Paleontology: A Brief History of Life

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Overview

"Endlessly absorbing and informative. It would be hard to imagine a better introduction to this most important and fascinating field.”—Bill Bryson, author of A Short History of Nearly Everything

Paleontology: A Brief History of Life is the fifth title published in the Templeton Science and Religion Series, in which scientists from a wide range of fields distill their experience and knowledge into brief tours of their respective specialties.
In this volume, Ian Tattersall, a highly esteemed figure in the fields of anthropology, archaeology, and paleontology, leads a fascinating tour of the history of life and the evolution of human beings.
 
Starting at the very beginning, Tattersall examines patterns of change in the biosphere over time, and the correlations of biological events with physical changes in the Earth’s environment. He introduces the complex of evolutionary processes, situates human beings in the luxuriant diversity of Life (demonstrating that however remarkable we may legitimately find ourselves to be, we are the product of the same basic forces and processes that have driven the evolutionary histories of all other creatures), and he places the origin of our extraordinary spiritual sensibilities in the context of the exaptational and emergent acquisition of symbolic cognition and thought.
 
Concise and yet comprehensive, historically penetrating and yet up-to-date, responsibly factual and yet engaging, Paleontology serves as the perfect entrée to science's greatest story.

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Product Details

Meet the Author

Ian Tattersall is a curator in the division of anthropology at the American Museum of Natural History in New York City. Trained in archaeology and anthropology at Cambridge, and in vertebrate paleontology at Yale, Tattersall has concentrated his research since the 1960s in three main areas: the analysis of the human fossil record and its integration with evolutionary theory, the origin of human cognition, and the study of the ecology and systematics of the lemurs of Madagascar. He is also a prominent interpreter of human paleontology to the public.

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Read an Excerpt

Paleontology

A Brief History of Life


By Ian Tattersall

Templeton Press

Copyright © 2010 Ian Tattersall
All rights reserved.
ISBN: 978-1-59947-342-0



CHAPTER 1

Rocks, Time, and Fossils


Whether or not living forms exist elsewhere in the cosmos, for all practical purposes life as we know it was born here on Earth, several billion years ago. An awful lot has happened since then, and it is in the rocks composing the surface of our planet that we find the fossils that document the long history of living things. So it seems appropriate to start this book on paleontology, the science that studies those fossils, with a few words about the planet that we take so much for granted.

The geologist Preston Cloud once neatly described our Earth as an "Oasis in Space," which is, I think, about as apt a short description as it's possible to achieve. Our planet today really is an extraordinary place, with an oxygen-rich atmosphere, abundant water, a hospitable range of surface temperatures, and all the other necessities for the maintenance of life as it is familiar to us today. This amazing and comfortable environment exists, moreover, in the midst of a vast, hostile emptiness. Yet life itself came into being under very different—and very much more extreme—conditions.

The matter of origins goes back in an infinite recession, to a point that lies beyond the bounds of today's science. But scientists know the general outlines of how the Earth first began to form, some 4.5 billion years ago, out of a roiling cloud of hot dust and gases that eventually condensed to form our solar system. In early days the Earth's surface was an inferno, assailed from below by raging radioactive heat and from above by a constant bombardment of asteroids, as the remains of the debris cloud were "mopped up." Volcanoes on the hardening surface vigorously exhaled gases such as carbon dioxide, ammonia, and methane into an atmosphere initially consisting largely of hydrogen and helium. In brief, the early atmosphere was a toxic mixture of gases that would have been hostile to almost all forms of life that we know today. Equally inhospitable were the noxious oceans, which started to form as soon as the Earth's surface had cooled sufficiently to support liquid water, initially gassed out as vapor.

Still, the formation of the planet's solid outer crust proceeded rapidly as the fireball lost its initial heat. The earliest rocks known may be as much as 4.3 billion years old and are believed to be witnesses to the early operation of the processes that have governed the form of the Earth's surface ever since. Once the crust had hardened sufficiently, its surface began to be cracked by the motion of the hot, molten rock below. Imagine a pot of thick soup simmering on a stove. Warm soup rises from the bottom of the bowl at the middle, where it is hottest. On reaching the surface it flows outward to the sides of the pot, where it cools and sinks once more, ultimately to be reheated and rise again. Driven by the radioactive furnace in the planet's interior, an identical process was established under our feet well over 4 billion years ago. The upshot is that the surface of the planet was, and continues to be, divided into a varying number of more or less rigid tectonic plates that are forever in motion. New, hot magma is added on one side of each plate as it is erupted along the linear structures known as mid-ocean ridges, while old, cold rock is returned to the depths along subduction zones at the other side.

The basaltic rocks of the oceanic crust are relatively heavy. As a result the lighter rocks that compose the continents "float" above them and stand high above the ocean basins like giant icebergs. The floating continents are passively carried along on the "conveyor belts" below, like logs in a current. When one of them reaches the side of the plate on which it is sitting, it may bump into and crumple the continental mass on the adjacent plate. Forceful collisions of this kind have produced the great linear mountain chains of the world such as the Himalayas, the Rockies, and the Alps. In this way, continental topography has constantly been renewed, in the face of the erosion that constantly threatens to flatten it.

