The Human Lineage - Matt Cartmill - E-Book

The Human Lineage E-Book

Matt Cartmill

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"This textbook, aimed at advanced undergraduates and postgraduatesin paleoanthropology courses, tackles a rather difficulttask--that of presenting the substantial body ofpaleontological, genetic, geological and archaeological evidenceregarding human evolution, and the associated scientific history,in a logical and readable way without sacrificing either clarity ordetail... the sheer quality of the writing and explanatorysynthesis in this book will undoubtedly make it a valuable resourcefor students for many years." --PaleoAnthropology, 2010 This book focuses on the last ten million years of humanhistory, from the hominoid radiations to the emergence anddiversification of modern humanity. It draws upon the fossil recordto shed light on the key scientific issues, principles, methods,and history in paleoanthropology. The book proceeds through thefossil record of human evolution by historical stages representingthe acquisition of major human features that explain the successand distinctive properties of modern Homo sapiens. Key features: * Provides thorough coverage of the fossil record and sites, withdata on key variables such as cranial capacity and body sizeestimates * Offers a balanced, critical assessment of the interpretativemodels explaining pattern in the fossil record * Each chapter incorporates a "Blind Alley" box focusing on onceprevalent ideas now rejected such as the arboreal theory,seed-eating, single-species hypothesis, and Piltdown man * Promotes critical thinking by students while allowinginstructors flexibility in structuring their teaching * Densely illustrated with informative, well-labelled anatomicaldrawings and photographs * Includes an annotated bibliography for advanced inquiry Written by established leaders in the field, providing depth ofexpertise on evolutionary theory and anatomy through to functionalmorphology, this textbook is essential reading for all advancedundergraduate students and beginning graduate students inbiological anthropology.

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Contents

Chapter 1 The Fossil Record

Changing Ideas about the Changing Earth

Neptune vs. Vulcan

A Brief Guide to Sedimentology

Dating the Rocks

The Succession of Faunas

Radiation-Based Dating Techniques

Other Dating Techniques

Dating Based on the Cycles of the Earth

The Problem of Orogeny

Continental Drift

Life: The First Three Billion Years

Multicellular Life

The Cambrian Revolution

Jaws, Fins, and Feet

The Reptilian Revolutions

The Two Great Extinctions

The Mammals Take Over

Chapter 2 Analyzing Evolution

Parsimony and Pigeons

Darwin’s Theory

Problems with Darwinism

The Concept of Species

Evidence for Anagenesis and Cladogenesis

The Tempo of Speciation

Semispecies, Hybrids, and Isolating Mechanisms

“Races”

Species and Fossils

Morphospecies

Microevolution and Macroevolution

The Politics of Macroevolution

Reconstructing the Tree of Life

Sources of Error in Phylogenetics

Linnaean Classification

Evolutionary Systematics

Phenetics and Cladistics

Pros and Cons of Phylogenetic Systematics

Chapter 3 People As Primates

Early Mammals

Allometry

Allometry and Early Mammals

Death and Molar Occlusion

Allometry, Motherhood, and Milk

Respiration and the Palate

The Tribosphenic Molar

Live Birth and Placentation

Cretaceous Mammals

The Order Primates

The Living Strepsirrhines

Anthropoid Apomorphies: Ears, Eyes, and Noses

Tarsiers

Platyrrhines: The New World Anthropoids

Cercopithecoids: The Old World Monkeys

Hominoids: The Living Apes

Pongids and Hominids

Bonobos and Chimpanzees

Humans vs. Apes: Skulls and Teeth

Primate Origins: The Crown Group

Fossil Primates: The Stem Group

The First Fossil Euprimates

Eocene “Lemurs” and “Tarsiers”

The First Anthropoids

Anthropoid Radiations

Chapter 4 The Bipedal Ape

Being Human vs. Becoming Human

The Taung Child

Australopithecus Grows Up

Bipedal Posture and the Vertebral Column

Bipedal Posture and the Pelvis

Bipedal Locomotion: Knees

Bipedal Locomotion: The Hip Joint

Bipedal Locomotion: Feet

Australopithecus Stands Up

The Skull of Australopithecus africanus

Australopithecus robustus

Man-Apes, Just Plain Apes, or Weird Apes?

Postcranial Peculiarities

Louis Leakey and East Africa

Olduvai Gorge

Sahelanthropus: The Oldest Hominin?

Mio-Pliocene Enigmas: Orrorin and Ardipithecus

Australopithecus anamensis

Australopithecus afarensis

Lucy’s Locomotion: The View from Stony Brook

Lucy’s Locomotion: The Rebuttal

Lucy’s Locomotion: Persistent Questions

Australopithecus bahrelghazali?

Australopithecus platyops

Australopithecus garhi

Australopithecus aethiopicus

Australopithecus boisei

Fitting in South Africa: The Problem(s) of Sterkfontein

Fitting in South Africa: Some robustus Questions

The Phylogeny of Australopithecus

What Did Australopithecus Eat?

Australopithecus and the Ecosystem

Two Species or Two Sexes?

Hunting, Gathering, and Dimorphism

Dinichism: A Possible Synthesis

Explaining Hominin Origins

Primitive Homo—Or “Advanced” Australopithecus?...........................

Dating and Geological Context of the Habilines from Olduvai, Omo, and Koobi Fora

Habiline Skulls

Habiline Teeth

Habiline Postcranial Remains

Advanced Australopithecus: The Frustrations of Variation

Advanced Australopithecus: Back to South Africa

Advanced Australopithecus or Early Homo? Phylogenetic Issues

Chapter 5 The Migrating Ape: Homo erectus and Human Evolution

The “Muddle in the Middle”

A Brief History of Homo erectus: 1889–1950

Later Discoveries in Africa and Eurasia

Erectine Chronology and Geographic Distribution

Cranial Vault Morphology of Homo erectus

Cranial Capacity and the Brain

Faces and Mandibles of Asian Homo erectus

The Erectine Dentition

Erectine Postcranial Remains

Early African Erectine Skulls and the Ergaster Question

Early African Erectine Postcranial Morphology

Early Erectine Adaptations: Anatomy and Physiology

Early Erectine Adaptations: The Archaeological Evidence

Patterns of Development and Evolutionary Change in Erectines

Early Erectine Radiations in Africa

Out of Africa I: The Erectine Radiation

Indonesian Erectines and the Specter of “Meganthropus”

Chinese Erectines

Dmanisi—Humans at the Periphery of Europe

The Initial Occupation of Europe

Major Issues: A Summing Up

Taxonomy

Dates and Additional Evidence

Evolutionary Patterns

Chapter 6 The Big-Brained Ape: Regional Variation and Evolutionary Trends in the Middle Pleistocene.......................

