era, the Cenozoic, has consisted of three periods, of which the most recent is the Quaternary. The next-smallest subdivision of geologic time is the epoch.

PHOTOSYNTHESIS:

The biological conversion of light energy (that is, electromagnetic energy) from the Sun to chemical energy in plants.

PROKARYOTE:

A cell without a nucleus or organelles bound by membranes.

VERTEBRATE:

An animal with an internal skeleton.

Paleontology

Invertebrate paleontology

Resources

Paleontology is the study of ancient animal life and how it developed. It is divided into two subdisciplines, invertebrate paleontology and vertebrate paleontology. Paleontologists use two lines of evidence to learn about ancient animals. One is to examine animals that live today, and the other is to study fossils. The study of modern animals includes looking at the earliest stages of development and the way growth occurs (embryology), and comparing different organisms to see how they are related evolutionarily (cladistics). The fossils that paleontologists study may be the actual remains of the organisms, or simply traces the animals have left (tracks or burrows left in fine sediments). Paleontology lies at the boundary of the life sciences and the earth sciences. It is thus useful for dating sediments, reconstructing ancient environments, and testing models of plate tectonics, as well as understanding how modern animals are related to one another.

Invertebrate paleontology

An invertebrate is essentially a multicellular animal that lacks a spinal column encased in vertebrae and a distinct skull. There are about 30 phyla, or groups, of invertebrates, and roughly 20 of these have been preserved as fossils. Still other phyla probably existed, but are not represented in the fossil record because the animals’ soft bodies were not preserved. Only one invertebrate phylum is known to have become extinct—the Archaeocyathida. These organisms, which were superficially similar to sponges, did not survive past the Middle Cambrian period (530 million years ago).

The different body plans of invertebrates most likely evolved during the Precambrian, between 1,000 and 700 million years ago. There was an “explosion” of invertebrate evolution in the Lower Cambrian (beginning about 570 million years ago), which lasted perhaps only 10 million years. During this time the different phyla, including those existing today, developed. Meanwhile, the glaciers from a Proterozoic ice age were melting, raising sea levels above the continental shelves. This gave invertebrates more places to live. There are no fossils showing how the first invertebrates evolved; they just suddenly appear in the fossil record. This may be because they were evolving so quickly, and because they developed hard shells, allowing them to be preserved. Many organisms from this period were preserved in the Burgess Shale formation (530 million years ago) in British Columbia, Canada.

The sponges (phylum Porifera) appeared in the Middle Cambrian. The bodies of these “lower” invertebrates are neither symmetrical nor differentiated into tissues. Sponges are less evolutionarily advanced than members of the phylum Cnidaria, which includes jellyfish, corals, and sea anemones. These two phyla may have arisen directly and independently from the protists (simple one-celled organisms such as bacteria, algae, etc.).

The appearance of bilateral symmetry (two halves which are mirror images of each other) was an important evolutionary breakthrough. The most primitive bilaterally symmetrical animals are the flatworms (phylum Platyhelminthes). Platyhelminthes gave rise to the coelomates, which have a coelom, or internal body cavity. The coelomates split into two evolutionary

lines, the protostomes (molluscs, annelids, and arthropods) and the deuterostomes (echinoderms and chordates). A few phyla, such as the phylum Bryozoa, are intermediate between the two lines. Of the nearly 20,000 species of bryozoans known, only 3,500 are still living.

The molluscs are a very diverse group of invertebrates. They include snails, chitons, and cephalopods (squids and octopuses). Recent studies of invertebrate genetic material has shown that molluscs and annelids (segmented worms) probably evolved from arthropods. Neopilina, which was discovered in 1957, is a modern, “primitive mollusc.” It is similar to what the first molluscs are believed to have looked like. One of the main reasons molluscs have evolved so many different forms is that they have diverse methods of eating and of avoiding being eaten.

The arthropods (jointed foot) are the most successful group of organisms ever. They include centipedes, insects, crustaceans, horseshoe crabs, spiders, scorpions, and the extinct trilobites (Figure 1) and eurypterids (Figure 2). Arthropods evolved 630 million years

ago. The trilobites lived for 350 million years, from the Lower Cambrian to the Late Permian, and developed into over 1500 genera. They had compound eyes with thin, biconvex lenses made of calcite. The last of the trilobites died out at the end of the Permian. The eurypterids were ancient water-scorpions. Most eurypterids were less than 7.9 in (20 cm) long, but some giant forms grew nearly 6.5 ft (2 m) long, making them the largest arthropods ever.

Insects colonized the land just after plants did, about 410 million years ago. Cockroaches and dragonflies appeared over 300 million years ago, and for the next 100 million years, insects were the only animals that could fly. The chelicerates (spiders, mites, scorpions, horseshoe crabs, and eurypterids) evolved in the Cambrian.

The echinoderms (phylum Echinodermata) include starfish, sea-urchins, sea cucumbers, and crinoids. A great many of these organisms were fossilized because they have skeletons made of calcite plates. The greatest number of different genera of echinoderms lived during the Carboniferous (360-286 million years ago). The embryology of modern echinoderms suggests that they are related to the chordates. Modern echinoderm larvae have a ciliated band that runs along both sides of their bodies. The “dipleurula theory” suggests that ancient adult echinoderms had this band also, and that in the ancestors of the chordates, it fused along the back to form the beginnings of the dorsal nerve. Pikaia (Figure 3), a worm-like creature found in the Burgess Shale, is the oldest known chordate. It lived in the Middle Cambrian (about 530 million years ago).