For the paleontologist, the main implication of plate tectonics is that the geography of the world is constantly changing. Today we recognize seven continents and a host of large islands scattered across the Earth's surface. But 180 million years ago, virtually all of the earth's dry land was assembled into one single supercontinent that geologists call Pangaea ("all lands"). Heat building up below it eventually split Pangaea into two giant continents: Laurasia in the north, and Gondwana in the south. Each of these then fragmented, ultimately to produce the various landmasses that we know today.

During these great movements, climates changed and biological forms shifted. Living populations were isolated or thrown into new states of competition. Species emerged and became extinct, and regions of the world developed their own distinctive assemblages of animals and plants.


Rocks and fossils

The rocks that make up the continents of the world come in three different kinds. First there are the igneous rocks, derived from the cooling of molten magma. These include basalts and andesites and tephra ejected by volcanoes on the Earth's surface, and granites that cooled at high depths and pressures, sometimes eventually to be exposed at the surface by weathering. Over the vastness of time, weathering has operated on a grand scale: if you ever find yourself looking at an outcropping of granite, just try imagining that it probably once lay beneath several miles' thickness of rock.

Then there are sedimentary rocks, composed of particles weathered from preexisting rocks before being transported by wind and water, collected, and compacted. Finally, there are metamorphic rocks, which have been reheated enough to flow and recrystallize, as when rough limestone turns to shiny marble.

Fossils are technically any and all traces of past life, not just bones and teeth and shells. Since they are almost exclusively found in sedimentary rocks, these are the only ones we need to dwell on here, except for a passing glance at the volcanic rocks that have proven vital in dating many fossils. When rapidly accumulating sediments cover the remains of dead animals or plants, there is a chance that they will be fossilized. Typically, only the hard tissues such as teeth, bones, or shells undergo fossilization, as their original constituents are replaced by minerals. But occasionally, even soft parts may leave impressions—sometimes amazingly detailed ones—in fine-grained sediments around them.

In the ocean, where sedimentation is relatively continuous, the remains of organisms are routinely trapped in clays, muds, sands, and so forth. On land the process is a bit chancier, and fossils are most often incorporated into the sedimentary record on riverbanks and floodplains, and at the shallow edges of lakes. Such spots also have the advantage—for the paleontologist—of being favorite places for predators to attack prey that have come to drink.

When a terrestrial mammal dies, its remains are likely to be devoured and dismembered by scavengers, and its bones broken and scattered around the landscape. Factors ranging from sun, wind, and water to beetles and bacteria will usually do the rest. If a bone by chance escapes all of these vicissitudes and finds its way to a place of deposition, it will often be further battered en route. This is why most mammal fossils in museum collections are incomplete or damaged in some way, and the most commonly found vertebrate fossils are simply isolated teeth—the hardest tissues of the body.

Occasionally a carcass will be covered by sediments where it lies, and its skeleton preserved intact—naturally enough, the paleontologist's dream. But even this best-case scenario is no guarantee of preservation. As it lies in the rock pile, the fossil must be reasonably undisturbed by earth movements. To be of any use to the paleontologist it has to be uncovered at the surface again by further erosion—where it will be rapidly obliterated by erosion unless it is quickly found and preserved. All in all, a rather chancy proposition, which explains why fossils of many species—especially those species that are thin on the ground in the first place—are rare indeed.


The Geological Time Scale

For the paleontologist, the most important thing about rocks is the historical record they contain. Ever since the Earth began taking on its familiar form, its continental crust has faithfully registered events happening on local and global scales. Some of this history can still be read, even though much evidence has subsequently been removed by erosion, covered by deposition, or altered by earth movements and metamorphism, sometimes on a gigantic scale.

Once it was established that the Earth was truly ancient, and had not simply been created more or less as we know it today, the first task of the early geologists was to reconstruct the historical sequence encoded in the rocks. This was not easy: for all that the working geologist could see were the rocks that happened to be exposed in any one place. And every local sedimentary basin, let alone each continent, has had its own geological history. Two basic questions thus emerged. One, at the front of every field geologist's mind, was "what was the sequence of events here?" And the other, usually asked when the geologist had returned home, or had at least struggled as far as the nearest pub, was, "How do I match it up with the sequences we see in other places?"

To approach the first question, early stratigraphers followed two rules. Sedimentary rocks accumulate in piles, one layer atop another, so the first rule was that the sediments at the bottom of any particular pile are older than the strata above. The second axiom was that these layers were originally laid down flat, no matter how earth movements might have tilted or buckled them since. Because most piles of sedimentary rock have undergone at least some deforming and tilting, together with displacement along faults that misalign the layers, stratigraphers first needed to establish the original relationships of the strata. That done, it was time to match up the sequence seen in one place with sequences seen elsewhere.