Of “Archaic Homo sapiens” and Homo heidelbergensis.......

Early Models of Later Human Evolution

The Recent African Origin Model

The Multiregional Evolution Model

European Heidelbergs

African Heidelbergs

Kabwe

Bodo and Ndutu

African Heidelberg Mandibles

Other African Heidelbergs

North Africans

Asian Heidelbergs?

Mugharet El-Zuttiyeh

Other West Asian Candidates

South Asia

East Asia

Australasia

Ngandong

Liang Bua

Supraorbital Tori, Chins, and Projecting Faces

Major Issues: Speciation, Migration, and Regional Continuity

Chapter 7 Talking Apes: The Neandertals

Neandertals—Early Discoveries and Ideas (1829–1909)........

Ideas about Neandertals—From Boule to the 21st Century

Neandertal Chronology and Distribution

Neandertal Morphology—The Cranial Vault

Neandertal Faces

Neandertal Mandibles

Neandertal Dentition

Body Size and Proportions

Neandertal Life History

Neandertal Genetics

Neandertal Technology

Diet and Subsistence Behavior

Neandertals and Language

Symbolic Behavior

Early European Neandertals

Würm Neandertals from Western Europe

Western and Central Asian Neandertals

Late Neandertals

Major Issues

Chapter 8 The Symbolic Ape: The Origin of Modern Humans

A “reative Explosion”?

Modern Human Anatomy—The Skull

Modern Human Anatomy—Cranial Capacity

Modern Human Anatomy—The Postcranial Skeleton

The Geochronology of Modern Human Origins

The African Transition: Background and Dating

The African Transition: Vault Morphology

The African Transition: Facial Morphology

The African Transition: Additional Bones, Archaeology, and Other Matters

East Asian Archaic Humans: Background and Context

East Asian Archaic Sites and Specimens

East Asian Archaics: Continuity or Someone New?

Early Modern Humans: The East African Record

Out of (East) Africa: Early Modern People in North and South Africa

The First Modern People Outside Africa: The Near Eastern Evidence

African and Circum-Mediterranean Gene Flow and Modern Human Origins

Modern Human Origins in East Asia

Modern Human Origins in Australasia

Europe: The Last Frontier

Recent Human Genetics and Modern Human Origins

Ancient DNA in Early Modern Humans

Modern Human Origins: The Models vs. the Facts

Assimilation and Interactions Between Modern and Archaic Humans

Appendix: Cranial Measurements

Bibliography

Index

Copyright © 2009 by Wiley-Blackwell. All rights reserved.

Original drawings copyright © 2009 by Matt Cartmill. All rights reserved.

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Published by John Wiley & Sons, Inc., Hoboken, New Jersey.

Published simultaneously in Canada.

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Library of Congress Cataloging-in-Publication Data:

Cartmill, Matt.

The human lineage / Matthew Cartmill, Fred H. Smith.

p. cm.

Includes bibliographical references and index.

ISBN 978-0-471-21491-5 (cloth)

1. Fossil hominids. 2. Human evolution. I. Smith, Fred H. II. Title.

GN282.C37 2009

599.93’8—dc22

2008055229

Preface

Unlike other animals, people wonder how things got to be the way they are; and one of the things we wonder about most is how we got to be so unlike other animals. The science of human origins therefore attracts a lot of interest and attention from a wide range of audiences, from average readers to dedicated researchers. This book about paleoanthropology has been written for readers at the scholarly end of that spectrum, especially for students who have already had a beginning course in the subject. But we have tried to keep it accessible to any educated reader, because we think it is important in today’s political and cultural environment to make a substantial account of human evolution available to anyone who has serious questions about it.

As its title implies, this is book about the evolutionary lineage of the human species, Homo sapiens. Because it is specifically centered on the human lineage, it touches on other aspects of the history of life on earth only to the extent that they bear on human origins. And because this book focuses on reconstructing the human lineage, it deals mainly with the facts of paleontology, which furnishes the only direct evidence we have of that lineage. Comparative anatomy, molecular biology, historical geology, archaeology, and other fields of science are brought in mainly to provide necessary background and context for the study of the human fossil record.

Opponents of scientific biology are fond of dismissing that record as a pathetic handful of controversial fragments. If that were so, this book would be a lot shorter. An often-repeated creationist canard insists that all known human fossils would fit on a billiard table. This was probably true in the late 19th century but it has not been true for a hundred years. Known human fossils number in the thousands and represent the remains of hundreds of individuals. They are more numerous and better-studied than the fossils of any comparable vertebrate group, because the intense interest that people have in the bones of their ancestors has driven them to devote far more effort to collecting and studying fossil humans than (say) fossil horses or herring. Having seen most of the major collections of human fossils in the world’s museums, we can assure our readers that those collections can no longer be laid out on a billiard table. It would be hard to cram them all into a boxcar.

The growth of the human fossil record has been especially rapid over the past half- century. In 1959, W. W. Howells could still provide a basic exposition of almost all of the significant human fossils then known in a relatively slim (384-page) volume entitled Mankind in the Making. Only three years later, Carleton Coon took 724 pages to present an only slightly more detailed account in his book The Origin of Races. Any book that

tried to survey today’s human fossil collections in the same detail would not fit between a single pair of covers. A recent catalog of most of the major cranial and dental remains of currently known fossil hominins—just photographs and descriptions of skulls and teeth—occupies three quarto volumes totaling more than 1500 pages (Schwartz and Tattersall 2002, 2003, 2005). Most of the decisions involved in writing a paleoanthropology textbook thus concern what to leave out, not what to put in. In making such decisions about the fossil evidence, we have tried to focus not on the details of particular fossils, but on the temporal, regional, or taxonomic patterns that they reveal. Conversely, in covering the theoretical aspects of evolutionary biology, we have stressed those facets of the theory that are deeply involved in current paleoanthropological debates and have skimped on others that are not.

Debates in paleoanthropology are often vigorous and contentious. Some writers would have you believe that such heated debates are both an idiosyncracy and a shortcoming of our discipline. We think they are wrong on both counts. The scientific enterprise is grounded in the assumptions that all knowledge is provisional and that knowledge increases through the refutation of old ideas and their replacement by new ones. Each generation of scientists makes its mark by overthrowing the received wisdom of the previous generation or transcending its limitations. Active and lively sciences are arenas in which ideas and claims compete for survival. Because reputations and egos are involved in these clashes, arguments are often heated and sometimes intemperate. In these respects, debates over the meaning of fossil skulls are not different from similar debates in other sciences over such issues as the reality of poly-water or the planetary status of Pluto.