Vertebrates are a subphylum of the Chordata (chordates). Lower vertebrates have a notochord, which is a flexible cartilage rod that runs along their backs; this is replaced in higher vertebrates with a vertebral column. Vertebrates are also called craniate chordates because they are the only animals with a distinct cranium (skull). The oldest known vertebrate remains date from the Upper Cambrian and Lower Ordovician (around 505 million years ago).

The first vertebrates were fishes. They evolved primarily during the Devonian (408-360 million years ago). The earliest fishes did not have jaws (Figure 4). Unlike modern jawless fishes (hagfishes and lampreys), the extinct forms were heavily armored and had pairs of fins. Jaws may have evolved from gill arches near the head, but there are no transition fossils which show this. Rays and sharks, which have skeletons of cartilage instead of bone, are the most ancient living fishes with jaws. Bony fishes, such as coelacanths and the ancestors of most modern fresh and saltwater fishes, probably evolved early in the Devonian. Coelacanths (Figure 5)

were believed to be extinct until a living one was discovered in 1938 in the Indian Ocean. Fishes thrived until the Late Devonian (360 million years ago), when a mass extinction wiped out 76% of fish families.

The amphibians were the first vertebrates to leave the seas for dry land. They evolved from fishes about 360 million years ago. One of the challenges to living on land is that animals must be able to support their own weight rather than simply allowing water to support them. The lobed fins of the bony fishes already contained the major bones that became the limbs of the early tetrapods (four feet); many of these bones are still used in our own limbs today.

The earliest-known amphibian was the Ichthyostega (Figure 6). It and other early tetrapods probably ate invertebrates such as cockroaches and spiders, as well as little fishes. The diadectomorphs appear to have been somewhat transitional between amphibians and reptiles. They belong to a category of amphibians called reptiliomorphs (as opposed to the batrachomorphs, or “true” amphibians).

The first reptiles, which were about the size of small lizards, emerged about 300 million years ago. They had a crucial advantage over amphibians in that their eggs could hatch on land, freeing them from spending part of their lives in the water. Among these early reptiles were the ancestors of modern birds and mammals. The pelycosaurs (Figure 7) were the most varied of the Early Permian reptiles. Some had tall, skin-covered “sails” that may have helped regulate their body temperature.

The main herbivores in the Late Permian were the dicynodonts, and the main carnivores were the gorgonopsians such as Arctognathus. Arctognathus had huge canines and could open its jaws 90^°. The ancestors of the crocodilians arose in the late Triassic. Terrestrisuchus was small (1.64 ft [0.5 m] long), probably ate insects and small reptiles, and may have walked bipedally on its long hind legs. Unlike its descendants, it did not live in the water.

Triassic oceans were filled with placodonts, nothosaurs, and ichthyosaurs (Figure 8). Placodonts had heavy teeth, which were probably used to crush the hard shells of mollusks. Nothosaurs had pointed teeth in their small heads, and may have eaten fish. Ichthyosaurs (fish lizards) were shaped somewhat like giant porpoises with long, narrow jaws. They grew up to 49 ft (15 m) long in the Late Triassic, but were smaller during the Jurassic and Cretaceous. Plesiosaurs, along with ichthyosaurs, ruled the Jurassic and Cretaceous seas (Figure 9). Plesiosaurs were probably related to nothosaurs. Their paddles, however, were flat and like an airplane wing in cross-section, so that these animals may have moved through the water by “flying,” the way penguins and sea turtles do.

The largest mass extinction of all time occurred at the end of the Permian (248-238 million years ago). All but about 10 tetrapod families died out, as well as 96% of marine species.

The dinosaurs arose in the Late Triassic (230 million years ago). The earliest dinosaurs walked upright on two legs and were carnivorous. Dinosaurs are divided into two groups, the Saurischia and the Ornithischia, based on their hips. The saurischians had “lizard hips” and the ornithischians had “bird hips.” The lizard hips arose first; Triassic dinosaurs were saurischians. While saurischians included both carnivores (meat eaters) and herbivores (plant eaters), all of the ornithischians were herbivores. The carnivorous saurischians are known as the theropods. They included the Late Jurassic Allosaurus and Late Cretaceous Tyrannosaurus, which probably was the largest carnivore ever to walk Earth. Brachiosaurus

was a herbivorous saurischian. This sauropod had a long neck and short, thick legs like an elephant’s, which were designed for bearing its enormous weight. There were two kinds of bird-hipped dinosaurs, the Late Cretaceous Cerapoda and the Late Jurassic Thyreophora. The former included hadrosaurs (duck-billed dinosaurs) and ceratopsians (“horned faces”), and the latter included ankylosaurs and stegosaurs.

One of the major debates among paleontologists is whether or not dinosaurs were warm-blooded. Some scientists suggest that a four-chambered heart would have been necessary to pump blood up the long necks of the sauropods to their brains. Mammals and birds, which are warm-blooded, have four-chambered hearts, but so do the cold-blooded crocodilians. The debate has not been conclusively resolved.

