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.
The basic principles of the study of sedimentary rocks were established by the Danish-born naturalist and physician Nicolaus Steno (1638686) in the mid seventeenth century. These principles state that all stratified sedimentary rocksowever distorted they may subsequently have becometarted 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 (1769839) demonstrated in England that sedimentary units could be identified by their distinctive fossil content. At about the same time in France, naturalist Georges Cuvier (1769832) 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 (1809882) 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 (1804892), who appropriated the study of the remarkable fossil reptile bones discovered by Gideon Mantell (1790852) 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 (1825895), took up the cudgels on Darwin's side, most famously in his debate with Bishop Samuel Wilberforce (1805873) in Oxford on June 30, 1860. Huxley supported Darwin's view of fossils as witnesses to a process of gradual transformation of organisms over timelthough, 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 workf sorting out the relationships among the myriad life forms represented in the fossil recordargely 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 contextreation, if you willf 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 (1881955), 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 (1887975) 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
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