In grappling with the important debates in our discipline, we have tried to do an even-handed job of laying out the core arguments and key facts that support various currently conflicting interpretations of the fossil record. It will not be difficult to figure out where we stand on most of these issues. But it should also be possible for the reader to understand why others read the facts differently, and even to come to conclusions that differ from ours. In general, we have not tried to articulate the reasoning behind ideas that nobody espouses any longer; but we have included a few such discarded ideas that seem to us to have special historical importance, set off inside boxes and labeled “Blind Alleys.”

The first three chapters of this book situate the human fossil record in the larger framework of evolutionary biology and provide the necessary background for what follows. Chapter 1 surveys the development of historical geology, including brief summaries of geological dating techniques and the fossil record of life up through the early radiation of mammals. Chapter 2 lays out a sketch of the underpinnings of evolutionary theory, with emphasis on its paleontological applications. In Chapter 3, we discuss the mammalian background and evolutionary history of the order Primates. We have undertaken a relatively complete survey of the other living members of our order, because we think that doing so helps put humans in heir proper biological context. Our overview of fossil primates focuses more narrowly on those that have some relevance to the earlier stages of the human lineage, either as potential ancestors or as comparative cases that illustrate relevant concepts and phenomena.

The final five chapters cover the specifically human (hominin) part of the lineage of our species. The initial hominin radiation in Africa, including the origin of the genus Homo, is reviewed and discussed in Chapter 4. Chapters 5 and 6 deal respectively with fossils commonly assigned to Homo erectus and Homo heidelbergensis. This division is not intended as a proxy for a taxonomic distinction or as a presentation of “stages” of human evolution, but only as a heuristic structure to organize the relevant material. A full chapter (Chapter 7) is devoted to the Neandertals, because these are the best-known pre-modern humans and offer unique insights into the pattern of later human evolution. Chapter 8 deals with the emergence and radiation of modern humans—people fundamentally like us. Finally, we provide an appendix detailing the anatomical points and measurements used in the book.

Unless otherwise indicated, all illustrations were drawn by one of us (MC). Some of them are diagrammatic or conceptual, but most were redrawn from photographs, with an eye to both anatomical accuracy and ease of interpretation. Figures for which no published source is credited are based on our own ideas, observations, and photographs.

Most of this book’s contents represent other people’s work. We have relied heavily on the published work and private thoughts of our colleagues, which have of course greatly enriched our own ideas and interpretations as well. In producing the text and figures for this book, we have tried diligently to give credit to our colleagues for their work and ideas and have attempted to be as thorough as possible in providing citations and references. Despite our best efforts, it is inevitable that we have overlooked some important sources and misinterpreted some others. We accept responsibility for such errors and omissions, and we ask that our colleagues bring them to our attention.

Writing a book like this one brings clearly into focus for us the high quality of research and researchers in paleoanthropology. Of all the pleasures involved in working on this book, none has been greater than our interactions with colleagues. This project could not have been completed without their input, information, and encouragement. Many of them have kindly provided

access to fossils and other material critical to the production of this volume. For general assistance we thank: J. Ahern, S. Bailey, O. Bar-Yosef, M. Bolus, D. Boyd, C.L. Brace, G. Bräuer, A. Busby, J. Calcagno, R. Cann, R. Caspari, S. Churchill, R. Clarke, M. Cole, T. Cole, G. Conroy, D. Curnoe, S. Donnelly, A. Durband, A. Falsetti, R. Franciscus, D. Frayer, J. Gaines, J. Gardner, D. Gebo, D. Glassman, A. Grauer, M. Green, L. Greenfield, F. Grine, P. Habgood, T. Holliday, R. Hollo-way, N. Holton, J.-J. Hublin, K. Hunt, V. Hutchinson, W. Hylander, I. Janković, R. Jantz, D. Johanson, C. Jolly, W. Jungers, R. Jurmain, I. Karavanić, R. Kay, W. Kimbell, J. Kidder, R. Klein, J. Kondrat, L. Konigsberg, A. Kramer, C.S. Larsen, S. Leigh, D. Lieberman, M. Liston, F. Livingstone, D. Lordkipandze, C.O. Lovejoy, A. Mann, J. McKee, N. Minugh-Purvis, J. Monge, S. Myster, L. Nevell, S. Pääbo, M. Parrish, R. Quam, M. Ravosa, J. Relethford, G.P.Rightmire, C. Ruff, D. Rukavina, P. Schmid, D. Serre, J. Simek, T. Simmons, E. Simons, F. Spencer, F. Spoor, C. Steininger, M. Stoneking, A. Thorne, A.-M. Tillier, E. Trinkaus, H. Ullrich, K. Valoch, C. Vinyard, P. Vinyard, A. Walker, C. Wall, C. Ward, S. Ward, K. Weiss, M. Weiss, T. White, M. Wolpoff, B. Wood, and T. Yokley. We would like to credit K. Porter for her initial ideas about dinich-ism in Australopithecus. We are grateful to the following individuals for permission to study fossil hominins, as well as for other forms of assistance: T. Akazawa (Tokyo), B. Asfaw (Addis Ababa), G. Avery (Capetown), H. Bach (Jena), D. Brajković (Zagreb), J. Brink (Bloem-fontein), N. Conard (Tübingen), Y. Coppens (Paris), I. Crnolatac (Zagreb), A. Czarnetzki (Tübingen), H. Del-porte (Saint-Germain-en-Laye), J. DeVos (Leiden), R. Feustel (Weimar), J.-L. Heim (Pairs/Leipzig), F.C. Howell (Berkeley), WW. Howells (Cambridge, MA), J. Jelínek (Brno), H.-E. Joachim (Bonn), R. Kraatz (Heidelberg), R. Leakey (Nairobi), A. Leguebe (Brussels), J. Lenardić (Zagreb), R. Macchiarelli (Rome/Poitiers), M. Malez (Zagreb), D. Mania (Halle), G. Manzi (Rome), F. Masao (Dar es Salaam), E. Mbua (Nairobi), J. Melentis (Thessa-lonika), A. Morris (Capetown), D. Pilbeam (Cambridge, MA), J. Radovčić (Zagreb), Y. Rak (Tel Aviv), M. Sakka (Paris), M. Schmauder (Bonn), R. Schmitz (Tubingen), B. Senut (Paris), C. Stringer (London), J. Szilvássy (Vienna), M. Teschler-Nikola (Vienna), T. Thackeray (Pretoria), M. Thurzo (Bratislava), P. Tobias (Johannesburg), B. Vandermeersch (Paris/Bordeaux), R. von Koe-nigswald (Frankfurt-am-Main), E. Vlčcek (Prague), J. Zias (Jerusalem), and R. Ziegler (Stuttgart).