There are two main models that attempt to explain the success of the dinosaurs. According to one, dinosaurs out-competed the mammal-like reptiles over a long period of time due to superior adaptations such as upright walking. The other model, which is

supported by fossil evidence, says that the dinosaurs took advantage of openings created by two mass extinctions. By the end of the Triassic, dinosaurs had taken over the land. They were dominant for 165 million years, from the Late Triassic until their extinction at the Cretaceous-Tertiary (K-T) boundary some 65 million years ago. This massive extinction may have taken place in only a week or lasted for tens of thousands of years; this has not been determined yet, but further study may provide an answer. One prominent theory for the cause of this event is that a meteorite hit Earth 65 million years ago, creating a cloud of dust which obscured the sun and prevented photosynthesis for several months. This set off a chain reaction which culminated in the death of many life forms on earth.

The pterosaurs (winged reptiles) were closely related to the dinosaurs, and lived at the same time (Figure 10). They had short bodies with long necks, and pointed jaws. They ranged from the pigeon-sized Eudimorphodon to the largest of all flying creatures, Quetzalcoatlus, which is believed to have had a wing span of up to 49 ft (15 m). Thanks to some well-preserved pterosaurs, we know they had hair, and were, therefore, possibly warm-blooded.

Birds evolved from the theropod dinosaurs. The first bird, Archaeopteryx (Figure 11), lived in the Late Jurassic (150 million years ago). This small, magpie-sized bird had sharp teeth on both jaws, and feathers. The presence of teeth, claws on the fingers, and its bony

tail are reptilian characteristics, whereas the feathers and “wishbone” (fused collarbones) are bird characteristics.

The ancestors of mammals were mammal-like reptiles called cynodonts (Figure 12). Cynodonts arose in the Late Permian (about 245 million years ago). Some were dog-sized carnivores, and others were herbivores. One way the transition to mammals can be seen is in the manner in which the jaws are joined. Mammals have a new joint not present in reptile jaws, which allows for the side-to-side action of chewing. The earliest true mammals appeared in the Late Triassic (230 million years ago). One was the tiny, shrew like Megazostrodon. Because of its size and the fact that it had pointed teeth, it was probably an insectivore. Mammals radiated widely in the Paleocene Epoch (66-58 million years ago), taking advantage of the openings left by the dinosaurs when they died out. The mammals were intelligent and took care of their young for an extended period of time, which probably gave them an edge over other animals. Mammals now appear in many very different forms, including the orders Insectivora (shrews), Carnivora (cats, dogs, seals), Chiroptera (bats), Cetacea (whales), Proboscidea (elephants), Tubulidentata (aardvarks), and Primates.

Resources

BOOKS

Benton, Michael J. Vertebrate Palaeontology, 3rd ed. London: Blackwell Publishing Professional, 2004.

Gould, Stephen Jay. Wonderful Life. The Burgess Shale and the Nature of History. New York: W.W. Norton & Company, 1989.

Palmer, Douglas. The Marshall Illustrated Encyclopedia of Dinosaurs & Prehistoric Animals: A Comprehensive

KEY TERMS

Cladistics —The study of evolutionary relationships between organisms based on analysis of the similarities and differences of their physical traits, that is, based on evolutionary divergence.

Coelom —An internal body cavity in which the digestive organs are suspended.

Embryology —The study of the development and early growth of living organisms.

Extinction —The condition in which all members of a group of organisms have ceased to exist.

Invertebrate —A multicellular animal that lacks a spinal column encased in vertebrae and a distinct skull.

Notochord —A flexible cartilage rod that runs along the back in chordates.

Phylum —A taxonomic division of animals, one level below kingdom in the taxonomic hierarchy (plural: phyla).

Vertebrate —Includes all animals with a vertebral column protecting the spinal cord such as humans, dogs, birds, lizards, and fish.

Color Guide to over 500 Species. New York: New Line Books, 2003.

Prothero, Donald R. Bringing Fossils To Life: An Introduction To Paleobiology. Columbus, OH: McGraw-Hill Science/Engineering/Math, 2003.

Kathryn M. C. Evans

Paleontology

Paleontology is the branch of science devoted to the understanding of past life as revealed by the fossil record. Normally, when an organism dies, its physical remains are scattered and destroyed by the elements in a short span of time. Such elements include not only wind, weather, and decay, but also water and the activities of carnivores and scavengers of many kinds. Occasionally, however, bony remains (and, very exceptionally, some soft tissues) lying on the surface or in superficial cavities may be covered by accumulating sediments (most often river or lake muds in the case of terrestrial organisms, and sea-bottom particulates in that of marine forms) before they are totally destroyed. Among the remains that escape destruction, complete articulated skeletons are extremely rare; more commonly preserved are individual bones and teeth, often broken. Unless the enclosing sediments are chemically hostile, or become melted by heat from the Earth's interior or pressure from above, bones thus incorporated into the accumulating sediment pile can survive more or less indefinitely, though their distortion due to local Earth movements is not uncommon. As water carries minerals in solution through both the sediments and the contained animal or plant remains, the organic constituents of those remnants become replaced by minerals, in a process most often known as mineralization. If erosion of the enclosing sediment pile subsequently sets in, the now-fossilized remains may become exposed once more at the Earth's surface, where they are yet again subject to the forces of natural destruction. However, for a brief period they are also available to be collected by human beings, who have been picking up unusual re-exposed objects for the last few hundred thousand years, at least.