To some extent, this could be done through lithology—the characteristics of the rock layers themselves. It turned out, though, that this worked only within local sedimentary basins, because each basin has its own geological history. Basins can be large, which is why sheep in southern England graze on the same limestone soils that support the grapevines of Champagne. But every basin has its limits, so stratigraphers found another way to correlate rock formations over broader areas. They recognized certain widely dispersing organisms as "index fossils," characteristic of particular periods. The resulting correlations made possible the development of a standard timescale.

While the succession of major geological periods was basically established by the end of the nineteenth century, means of calibrating that sequence in years are quite recent. As Figure 1.2 shows, Earth history over the last 3.8 billion years is nowadays organized into three major eons that follow the initial period that is informally known as the Hadean, in acknowledgment of the fiery nature of the planet's surface in its earliest days. The first two post-Hadean eons compose the long stretch prior to the earliest fossils known to the early geologists and are grouped together in a larger unit called the Precambrian. The third eon, the Phanerozoic, covers the last 542 million years. Each eon is divided into eras. These are subdivided into periods, which are in turn composed of smaller time units known as epochs.


Chronometric Dating

Most current methods of applying real time (in years) to the geological record rely in one way or another on radioactivity. Chemical elements may exist in several alternative forms (isotopes), of which some (the radioactive ones) are unstable: their atomic nuclei spontaneously "decay" to stable states. Conversion takes place at a rate that is constant, measurable, specific to the isotope concerned, and unaffected by environmental factors. Some isotopes decay fast; others more slowly. Chemists express the rate of decay in terms of an isotope's half-life—the time it takes for half of the atoms present to decay. Geochronologists have used this property of radioactive isotopes to date rocks containing them.

There are two long-established approaches to such radiometric dating, both first developed in the mid-twentieth century. One embraces accumulation techniques, based on the buildup of stable daughter atoms. The classic accumulation technique is potassium/argon (K/Ar) dating, recently supplanted by the argon/argon (39Ar/40Ar) technique. Because the half-life involved is very long, these methods and others like them can be used to date very old rocks indeed—volcanic ones are preferred, because when laid down they are heated high enough to purge them of any daughter product and are often found interstratified with fossil-bearing sediments.

The opposite approach is represented by decay techniques, such as radiocarbon (14C), first introduced around 1950. The unstable carbon isotope 14C (radiocarbon) is produced in the upper atmosphere in a reaction governed by cosmic ray influx, and is incorporated into all living things. When an organism dies, it becomes isolated from the carbon cycle, and the 14C it contains begins to decay, diminishing steadily as a proportion of the total carbon present. At 5,730 years, the half-life of radiocarbon is rather short, which means that the method can only be used on samples up to about 40,000 years old. But whereas K/Ar is used to date rocks, 14C has the decided advantage of being able to date fossil specimens directly, provided enough bone protein (collagen) is preserved.

In recent years, the number of approaches to chronometric dating has multiplied, mostly for the fairly recent time frames that are of particular interest to paleoanthropologists. Most of these are "trapped-charge" methods that depend one way or another on the fact that electrons released by radioactivity may become trapped, at a measurable rate, in the lattice structure of various crystals. Good examples are thermoluminescence (TL) and electron spin resonance (ESR) dating.


What Fossils Can Tell Us

Once your dated fossil is sitting on your workbench, you need to extract as much information from it as possible. There are many different ways of going about this, involving specialists of many different kinds. The first step is to determine to what species your fossil belongs—and if necessary to create a new species to accommodate it. Then, of course, you need to situate that species in the great Tree of Life. These initial steps are absolutely fundamental to everything else that you do, and they may well prove to be the most difficult steps of all. But only when they are completed should you proceed to what most people regard as the really interesting stuff: reconstructing how your fossil lived back when it was alive, and what role it played in the ongoing soap opera of life.

Apart from its age and the location at which it was found, the most obvious information any fossil has to offer is its morphology—what it looks like. How you are built not only shows to whom you are related, but also determines how you can live. Every species is limited by its structure, both in what it can do right now and in its evolutionary potential for the future.

Of course, when you are confronted with nothing but bones or teeth, it is much easier to reconstruct what their owners might have done in life if you can find a living form whose lifestyle is reflected in features comparable to those of your fossil. The ichthyosaurs, for example, are extinct reptiles whose body form so clearly echoes those of fish that there has never been any doubt that they were swimmers—as is independently confirmed by the marine rocks in which their fossils are found.


(Continues...)

Excerpted from Paleontology by Ian Tattersall. Copyright © 2010 Ian Tattersall. Excerpted by permission of Templeton Press.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.

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Table of Contents

Contents

Introduction,
Chapter 1: Rocks, Time, and Fossils,
Chapter 2: Evolutionary Processes,
Chapter 3: The Tree of Life,
Chapter 4: In the Beginning,
Chapter 5: The Paleozoic: "Ancient Life",
Chapter 6: The Age of Dinosaurs,
Chapter 7: The Age of Mammals,
Chapter 8: Of Whales and Primates,
Chapter 9: Walkers and Toolmakers,
Chapter 10: A Cognitive Revolution,
Acknowledgments,
Bibliography,

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