Over the years, our work has been supported by a number of agencies and universities. These are the National Science Foundation (USA), the National Academy of Sciences (USA), the National Institutes of Health (USA), the Wenner-Gren Foundation, the National Geographic Society, the L. S. B. Leakey Foundation, Sigma Xi, the Alexander von Humboldt Foundation, Fulbright Foundation, the John Simon Guggenheim Memorial Foundation, Duke University, the University of Tennessee, the University of Tübingen, Northern Illinois University, Loyola University Chicago, the University of Zagreb, the University of Hamburg, the Croatian Natural History Museum, the Institute of Anthropology (Zagreb), the Institute for Quaternary Geology and Vertebrate Paleontology—Croatian Academy of Sciences and the Arts, the Frank H. McClung Museum, Boston University, and Illinois State University. We are grateful to all of these institutions for their support.

Several of our colleagues—J. Ahern (University of Wyoming), A. Durband (Texas Tech University), R. Franciscus (University of Iowa), P. Habgood (University of Queensland), A. Hartstone- Rose (Pennsylvania State University), I. Janković (Institute of Anthropology, Zagreb), K. Rosenberg (University of Delaware), and D. Schmitt (Duke University)—deserve special thanks for reading and making suggestions on parts of the manuscript. Their help was invaluable. We are particularly grateful to our Series Editor Kaye Brown for her editorial suggestions and other assistance; and we are indebted to Karen Chambers, our editor at Wiley-Blackwell, for her patience and help. Last and most importantly, we want to express our deep gratitude to our families (Kaye Brown, Maria O. Smith, Erica Cartmill, Burton T. Smith, and Maria K. Smith) for all the warm and generous support, patience, advice, understanding, and encouragement that they gave us during the researching and writing of this book.

                                                                   MATT CARTMILL

                                                                   FRED H. SMITH

CHAPTER ONE

The Fossil Record

CHANGING IDEAS ABOUT THE CHANGING EARTH

The past is no longer with us. To reconstruct it, we have to look at the traces that it has left in the present. Discerning those traces and figuring out their meanings is not a simple task. It has taken over 300 years for scientists to arrive at the methods that we rely on today in using the present to resurrect the distant past.

Ideas about the remote past began with pre-scientific speculation about the formation of the earth. When our ancestors looked at the landscape around them, it was not obvious to them that it contained a record of slow changes over an immense period of time. Most of the changes in the earth’s surface that are noticeable to a human observer are small, swift, and local. After a storm, water running off deforested hills may cut gullies in the soil. Further downstream, that same floodwater may cause a muddy river to overflow its banks and deposit a layer of silt for miles around. At long intervals, an earthquake may shift a piece of land by a few inches, or a volcano may shower a region with ash and spill lava down its slopes.

Changes of this sort have been noticed and commented on since the dawn of history. But few of us ever see all four of these processes—erosion, sedimentation, tectonics, and vulcanism—working to alter the landscape in the course of our own lifetimes. Until about 250 years ago, nobody seems to have thought about how these forces might combine over millions of years to produce the rocks, soils, mountains, and valleys of the earth.

Throughout most of the history of Western thought, speculation about the history of the earth has been constrained by the sacred poetry of the Hebrew scriptures, in which the voice of God from the whirlwind reproves would-be geologists in words of the gravest majesty:

Where wast thou when I laid the foundations of the earth?

declare, if thou hast understanding.

Who hath laid the measures thereof, if thou knowest?

or who hath stretched the line upon it?

Whereupon are the foundations thereof fastened?

or who laid the cornerstone thereof;

When the morning stars sang together, and all the sons of God shouted for joy?

These lines from the Book of Job represent God as a divine mason building a world with floor plans, foundations, and a cornerstone. Few people took this poetic metaphor literally. But the creation story in Genesis, which tells how God shaped the face of the earth and planted it with grass and trees in the course of a single day, was taken very literally indeed (and still is, in some quarters). Reckoning forward from the first day of that Creation by adding together the ages of patriarchs and the reigns of kings listed in the Old Testament, biblical scholars reasoned that the universe had to be less than 6000 years old. Some Christian authorities fixed on a date of 4004 B.C. for the beginning of the world. Jewish scholars numbered the years of their calendar from a supposed Creation date equivalent to 3760 B.C.

As long as this short time scale was accepted, floods, earthquakes, and volcanoes could not be thought of as having produced the rocks and topography of the earth. The changes wrought by these processes were too small and slow to have brought the present landscape into being in a few thousand years. The minor changes that people could actually see happening in the face of the earth tended to be regarded as blemishes on the original work: signs of decay presaging the approaching end of the world, or marks left by outbursts of God’s wrath.

The biggest such outburst was thought to have been the Flood of Noah, in which God had supposedly submerged the whole world under water in order to get rid of sinful humanity (see Blind Alley #1). As evidence of that universal Flood, many people pointed to the seashells and fish bones found in rocks far above the present level of the sea. Those fossil shells and bones seemed to bear out the truth of the Genesis

Blind Alley #1: The Flood Story

People all over the world recount tales of disasters long ago that ravaged the earth and threatened humanity. In the ancient Near East, the Sumerians, Babylonians, and Hebrews told stories about a great flood sent by the gods to wipe out the wicked human race. In the Babylonian version, the goddess Ishtar incites the gods to drown the world. Her plan is foiled by the god Ea, who instructs the hero Utnap- ishtim to build a huge vessel and load his family and all the world’s animals aboard it. Safe in the ship, the people and beasts weather a great storm that destroys all other living things. After sending out a dove, a sparrow, and a raven to seek land, Utnapishtim grounds his ark on Mt. Nisir and offers a burnt sacrifice. The gods gather around, repent their acts, and promise never to do it again.

The similar flood story in the Old Testament is another member of this family of Near Eastern legends. But because it was part of sacred scripture, Jews and Christians long accepted it as historical fact. When they found fossil shells in rocks lying hundreds of feet above sea level, they took them as confirmation of a great flood that had drowned the mountains and laid down the fossil-bearing strata. When they dug up ancient human artifacts and remains, they described them as “antediluvian”—that is, dating to a time before the Flood.

The biblical Flood story has implications that can be checked out. If all the fossilized organisms died at the same time a few thousand years ago, then all the fossil-bearing rocks should contain fossils of people and other extant creatures. But they don’t. If the Ark saved all the world’s animals from destruction, there shouldn’t be any extinct species in the fossil record. But there are. (Almost all fossil species are extinct.) If all the animals dispersed from the Ark’s landing site on Mt. Ararat, then the world’s faunas should grow less and less diverse the further away they are from Turkey. They don’t. If enough rain had fallen to cover all the land—raising the seas an additional seven miles— then the total rainfall would have amounted to some 1.4 billion cubic miles of fresh water. How did all that water dry up in 150 days (Genesis 7:24)? Where is it now?