Fossils are found all over the world, and are a vast storehouse of information about past life. Those who professionally find fossils and extract such information from them are known as paleontologists. From the very beginning, paleontology has been integral to the study of Earth history, which began with attempts to order the sedimentary rocks, laid down by water and wind, which contain fossils. The other grand categories of rocks exposed on the Earth's surface include igneous rocks (extruded from Earth's molten core by volcanoes and by the physical rising through the solid crust of lighter rocks such as granite), and metamorphic rocks, which are sedimentary rocks that have been recrystallized by pressure and heat. Volcanic rocks are particularly helpful in dating the ages of various events in Earth history because reliable techniques exist by which to measure the time that has elapsed since they last cooled. These techniques depend on the phenomenon of radioactivity, by which unstable forms of certain elements "decay" to stable states at known rates. Volcanic rocks do not contain fossils themselves (unless you count as fossils such things as the vacuities in the shape of human bodies found at Pompeii, or the ancient hominid footprints at Laetoli), but they represent single points in time and are often interleaved among sedimentary fossil-bearing layers, which can be dated by reference to them.

Early history

The basic principles of the study of sedimentary rocks were established by the Danish-born naturalist and physician Nicolaus Steno (1638–1686) in the mid seventeenth century. These principles state that all stratified sedimentary rocks—however distorted they may subsequently have become—started life as horizontal bands of sediments and that they were laid down in sequence, with the oldest layers at the bottom and the youngest at the top. Such layering is usually readily visible in local sedimentary basins, but there is a problem in correlating the strata that are exposed in different basins and geographical areas. This is the context within which fossils entered the picture. The fossil record clearly shows that past time has been characterized by a long succession of distinctive biotas, or communities of organisms; it was the resulting diagnostic assemblages of fossils that were seized upon by early stratigraphers as the key to ordering regionally exposed rocks into their temporal sequences.

During the early years of the nineteenth century, the engineer and geologist William Smith (1769–1839) demonstrated in England that sedimentary units could be identified by their distinctive fossil content. At about the same time in France, naturalist Georges Cuvier (1769–1832) worked out the sequence of sedimentary units in the Paris Basin using fossil terrestrial vertebrates as markers, and showed that many large animals had no living counterparts. In doing the latter, Cuvier made extinction a reality to be contended with. And he went farther, showing that as the rocks became younger they contained faunas steadily more similar to those of today. He concluded that this pattern revealed an advancing complexity of life, but he was unable to find gradations among the various faunas preserved in the Paris Basin (where stratigraphic discontinuities are in fact rife, as they are in most terrestrial situations). Instead, he found that distinctive faunas were replaced by other distinctive fossil associations. This suggested to Cuvier that a series of catastrophes had wiped out successive faunas, which were re-created anew after each extinction event. Popular opinion rapidly adopted the last such event as evidence of the Biblical deluge, and conveniently equated the earlier faunas with the biblical "days" of creation.

Thus, improbably, was paleontology born as a science. For several decades following the pioneering work of Cuvier and Smith, paleontologists labored within the confines of biblical constructs even as they gradually built up a robust picture of the Earth's sedimentary history based on an expanding fossil record. During this period the beginnings of specialization within paleontology began to appear, with today's division of the science into vertebrate and invertebrate branches emerging. The distinction is important, not simply because of the distinctiveness and the rapid swelling of the database in each branch, but because invertebrate paleontology came to be dominated by the study of marine organisms, just as vertebrate paleontology was dominated by terrestrial forms. Neither branch was (or is) exclusively focused on one side or the other of the marine-terrestrial dichotomy, but a subtle difference in outlook was almost inevitably introduced because the marine sedimentary record is much more continuous than its terrestrial equivalent, which is repeatedly interrupted by erosional cycles.

Paleontology and evolutionary ideas

By the time that Charles Darwin (1809–1882) published his epochal On the Origin of Species in 1859, the outline shape of the fossil record was fairly well established. Inconveniently well-established, in fact, as far as Darwin was concerned. For while Darwin favored an elegantly simple model of evolution as a more or less straight-line process involving gradual change in living populations from generation to generation under the guiding hand of natural selection, the fossil record itself showed a pattern of discontinuities among taxa (a generalized term given to taxonomic units at any rank: species, genera, and more inclusive groupings such as families and orders). There was much early debate over the application of Darwinian ideas to the fossil record. Some scientists, such as the eminent Victorian comparative anatomist Richard Owen (1804–1892), who appropriated the study of the remarkable fossil reptile bones discovered by Gideon Mantell (1790–1852) in the 1820s (and who coined the term dinosaur ), reacted negatively to Darwin's publication. Owen preferred to see the fossil record as evidence of the unfolding of a divine plan, and clung throughout his life to an essentialist view of species as fixed and unchanging. Others, such as the brash Thomas Henry Huxley (1825–1895), took up the cudgels on Darwin's side, most famously in his debate with Bishop Samuel Wilberforce (1805–1873) in Oxford on June 30, 1860. Huxley supported Darwin's view of fossils as witnesses to a process of gradual transformation of organisms over time—although, significantly, he never managed to place the newlydiscovered Neanderthal fossil into this perspective, preferring to interpret it as a lowly form of modern human.