And so on. None of the implications of the biblical Flood story check out. It was not until this old Mesopotamian legend had been set aside that the building of scientific theories about the history of the earth’s rocks, animals, and plants could begin in earnest.

story, but they also posed certain problems. For one thing, they were not really shells and bones. Most of them were composed of minerals like those found in the surrounding rocks. Some insisted that the seeming plants and animals imbedded in the rocks were not really the remains of once-living things, but just lusi naturae, “games of nature”—freakish, abortive organic forms produced by mysterious creative forces inherent in the stone itself. But the fossils’ detailed similarities to bones, shells, and leaves made this hard to credit.

In 1669, Nicolaus Steno, a Danish physician living in Italy, published a book that used fossils to help interpret the history of the earth. Rocks that contain fossils are always organized into strata or layers. We can see these layers exposed wherever a quarry or cliff face cuts a vertical section through such rocks. Different layers contain rocks of different composition: Beds of smooth slate may alternate with gritty sandstone or coarse conglomerates containing pebbles of yet another sort of stone. Steno argued that all these layers had been formed when sediments were discharged into lakes or seas by rivers, settled to the bottom, and became cemented together. Fast- moving streams carried coarse sediment that became sandstone or conglomerates; broad, slow rivers carried silt that gave rise to slate and other finegrained rocks.

The fossils imbedded in these rocks, Steno insisted, truly were the remains of creatures that had lived in or near the seas and lakes. He showed that strange triangular fossils long known as “tongue stones” were identical to the teeth of living sharks and that other fossils found with them were clearly the remains of other sea creatures. The rocks containing such fossils, he argued, must have been laid down on the ocean floor. Other rocks, containing the remains of land plants and animals, had probably been formed from fresh-water sediments.

The layers of sedimentary rock are usually more or less horizontal, as you would expect beds of sediment to be. However, Steno recognized that some of them are tilted or folded. He concluded that the originally flat layers of sediment must have been worked on later by powerful disruptive forces capable of bending or breaking thick beds of solid stone. Steno suggested that these layers had been first lifted above the sea and then undermined by the formation of huge internal caverns. When their roofs fell in, these collapsed caverns became valleys, in which the fallen roof fragments lay as beds of tilted rock. The remaining elevations were left as mountains. Streams running down these mountains then carried silt and gravel down into the valleys, which gradually filled with new sediments.

All these ideas were generally rejected in Steno’s own day, but they influenced later geologists (Gillespie 1959, Gohau 1990). Three of Steno’s basic postulates— the sedimentary origin of rock strata, the organic nature of fossils, and the rule that overlying strata are younger than those below them—are still fundamental assumptions in reconstructing the histories of the earth, of prehistoric life, and of ancient human activity.

NEPTUNE VS. VULCAN

The rise of industrial economies in northern Europe in the 1700s lent new practical importance to theories about the history of the earth. As the prosperity of nations began increasingly to depend on mineral wealth, it was worth money to understand how these resources were distributed in the rocks. It was obvious that there were patterns there to be understood. The coal and metal ores that the new industrial order needed were not scattered around in random pockets. They tended to occur in particular strata, sandwiched in between layers of other sorts of rock. Miners had long ago learned to identify these mineral-bearing strata on the surface and dig down after them into the earth.

Amateur scientists began to trace and study these stratigraphic patterns. In the 1740s, a French physician named Jean Guettard, whose hobby was collecting plants, noticed that certain species of plants were found only where the underlying rocks were of certain sorts, such as limestone or chalk. Mapping the distributions of these plants, he found that their associated rocks were arranged in bands that ran across the map of Europe, sometimes for hundreds of miles. Guettard concluded that these bands represented the exposed edges of superimposed sheets or layers of rock, which had been laid down atop one another as sediments at the bottom of the sea.

If the landscape really had been carved out of stacked-up layers of sedimentary rock, then it might be possible to reconstruct the story of the earth’s creation by reading upward through those layers, starting at the bottom with the oldest rocks of all. One of the first people to try this was a German scholar named Johann Lehmann. Lehmann distinguished three major phases in the formation of the earth. He thought that the oldest rocks, which had no fossils in them, had precipitated out of a planetary suspension of liquid mud during a primitive period before the creation of life. This “Primary” period corresponded to Day 1 of the Genesis creation story, when “the earth was without form, and void.” Later rocks, containing fossil remains of plants and animals, had been laid down over the Primary rocks during the Flood of Noah. Overlying these “Secondary” strata was a third, superficial layer of recent deposits formed by erosion and volcanic action.

These ideas formed the nucleus of the first systematic theory of earth history, developed and promulgated from the 1780s on by Abraham Werner, a professor at Freiburg in Germany. Werner adopted Lehmann’s account of the series of Primary and Secondary rocks. His main contribution was his explanation of volcanoes. Werner taught that the earth’s deposits of coal and petroleum had been laid down during the Secondary period. When some of these flammable minerals caught fire, the heat had melted the surrounding rocks and produced volcanic eruptions, spreading sheets of lava in places on top of the Secondary strata. These strata of igneous rocks (rocks formed by melting) represented a brief third or Tertiary period of rock formation. Finally, erosion and floods had deposited soil, gravel, and other loose sediments on top of everything else during the most recent period, the Quaternary. Because Werner and his followers thought that almost all geological formations had been formed by the action of water, they came to be known as Neptunists, after the Roman god of the sea.

Their opponents argued that certain ancient rocks that the Neptunists called sedimentary were in fact volcanic in origin. For this reason, the anti-Neptunist school were sometimes referred to as Vulcanists. Rejecting the notion that different rock-forming processes had operated at different periods of the earth’s history, the Vulcanists insisted that the forces seen at work on the landscape today—erosion, sedimentation, volcanoes, and earthquakes—had worked together throughout geological time in the same way they do now and that together they were sufficient to account for all the observable features of the earth.

This doctrine—that the laws and processes that operated in the past are the same ones we see operating in the world around us—is called uniformitarianism. The eventual acceptance of the uniformitarian principle was crucially important in the maturation of geology into a genuine science. It also helped to lay the groundwork for Charles Darwin’s application of a similar uniformitarianism to the study of biology (Gould 1986). However, scientists today do not regard the uniformitarian principle as entirely valid over the long run. For example, the universe is now thought to have originated in an explosive cosmic event called the Big Bang, which involved processes and forces no longer at work in the world we observe. The formation of the solar planets from condensations of interstellar dust surrounding the newborn sun was another one-time-only process that has no counterpart in today’s solar system, though we can get some insight into it by studying other stars younger than our own. The history of the earth and its living organisms has been profoundly affected by asteroid impacts and other catastrophic global events, which we (luckily enough) have never had a chance to witness in the brief span of human history.