Interestingly, following a scandalized initial reaction to his evolutionary ideas, Darwin's central tenet of "descent with modification," whereby all life forms are related by descent from a common ancestor, became quite rapidly accepted by scientists and public alike. What was not so readily accepted was the mechanism of natural selection, which involves the gradual modification of population gene frequencies over long periods of time due to the greater reproductive success of fitter individuals, those best adapted to prevailing environments. Indeed, natural selection did not assume its current central place in paleontology and other branches of evolutionary biology until the second quarter of the twentieth century, when the Evolutionary Synthesis took biology by storm. The product of agreement among influential geneticists, paleontologists, and systematists, the Synthesis eventually succeeded in reducing virtually all evolutionary phenomena to generation-by-generation changes in gene frequencies. This notion emphasized the linear, transformational, dimension of evolution at the expense of the histories of taxa, and it encouraged paleontologists to ignore the discontinuities in the fossil record that Darwin had been aware of, but had ascribed to the record's incompleteness.

It was not until the 1970s that paleontologists started to realize that perhaps the gaps in the fossil record were actually revealing something after all. Thus was born the notion of punctuated equilibria (long periods of stasis interrupted by brief bursts of change), which was presented as an alternative to the phyletic gradualism preached by the Synthesis. Paleontologists in general began to realize that the Synthesis, elegant though its simplicities might have been, was incomplete as an explanatory framework for all of the evolutionary phenomena evoked by the fossil record. It turned out, indeed, that although natural selection undoubtedly plays a role in the differentiation of species and in their accommodation to local environments, many other influences enter the evolutionary equation. These include speciation, the set of mechanisms by which new species come about, and competition among closely and more distantly related species, which involves extinction as a regular event. All of this occurs, moreover, within a context of constantly fluctuating environmental conditions. Modern paleontologists are hence much more acutely aware than their predecessors were of the complexities of the evolutionary process and of the roles played in it by competing taxa, as well as by competing individuals.

Hypothesis formation in paleontology

For many years paleontologists pursued their work—of sorting out the relationships among the myriad life forms represented in the fossil record—largely by intuition and the assessment of overall resemblance. Admittedly, this process got them a long way in sketching the outlines of the tree of life, but it did lead to some anomalies. Thus while it may appear counterintuitive to claim that lunfgishes are more closely related to cows than to salmon, in terms of ancestry and descent this claim is demonstrably true. Paleontology was thus revolutionized during the 1970s by the widespread introduction of cladistics, an approach to comparative biology that provided an explicit recipe for recognizing relationships among taxa. In a nutshell, cladistics argues that only the common possession of derived characters, those inherited from an immediate ancestral taxon, is useful in deducing relationships among taxa. The common possession of primitive attributes, those inherited from more remote common ancestors, shows only that two taxa belong to the wider group descended from that ancestor. Thus having a spine shows simply that you belong to the large taxon of vertebrates, while having three bones in the middle ear indicates that you are a mammal, a member of a taxon that is nested inside that group. The distribution of derived characters within a group is summarized in a branching diagram known as a cladogram, which in its simplest form states nothing more than that "taxon A and taxon B are more closely related by common ancestry than either is to related taxon C. " Cladograms are the only statements in systematics that are truly testable.

More elaborate is the evolutionary tree, which adds ancestry and descent as well as time to the mix. Trees are more interesting than cladograms, but cannot be tested since the age of fossils has no direct connection to their relationships, and because in theory an ancestor has to be primitive in all respects relative to its descendant, in which case there is nothing to link them. Yet more complex (and more interesting) is the evolutionary scenario, in which paleontologists add everything they know about function, environment, adaptation, and so forth to the information present in the tree. Competing scenarios are comparable only on the basis of their plausibility, which makes them inherently unscientific; yet their plausibility can be reasonably objectively judged if they are based on specifically stated cladograms and trees. Scenarios are constructed using a bewildering variety of types of information derived from many different sciences. Paleontologists take into account information derived from paleoclimatology (the study of past climates), taphonomy (the science of what happens to organic remains after death), sedimentology (environments in which fossils were deposited), stratigraphy (in the broadest sense, the sequence and relationships of rock strata, and their dating by a host of means), and functional anatomy (the study of the morphology of fossil forms, and how they may have functioned in life), to name but a few of the areas that contribute to the most complete understanding possible of the lives, environments, and relationships of fossil species. Molecular genetics has also begun to contribute to our knowledge of affinities among extinct species, and even newer technologies are on the way.

Paleontology in a wider context

Since about 1970, then, paleontology has vastly refined its abilities to teach us about our past and about the broader biological context from which our remarkable species has emerged. Literally, paleontology has played and continues to play a central role in establishing our own place in nature. And the lesson is a humbling one. The living world of today is mind-bogglingly diverse and marvelous indeed, heedless though so many of us are of its welfare. But looking around ourselves today we see only a single slice of time; and when we add the paleontological dimension we can at last begin to glimpse the truly extraordinary richness and majesty of the organic context—creation, if you will—of which we form part.

This recognition of the vastness of nature in time as well as in space has had a profound impact. Since medieval times and probably long before, people in the Western tradition had viewed Homo sapiens as the center of earthly creation, around which all else revolved. But paleontology, especially in concert with the more recent revelations of cosmology, has demonstrated that our species is in fact an infinitesimal part of an enormous and still-enlarging universe. Among many members of our egotistical species, the tendency has been to ignore this uncomfortable fact. But it is nonetheless a fact to be faced, and some theologians have sought to reconcile the findings of paleontology with the traditions of Christian theology. The best-known of these was the Jesuit Pierre Teilhard de Chardin (1881–1955), a practicing geologist and paleontologist, whose posthumously published The Phenomenon of Man had a particularly broad impact. Teilhard viewed the process through which humanity emerged in teleological terms, envisioning the appearance of human consciousness as the outcome of directed change from a more generalized state, in pursuit of an ultimate union with the "Omega." This latter was taken by many to represent a "cosmic Christ," as the biologist Julian Huxley (1887–1975) put it. Teilhard's arguments are often obscure, but his wide following bears witness to the profound urge that exists among so many to incorporate the perspectives of paleontology into a wider worldview.

See also Darwin, Charles; Evolution, Human; Gradualism; Life, Origins of; Paleoanthropology; Punctuated Equilibrium; Teilhard de Chardin, Pierre

Bibliography

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eldredge, niles. "rates of evolution revisted." paleobiology 2 (1976): 174–177.

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tattersall, ian. the fossil trail: how we know what we think we know about human evolution. new york: oxford university press, 1995.

tattersall, ian. becoming human: evolution and human uniqueness. new york: harcourt, 1998.

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teilhard de chardin, pierre. the phenomenon of man. new york: harper, 1976.

ian tattersall
kenneth mowbray

Paleontology

Paleontology is the study of ancient animal life and how it developed. It is divided into two subdisciplines, invertebrate paleontology and vertebrate paleontology. Paleontologists use two lines of evidence to learn about ancient animals. One is to examine animals that live today, and the other is to study fossils. The study of modern animals includes looking at the earliest stages of development and the way growth occurs (embryology ), and comparing different organisms to see how they are related evolutionarily (cladistics). The fossils that paleontologists study may be the actual remains of the organisms, or simply traces the animals have left (tracks or burrows left in fine sediments). Paleontology lies at the boundary of the life sciences and the earth sciences. It is thus useful for dating sediments, reconstructing ancient environments, and testing models of plate tectonics , as well as understanding how modern animals are related to one another.

Invertebrate paleontology

An invertebrate is essentially a multicellular animal that lacks a spinal column encased in vertebrae and a distinct skull. There are about 30 phyla, or groups, of invertebrates , and roughly 20 of these have been preserved as fossils. Still other phyla probably existed, but are not represented in the fossil record because the animals' soft bodies were not preserved. Only one invertebrate phylum is known to have become extinct—the Archaeocyathida. These organisms, which were superficially similar to sponges , did not survive past the Middle Cambrian period (530 million years ago).

The different body plans of invertebrates most likely evolved during the Precambrian, between 1,000 and 700 million years ago. There was an "explosion" of invertebrate evolution in the Lower Cambrian (beginning about 570 million years ago), which lasted perhaps only 10 million years. During this time the different phyla, including those existing today, developed. Meanwhile, the glaciers from a Proterozoic ice age were melting, raising sea levels above the continental shelves. This gave invertebrates more places to live. There are no fossils showing how the first invertebrates evolved; they just suddenly appear in the fossil record. This may be because they were evolving so quickly, and because they developed hard shells, allowing them to be preserved. Many organisms from this period were preserved in the Burgess Shale formation (530 million years ago) in British Columbia, Canada.

The sponges (phylum Porifera) appeared in the Middle Cambrian. The bodies of these "lower" invertebrates are neither symmetrical nor differentiated into tissues. Sponges are less evolutionarily advanced than members of the phylum Cnidaria, which includes jellyfish , corals, and sea anemones . These two phyla may have arisen directly and independently from the protists (simple onecelled organisms such as bacteria , algae , etc.).

The appearance of bilateral symmetry (two halves which are mirror images of each other) was an important evolutionary breakthrough. The most primitive bilaterally symmetrical animals are the flatworms (phylum Platyhelminthes). Platyhelminthes gave rise to the coelomates, which have a coelom, or internal body cavity. The coelomates split into two evolutionary lines, the protostomes (molluscs, annelids, and arthropods ) and the deuterostomes (echinoderms and chordates ). A few phyla, such as the phylum Bryozoa, are intermediate between the two lines. Of the nearly 20,000 species of bryozoans known, only 3,500 are still living.

The molluscs are a very diverse group of invertebrates. They include snails , chitons , and cephalopods (squids and octopuses). Recent studies of invertebrate genetic material has shown that molluscs and annelids (segmented worms ) probably evolved from arthropods. Neopilina, which was discovered in 1957, is a modern, "primitive mollusc." It is similar to what the first molluscs are believed to have looked like. One of the main reasons molluscs have evolved so many different forms is that they have diverse methods of eating and of avoiding being eaten.

The arthropods (jointed foot) are the most successful group of organisms ever. They include centipedes , insects , crustaceans, horseshoe crabs , spiders, scorpions, and the extinct trilobites (Figure 1) and eurypterids (Figure 2). Arthropods evolved 630 million years ago. The trilobites lived for 350 million years, from the Lower Cambrian to the Late Permian, and developed into over 1500 genera. They had compound eyes with thin, biconvex lenses made of calcite. The last of the trilobites died out at the end of the Permian. The eurypterids were ancient water-scorpions. Most eurypterids were less than 7.9 in (20 cm) long, but some giant forms grew nearly 6.5 ft (2 m) long, making them the largest arthropods ever.