But over the span of time that concerns biologists, from the origin of life some four billion years ago down to the present, the uniformitarian principle is true enough for most practical purposes. The small, local forces that we can see reshaping our landscapes today suffice to account for the formation of the rocks of the earth over the vast duration of geological time. They provide the geological background for the history of life.

A BRIEF GUIDE TO SEDIMENTOLOGY

Most of the fossils that paleontologists study are the remains of marine life, buried ages ago in the rain of fine mineral particles and organic detritus that falls ceaselessly through the oceans of the earth to accumulate as mud and sand on the sea bottom. When mud and sand are themselves buried and compressed by overlying sediments, they tend over millions of years to become cemented together by chemical solution and recrystallization, forming hard, dense rock. In much the same way, sugar granules in a bowl will “petrify” into a single stony lump if they get damp and then dry out again. The process takes vastly longer for sand than it does for sugar because silica is far less soluble than sucrose.

The texture of the resulting rock, as well as its suitability as a medium for the preservation of fossils, depends mainly on the size of its constituent particles. The size of those particles depends in turn on how fast the water around them was moving when they were deposited. A swiftly rushing mountain creek will wash away pebbles and gravel together with sand and silt and carry them all downstream. As it flows into flatter country and slows down, it no longer has enough kinetic energy to move the larger bits of rock. One by one, they tumble to the bottom and form deposits of coarse sediment. The smaller particles remain suspended in the water and travel further downstream. Wherever the stream speeds up, it picks up larger particles; wherever it slows down, it drops them. By the time it reaches the ocean as a broad, slow-flowing river, it may have nothing left in it but a thin suspension of clay minerals. Borne out to sea by the currents, this sediment eventually settles to the bottom as a fine-grained mud—the first stage in the formation of shale. Similar processes of sedimentation take place on a smaller scale at the bottom of lakes, or on plains flooded by overflowing rivers.

At the edges of the continents, the action of waves breaking against the rocks has the same sorts of effects that stream runoff has on dry land. High, energetic waves tear loose sizeable chunks of stone and rub them against each other in the surf until all their rough edges are worn away and they become smooth, rounded pebbles. A lot of the rock particles produced by wave action get carried away by ocean currents, in which the particles sort themselves out by size just as they do in a river. Big pieces drop out immediately and form stony bottoms or pebble beaches near the shoreline. Sand travels further, so sandy deposits extend further away from the zone of wave action. Far out to sea, most sediments are fine-grained and may contain a high proportion of organic material. Rocks formed from such sediments are often largely made up of fossil shells and other remains of sea life. For example, the thick beds of chalk that accumulated in many areas near the end of the age of the dinosaurs are composed mainly of the calcareous shells of tiny one-celled animals called foraminifera. In general, the finer the sediment, the more likely it is that the remains and impressions of dead organisms buried in it will be preserved as fossils.

Sediments can also be formed on land by the action of air or ice. Winds blowing across dry, dusty soils can carry small particles away and deposit them as sand dunes, or as deposits of fine silt called loess. The particles in such windborne deposits are small, since blowing air is far less energetic than rushing water.

Rivers of flowing ice, on the other hand, can carry huge boulders for long distances. These ice rivers, called glaciers, form wherever the snow that falls during the winter does not melt entirely during the summer. In these chilly places, snow builds up year after year, becoming compressed into thick layers of dense ice. The weight of the accumulating ice forces some of it to begin moving away from the areas of buildup, flowing downhill with what is appropriately called “glacial” slowness. As it flows, the moving ice breaks away big and small pieces of the rock beneath it, scouring out characteristic U-shaped valleys like a carpenter’s gouge running across the earth.

When it reaches a warmer area where snow melts faster than it accumulates, the glacier melts too. As the rock fragments imbedded in a flowing glacier reach its melting edge, they fall to the ground, depositing a jumble of boulders and gravel known as a glacial moraine. If a flowing glacier reaches the sea before it melts, it may form or add to an oceanic ice sheet, which rides on top of the waves (because ice floats in water). The edges of such ice sheets eventually break off as icebergs and drift out to sea. Melting, they may drop big intrusive chunks of continental rock called dropstones into the fine sediments of the sea floor many miles from land. Like moraines, dropstones in fine-grained sedimentary rocks are an indicator of the presence of ancient glaciers.

Near the poles, where temperatures remain frigid year-round, sheets of glacial ice may cover the sea permanently and become three or four kilometers thick over land areas. There have been several periods in the earth’s history when these polar ice caps spread toward the equator, covering large areas of the temperate zones with ice. These periods are called ice ages or continental glaciations. The continental ice sheets that form during an ice age swell and shrink periodically, retreating toward the poles during interglacial phases and advancing again during glacial maxima. The effects that these fluctuations had on the course of human evolution during the last great ice age, the Pleistocene epoch, are debated by scientists. We will return to these debates in the later chapters of this book.

At the moment, the only large land areas covered by continental ice sheets are Greenland and Antarctica. But Northern Hemisphere ice sheets extended far south into North America and Eurasia during the last glacial maximum, about 20,000 years ago. Geologists infer the former presence and extent of these ice sheets from signs of glacial erosion and from moraines deposited at the melting edges of long-vanished ancient glaciers. In reviewing the prehistory of human populations in northern Eurasia, it should be borne in mind that some evidence of early human presence at high latitudes may have been destroyed by the erosive forces of continental glaciation.

DATING THE ROCKS

The dispute between the Neptunists and the Vulcanists hinged largely on the question of the age of the oldest igneous rocks. This issue could not be entirely settled until there was a generally agreed way of telling which rocks were older. It was easy enough to do this as long as geologists looked at a single locality where rock strata lay on top of each other. In a given geological section, the strata higher up are always younger than those lower down in the section (if you make allowances for occasional folding and twisting). But no single section contained a sample of all time periods and rock types. It was therefore hard to say whether a layer of shale in, say, Scotland was of the same age as a similar-looking shale in Pennsylvania.