Insects colonized the land just after plants did, about 410 million years ago. Cockroaches and dragonflies appeared over 300 million years ago, and for the next 100 million years, insects were the only animals that could fly. The chelicerates (spiders, mites , scorpions, horseshoe crabs , and eurypterids) evolved in the Cambrian.

The echinoderms (phylum Echinodermata) include starfish , sea-urchins, sea cucumbers , and crinoids. A great many of these organisms were fossilized because they have skeletons made of calcite plates. The greatest number of different genera of echinoderms lived during the Carboniferous (360-286 million years ago). The embryology of modern echinoderms suggests that they are


related to the chordates. Modern echinoderm larvae have a ciliated band that runs along both sides of their bodies. The "dipleurula theory" suggests that ancient adult echinoderms had this band also, and that in the ancestors of the chordates, it fused along the back to form the beginnings of the dorsal nerve. Pikaia (Figure 3), a worm-like creature found in the Burgess Shale, is the oldest known chordate. It lived in the Middle Cambrian (about 530 million years ago).

Vertebrates are a subphylum of the Chordata (chordates). Lower vertebrates have a notochord, which is a flexible cartilage rod that runs along their backs; this is replaced in higher vertebrates with a vertebral column. Vertebrates are also called craniate chordates because they are the only animals with a distinct cranium (skull). The oldest known vertebrate remains date from the Upper Cambrian and Lower Ordovician (around 505 million years ago).

The first vertebrates were fishes. They evolved primarily during the Devonian (408-360 million years ago). The earliest fishes did not have jaws (Figure 4). Unlike modern jawless fishes (hagfishes and lampreys), the extinct forms were heavily armored and had pairs of fins. Jaws may have evolved from gill arches near the head, but there are no transition fossils which show this. Rays and sharks , which have skeletons of cartilage instead of bone, are the most ancient living fishes with jaws. Bony fishes, such as coelacanths and the ancestors of most modern fresh and saltwater fishes, probably evolved early in the Devonian. Coelacanths (Figure 5) were believed to be extinct until a living one was discovered in 1938 in the Indian Ocean. Fishes thrived until the Late Devonian (360 million years ago), when a mass extinction wiped out 76% of fish families.

The amphibians were the first vertebrates to leave the seas for dry land. They evolved from fishes about 360 million years ago. One of the challenges to living on land is that animals must be able to support their own weight rather than simply allowing water to support them. The lobed fins of the bony fishes already contained the major bones that became the limbs of the early
tetrapods (four feet); many of these bones are still used in our own limbs today.

The earliest-known amphibian was the Ichthyostega (Figure 6). It and other early tetrapods probably ate invertebrates such as cockroaches and spiders, as well as little fishes. The diadectomorphs appear to have been somewhat transitional between amphibians and reptiles . They belong to a category of amphibians called reptiliomorphs (as opposed to the batrachomorphs, or "true" amphibians).

The first reptiles, which were about the size of small lizards, emerged about 300 million years ago. They had a crucial advantage over amphibians in that their eggs could hatch on land, freeing them from spending part of their lives in the water. Among these early reptiles were the ancestors of modern birds and mammals . The pelycosaurs (Figure 7) were the most varied of the Early Permian reptiles. Some had tall, skin-covered "sails" that may have helped regulate their body temperature . The main herbivores in the Late Permian were the dicynodonts, and the main carnivores were the gorgonopsians such as Arctognathus. Arctognathus had huge canines and could open its jaws 90°. The ancestors of the crocodilians arose in the late Triassic. Terrestrisuchus was small (1.64 ft [0.5 m] long), probably ate insects and small reptiles, and may have walked bipedally on its long hind legs. Unlike its descendants, it did not live in the water.

Triassic oceans were filled with placodonts, nothosaurs, and ichthyosaurs (Figure 8). Placodonts had heavy teeth, which were probably used to crush the hard shells of mollusks . Nothosaurs had pointed teeth in their small heads, and may have eaten fish. Ichthyosaurs (fish lizards) were shaped somewhat like giant porpoises with long, narrow jaws. They grew up to 49 ft (15 m) long in the Late Triassic, but were smaller during the Jurassic and Cretaceous. Plesiosaurs, along with ichthyosaurs, ruled the Jurassic and Cretaceous seas (Figure 9). Plesiosaurs were probably related to nothosaurs. Their paddles, however, were flat and like an airplane wing in cross-section, so that these animals may have moved through the water by "flying," the way penguins and sea turtles do.

The largest mass extinction of all time occurred at the end of the Permian (248-238 million years ago). All
but about ten tetrapod families died out, as well as 96% of marine species.