The key to this puzzle lay in the fossils imbedded in the sedimentary rocks. In the 1790s, the young English surveyor and engineer William Smith began a systematic study of the rock strata of Somerset, where he had been hired to supervise the construction of a canal. He found that the stacked-up sequence of successive rock types was different in different sections through the earth, so that a bed of red sandstone that was overlain by a layer of gray shale in one canal cut might be covered by a layer of brown sandstone in another. But the sequence of fossils was always the same from cut to cut. Therefore, the distinctive index fossils that were restricted to a particular part of one section could be used to match that part up with different rocks in

FIGURE 1.1 William Smith’s principle of dating rocks by their fossils. If strata containing fossils of the species B are always younger (higher in the section) than those with A, and strata containing C are always younger than both, then these species can thus be used as index fossils to determine the relative ages of rocks containing them. The composite stratigraphic sequence diagrammed here comprises three successive units, the “A,” “B,” and “C” zones, though no one section contains all three. Section 3 does not contain a “B” zone, implying that it was a site of erosion rather than deposition during that period. The inferred gap or unconformity in this section (arrow) is usually also reflected in an interrupted pattern of sedimentation at that time horizon.

other areas and assign them all to the same time period (Fig. 1.1).

Armed with this insight, Smith began to classify and organize the rocks of southern Britain on the basis of the fossils they contained. With the help of a local minister who collected fossils as curiosities, he began in 1799 to publish a series of maps, charts, and books bearing such titles as A Delineation of the Strata of England and Wales. Other scientists soon began to build on Smith’s work and to extend his system to other lands and strata. Smith lived to see himself hailed on all sides as the pioneering founder of the science of stratigraphy before he died in 1839.

The use of index fossils is the basis of biostratigraphy, often referred to as faunal or floral dating. Since no single locality preserves the entire record of life on earth, scientists must correlate strata from different sites in order to determine their relative ages and fit them into a single time sequence. This process begins with the tallying up of the shifting lists of organisms found in successive strata at a single site. This sequence is then compared with similar sequences from other sites in the same area. Of course, two strata from the same time period will not always contain exactly the same suite of organisms, particularly if these strata represent different ecological zones or come from sites that are far apart from each other. Within a region, however, certain key organisms will usually be characteristic of a specific time period. By comparing the biostratigraphic sequences of the sites in a localized area, scientists can work out the time relationships between them and piece them together to form a regional biostratigraphic column. Then by comparing sites that overlap two contiguous regions (or ecological zones), a picture of relative stratigraphic relationships on a broader scale can be formed.

While Smith was walking along his canal cuts in Somersetshire and laying the foundations of stratigraphy in his mind, the Scottish gentleman farmer James Hutton was putting the final touches on his 1795 masterwork, Theory of the Earth. In this book, Hutton laid out detailed evidence for believing that a single set of rock- forming processes had operated throughout the history of the earth. Hutton contended that the small-scale geological processes that could be observed acting in the present were the same processes that had fashioned the large-scale features of the earth’s topography: Mountains were the result of slow uplift, and valleys and deltas were the result of gradual erosion and deposition by rivers and streams. He argued that the fossil-free “Primary” rocks, which the Neptunists saw as condensations from a worldwide suspension of some nebulous primordial ooze, showed clear signs of having been formed in the same ways as later rocks—either through the consolidation of silt and sand and pebbles washed into the world’s waters by waves and streams, or through the solidification of molten stuff belched out of the earth’s interior through volcanic vents. The same subterranean heat that had melted those rocks, Hutton suggested, had in some way produced buckling or heaving movements of the crust of the earth, raising former sea beds into the air to form mountains. He surmised correctly that some ancient rocks, which we now call metamorphic, represented sedimentary deposits that had been considerably altered by the action of heat and pressure inside the earth. Hutton insisted that all these processes had gone on throughout geological time and were still going on today. His arguments implied that the earth must be much older than was generally thought, because it would have taken an enormous amount of time for these processes to produce the current landforms. God, Hutton concluded, had created the earth as a perpetually working machine like the Newtonian solar system, perfectly designed and exquisitely balanced to keep turning over forever. In a famous phrase, he declared that the study of the earth disclosed “no vestige of a beginning, no prospect of an end.”

THE SUCCESSION OF FAUNAS

From the union of Smith’s stratigraphic principles with Hutton’s uniformitarianism, the science of historical geology emerged during the early 1800s. The canonical expression of this synthesis was the English geologist Charles Lyell’s massive compendium, Principles of Geology, published in 1830. Lyell’s Principles had a major influence on Darwin’s Origin of Species (1859) in at least three respects: it provided an exemplary model (of a big theoretical work supported by masses of empirical detail), it established gradual uniformitarian transformation as the normative mode of large-scale prehistoric change, and it demonstrated that the age of the earth was great enough to allow for the production of life’s diversity through the slow, imperceptible processes of evolution that Darwin postulated.

The great task facing the early geologists was the reconstruction of the geological column—the overall sequence of the rocks of the earth throughout its history (Fig. 1.2). No one locality preserves the column in its entirety, because there is no place on the face of the earth where sediment has been building up without interruption for four billion years. The longest single exposure is that seen in the cliff faces of the mile-deep Grand Canyon in Arizona, but even this sequence dates back only some 1.75 billion years—and about half of that time span is represented by gaps in the sequence, produced during periods in this region’s history when erosion outpaced the deposition of sediments.

The complete geological column therefore has had to be pieced together by collecting fossils and rock samples from all over the world and bringing them back for publication and comparison. Two centuries of dangerous, painstaking, difficult labor and the lives of thousands of scientists have gone into this reconstruction. The job is still far from being completed, but what has been accomplished so far is one of the greatest triumphs of the scientific enterprise.

It became obvious early on in this undertaking that the creation story in Genesis was not a satisfactory account of the history of the earth. First of all, it was clear that in spite of the supposed labors of Noah and his sons, most of the species that had once lived were now extinct. In fact, this was what made Smith’s stratigraphic methods work. If all the plants and animals that had ever lived were still alive today, rocks of all ages would contain the same species. Index fossils could be used to order the earth’s rocks only because species had finite life spans. The mortality of species was a disturbing discovery.

Different rocks not only contained different species, they contained different kinds of species. Only the very youngest rocks preserved the remains of creatures much like those living today. As one traced the history of life downward through the geological column, the fossil animals and plants became more and more alien. Human remains and artifacts, for example, were restricted to the Quaternary deposits at the very top of the column. Below these, the Tertiary rocks contained various sorts of fossil mammals—but the mammals found in lower Tertiary strata were not at all like those of today. Still further back in time, there were no fossil mammals whatever, and the only large land animals were fearsome dragonlike reptiles, sometimes of gigantic size. Before that, no land animals of any sort could be found, and all the earth’s organisms had apparently lived in the sea. And at the very bottom of the column, one encountered the fossil-free rocks of the Primary series, whose sediments seemed to bear witness to an unthinkable expanse of time when there had been no living things on the face of the earth.

The scientists of the early 19th century weighed two alternative interpretations of all these uncomfortable facts. One possibility was that ancient species had from

FIGURE 1.2 The geological column, showing the sequence and age of the principal stratigraphic units and some major events in the history of the human lineage.

time to time been obliterated and succeeded by specially created species belonging to new, more advanced types. The French paleontologist Georges Cuvier argued that the abrupt transitions from one stratum to another in the geological column represented brief periods of convulsive change in the earth’s surface, in which most of the planet’s life forms had been wiped out (Cuvier 1831). After each of these catastrophes, new species had been created to rule the earth during a long, peaceful period of uniformitarian change. Many followers of this doctrine of catastrophism did not hesitate to see the hand of God at work in this cycle of extinction and rebirth, and the final catastrophe in Cuvier’s scheme was often identified with the Flood of Noah.

There were two major problems with catastrophism. The first was that it violated the uniformitarian principle, by postulating processes of episodic destruction of a kind and magnitude unknown to human experience. The second was that it provided no explanation for the generation of new species. Some catastrophists invoked divine intervention to account for the periodic creation of new forms of life. But this sort of miraculous “explanation” was scientifically unacceptable. (A science of miracles is a contradiction in terms.) Even from a religious standpoint, it was not entirely satisfactory. Why, after all, would an all-powerful God obliterate all his living creatures from time to time and start afresh with new, improved versions, instead of producing the desired product in the first place? Surely he was not creating life over and over because he needed the practice.

The other way out of this dilemma was to adapt Hutton’s model of the earth as a perpetually working machine and to suppose that today’s life forms had come into being through gradual transformation of the earlier, more primitive species. This so-called development hypothesis was articulated in different ways by Charles Darwin’s grandfather Erasmus Darwin (1794) and a few thinkers of the early 1800s, including J.-B. Lamarck (1809), E. Geoffroy Saint-Hilaire (1830), and R. Chambers (1844). But the development hypothesis was not highly thought of by most experts, because it too seemed to violate the uniformitarian principle. No one, after all, had ever witnessed one species evolving into another one. Cuvier pointed to the mummies of animals recovered from Egyptian tombs, which were estimated to be 3000 years old but displayed no differences from their modern counterparts (Ferembach 1997). This fact, he argued, shows that species have no tendency to change through time. The development theorists could only reply that 3000 years was evidently not enough time to produce detectable changes, and that it must have taken hundreds or thousands of millions of years to accumulate all the transformations seen in the fossil record. Many people found it hard to accept these enormous expanses of time.

RADIATION-BASED DATING TECHNIQUES

How could this question be settled? The stratigraphic methods used to reconstruct the geological column would not do the job. All they could provide were dates for one stratigraphic unit relative to others, based on the principle of superposition (the rule that stacked-up strata had been deposited in chronological sequence, with the ones on the bottom being older than those overlying them). These relative dates allowed geologists to determine that, say, rocks containing fossils of trilobites were everywhere older than rocks containing dinosaurs and that strata containing a particular species of trilobites were probably contemporaneous with strata at other localities containing the same species. But bio- stratigraphy was of no help in determining how many thousands or millions of years had passed since a particular layer of rock had been laid down.

Some tried to attach such absolute dates to the geological column by estimating how many millimeters of sediment get deposited each year on the bottoms of today’s oceans and then comparing these estimates to the number of meters of rock found in various stratigraphic units. But sedimentation rates, erosion rates, and thicknesses of strata vary too much from place to place to be very useful as clocks. About all that could be said for sure was that it had taken many hundreds of millions of years to build the sedimentary rocks of the earth. To produce reliable absolute dates, geologists needed to find some physical process that proceeds at a constant rate in all times and places, producing changes in the sedimentary rocks that could be measured to determine how much time had elapsed since the rock was first laid down.

Such processes began to be identified in the 1950s, as scientists made use of the phenomena of radioactivity to develop radiometric dating techniques. Unstable atoms “decay” by emitting or absorbing subatomic particles and changing into something else. For example, the unstable carbon isotope carbon-14 (14C) undergoes “beta decay” by emitting an electron (beta particle) and changing into an atom of nitrogen-14 (which is stable). These decay events are unpredictable; but their average rate across a large sample of 14C atoms is constant. It takes 5730 years for 50% of the atoms in a sample of 14C to turn into nitrogen-14. After another 5730 years has gone by, half of the remaining C-14 will have turned into nitrogen, and the sample will be 75% nitrogen—and so on. The period of 5730 years is called the half-life of the 14C isotope.

If we found a sealed-up canister labeled “pure carbon- 14” and we wanted to know how long it had been sealed up, we could find out in two ways. First, we could take a sample and determine the ratio of nitrogen to 14C in it. (The more nitrogen in the canister, the older the contents must be.) Second, we could measure the rate at which the canister’s contents give off beta particles. The lower the rate of emissions, the smaller the percentage of 14C remaining must be—and therefore, the older the contents are.

We do not find sealed canisters of once-pure 14C in ancient archaeological sites, but we find something just as useful: tissues from dead animals and plants. Carbon- 14 is produced in the earth’s atmosphere at a more or less constant rate through the action of cosmic radiation on carbon dioxide molecules. Plants incorporate the radioactive CO2 into their tissues, and 14C moves up the food chain from there. When plants and animals die, they stop assimilating 14C from their environments. The 14C remaining in their dead bodies gradually disappears through beta decay, while the stable isotopes of carbon remain unchanged. We can therefore determine how old a site is by determining the ratio of 14C to other carbon isotopes in wood or bones from the site. We can do this either directly (through mass spectrometry) or indirectly (by measuring beta radiation).

Carbon-14 dating (also known as radiocarbon dating) has its complications and shortcomings. The rate of production of 14C is not constant, because it varies with the amount of CO2 in the atmosphere. Moreover, some organisms have physiologies or ways of life that cause them to assimilate less 14C than other organisms do. As a result, not all organisms have the same percentage of 14C in their tissues at death. Carbon-14 dates have to be corrected to take these sources of error into account. And even when all the error factors are compensated for, bone or wood that is older than about 50,000 years has too little 14C left in it to be used for dating. With a time depth of only 50,000 years, 14C dating is useful to archaeologists but has little utility in most paleontological contexts.

Carbon dating has other limitations. Because the death of an organism is required to start the 14C “clock” running, radiocarbon can only be used to estimate an age for bone, wood, or other organic materials. It cannot be used to directly date a mineral sample or a stone tool. Even in dealing with biological materials of suitable age, there is an ever-present risk of postmortem contamination of the sample by carbon compounds of later origin. There are techniques for detecting contamination in samples, but they do not always work. These sources of error introduce uncertainty into any radiocarbon date. A 14