The dinosaurs arose in the Late Triassic (230 million years ago). The earliest dinosaurs walked upright on two legs and were carnivorous. Dinosaurs are divided into two groups, the Saurischia and the Ornithischia, based on their hips. The saurischians had "lizard hips" and the ornithischians had "bird hips." The lizard hips arose first; Triassic dinosaurs were saurischians. While saurischians included both carnivores (meat eaters) and herbivores (plant eaters), all of the ornithischians were herbivores. The carnivorous saurischians are known as the theropods. They included the Late Jurassic Allosaurus and Late Cretaceous Tyrannosaurus, which probably was the largest carnivore ever to walk the earth. Brachiosaurus was a herbivorous saurischian. This sauropod had a long neck and short, thick legs like an elephant's, which were designed for bearing its enormous weight. There were two kinds of bird-hipped dinosaurs, the Late Cretaceous Cerapoda and the Late Jurassic Thyreophora. The former included hadrosaurs (duck-billed dinosaurs) and ceratopsians ("horned faces"), and the latter included ankylosaurs and stegosaurs.

One of the major debates among paleontologists is whether or not dinosaurs were warm-blooded. Some scientists suggest that a four-chambered heart would have been necessary to pump blood up the long necks of the sauropods to their brains. Mammals and birds, which are warm-blooded, have four-chambered hearts, but so do the cold-blooded crocodilians. The debate has not been conclusively resolved.

There are two main models that attempt to explain the success of the dinosaurs. According to one, dinosaurs out-competed the mammal-like reptiles over a long period of time due to superior adaptations such as upright walking. The other model, which is supported by fossil evidence, says that the dinosaurs took advantage of openings created by two mass extinctions. By the end of the Triassic, dinosaurs had taken over the land. They were dominant for 165 million years, from the Late Triassic until their extinction at the Cretaceous-Tertiary (K-T) boundary some 65 million years ago. This massive extinction may have taken place in only a week or lasted for tens of thousands of years; this has not been determined yet, but further study may provide an answer. One prominent theory for the cause of this



event is that a meteorite hit the earth 65 million years ago, creating a cloud of dust which obscured the sun and prevented photosynthesis for several months. This set off a chain reaction which culminated in the death of many life forms on Earth.

The pterosaurs (winged reptiles) were closely related to the dinosaurs, and lived at the same time (Figure 10). They had short bodies with long necks, and pointed jaws. They ranged from the pigeon-sized Eudimorphodon to the largest of all flying creatures, Quetzalcoatlus, which is believed to have had a wing span of up to 49 ft (15 m). Thanks to some well-preserved pterosaurs, we know they had hair, and were therefore possibly warm-blooded.

Birds evolved from the theropod dinosaurs. The first bird, Archaeopteryx (Figure 11), lived in the Late Jurassic (150 million years ago). This small, magpie-sized bird had sharp teeth on both jaws, and feathers. The presence of teeth, claws on the fingers, and its bony tail are reptilian characteristics, whereas the feathers and "wishbone" (fused collarbones) are bird characteristics.

The ancestors of mammals were mammal-like reptiles called cynodonts. Cynodonts arose in the Late Permian (about 245 million years ago). Some were dogsized carnivores, and others were herbivores. One way the transition to mammals can be seen is in the manner in which the jaws are joined. Mammals have a new joint not present in reptile jaws, which allows for the side-to-side action of chewing. The earliest true mammals appeared in the Late Triassic (230 million years ago). One was the tiny, shrew-like Megazostrodon. Because of its size and the fact that it had pointed teeth, it was probably an insectivore . Mammals radiated widely in the Paleocene Epoch (66-58 million years ago), taking advantage of the openings left by the dinosaurs when they died out. The mammals were intelligent and took care of their young for an extended period of time, which probably gave them an edge over other animals. Mammals now appear in many very different forms, including the orders Insectivora (shrews ), Carnivora (cats , dogs, seals ), Chiroptera (bats ), Cetacea (whales), Proboscidea (elephants), Tubulidentata (aardvarks), and Primates .

Resources

books

Benton, Michael J. Vertebrate Palaeontology. London: Uniwin Hyman, 1990.

Chaline, Jean. Paleontology of Vertebrates. New York: Springer-Verlag, 1990.

Enay, R. Paleontology of Invertebrates. New York: Springer-Verlag, 1993.

Gould, Stephen Jay. Wonderful Life. The Burgess Shale and the Nature of History. New York: W.W. Norton & Company, 1989.

Palmer, Douglas. The Marshall Illustrated Encyclopedia of Dinosaurs & Prehistoric Animals: A Comprehensive Color Guide to over 500 Species. New York: Todtri, 2002.

Prothero, Donald R. Bringing Fossils To Life: An Introduction To Paleobiology. Columbus: McGraw-Hill Science/Engineering/Math, 1997.

Kathryn M. C. Evans

KEY TERMS

Cladistics

—The study of evolutionary relationships between organisms based on analysis of the similarities and differences of their physical traits, that is, based on evolutionary divergence.

Coelom

—An internal body cavity in which the digestive organs are suspended.

Embryology

—The study of the development and early growth of living organisms.

Extinction

—The condition in which all members of a group of organisms have ceased to exist.

Invertebrate

—A multicellular animal that lacks a spinal column encased in vertebrae and a distinct skull.

Notochord

—A flexible cartilage rod that runs along the back in chordates.

Phylum

—A taxonomic division of animals, one level below kingdom in the taxonomic hierarchy (plural: phyla).

Vertebrate

—Includes all animals with a vertebral column protecting the spinal cord such as humans, dogs, birds, lizards, and fish.