paleobiology ii

PALAEOBIOLOGY II
The dinosaur Diplodocus as seen in the BBC’s acclaimed series ‘Walking with Dinosaurs’
—
the world’s first natural history of
dinosaurs. © BBC Worldwide Ltd, 1999.
PALAEOBIOLOGY II
EDITED BY
DEREK E.G. BRIGGS
Department of Geology & Geophysics
Yale University
New Haven
Connecticut 06511
USA
AND
PETER R. CROWTHER
Keeper of Geology
National Museums and Galleries of Northern Ireland
Ulster Museum
Botanic Gardens
Belfast BT9 5AB
OF THE PALAEONTOLOGICAL ASSOCIATION
To the memory of J.J. Sepkoski Jr
© 2001, 2003 by Blackwell Science Ltd,
a Blackwell Publishing company
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The right of the Authors to be identified as the Authors of the Editorial Material in
this Work has been asserted in accordance with the UK Copyright, Designs, and

Patents Act 1988.
All rights reserved. No part of this publication may be reproduced, stored in a
retrieval system, or transmitted, in any form or by any means, electronic,
mechanical, photocopying, recording or otherwise, except as permitted by the UK
Copyright, Designs, and Patents Act 1988, without the prior permission of the
publisher.
First published 2001
First published in paperback 2003
Library of Congress Cataloging-in-Publication Data
Palaeobiology II / edited by D.E.G. Briggs, P.R. Crowther; foreword by
E.N.K. Clarkson.
p. cm.
Includes bibliographical references and index.
ISBN 0-632-05147-7 (hb : alk. paper)—ISBN 0-632-05149-3 (pb. : alk.
paper)
1. Palaeobiology. I. Title: Palaeobiology two. II. Title: Palaeobiology 2.
III. Briggs, D.E.G. IV. Crowther, Peter R.
QE719.8 .P34 2001
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—
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List of Contributors, ix

Foreword, xv
e.n.k. clarkson
1 Major Events in the History of Life
1.1 Early Life, 3
1.1.1 Origin of Life, 3
a. lazcano
1.1.2 Exploring for a Fossil Record of
Extraterrestrial Life, 8
j.d. farmer
1.1.3 Life in the Archaean, 13
r. buick
1.1.4 Late Proterozoic Biogeochemical
Cycles, 22
g.a. logan and r.e. summons
1.2 The Cambrian Radiation, 25
1.2.1 Metazoan Origins and Early Evolution, 25
d.h. erwin
1.2.2 Significance of Early Shells, 31
s. conway morris
1.2.3 Cambrian Food Webs, 40
n.j. butterfield
1.2.4 The Origin of Vertebrates, 43
m.p. smith and i.j. sansom
1.3 Palaeozoic Events, 49
1.3.1 Ordovician Radiation, 49
a.i. miller
1.3.2 Rise of Fishes, 52
j.a. long
1.3.3 Evolution of Reefs, 57
r.a. wood

1.3.4 Early Land Plants, 63
d. edwards
1.3.5 Afforestation
—
the First Forests, 67
s.e. scheckler
1.3.6 Terrestrialization of Animals, 71
p.a. selden
1.3.7 Origin of Tetrapods, 74
m.i. coates
1.3.8 Carboniferous Coal-swamp Forests, 79
w.a. dimichele
1.3.9 Rise and Diversification of Insects, 82
c.c. labandeira
1.3.10 Origin of Mammals, 88
j.a. hopson
1.4 Mesozoic Events, 94
1.4.1 Mesozoic Marine Revolution, 94
p.h. kelley and t.a. hansen
1.4.2 Origin and Radiation of Angiosperms, 97
e.m. friis, k.r. pedersen and
p.r. crane
1.4.3 Rise of Birds, 102
l.m. chiappe
1.5 Cenozoic Events, 106
1.5.1 Evolution of Modern Grasslands and
Grazers, 106
t.e. cerling
1.5.2 Radiation of Tertiary Mammals, 109
c.m. janis

1.5.3 Rise of Modern Land Plants and
Vegetation, 112
m.e. collinson
1.5.4 Early Primates, 115
k.d. rose
1.5.5 Hominid Evolution, 121
b.a. wood
1.5.6 Neandertals, 127
l.c. aiello
2 The Evolutionary Process and
the Fossil Record
2.1 Species Evolution, 133
2.1.1 Speciation and Morphological
Change, 133
d.b. lazarus
2.1.2 Evolutionary Stasis vs. Change, 137
a.h. cheetham
2.1.3 Rapid Speciation in Species Flocks, 143
a.r. mccune
2.2 Evolution of Form, 147
2.2.1 Developmental Genes and the Evolution of
Morphology, 147
g.a. wray
2.2.2 Constraints on the Evolution of Form, 152
p.j. wagner
2.2.3 Occupation of Morphospace, 157
a.r.h. swan
Contents
v
2.3 Macroevolution, 162

2.3.1 Origin of Evolutionary Novelties, 162
d. jablonski
2.3.2 Controls on Rates of Evolution, 166
s.m. stanley
2.3.3 Competition in Evolution, 171
j.j. sepkoski jr
2.3.4 Biotic Interchange, 176
d.r. lindberg
2.3.5 Importance of Heterochrony, 180
k.j. mcnamara
2.3.6 Hierarchies in Evolution, 188
t.a. grantham
2.3.7 Phylogenetic Tree Shape, 192
p.n. pearson
2.3.8 Contingency, 195
s.j. gould
2.3.9 Selectivity during Extinctions, 198
m.l. mckinney
2.3.10 Biotic Recovery from Mass Extinctions, 202
d.j. bottjer
2.3.11 Evolutionary Trends, 206
d.w. mcshea
2.4 Patterns of Diversity, 211
2.4.1 Biodiversity through Time, 211
m.j. benton
2.4.2 Late Ordovician Extinction, 220
p.j. brenchley
2.4.3 Late Devonian Extinction, 223
g.r. mcghee jr
2.4.4 End-Permian Extinction, 226

p.b. wignall
2.4.5 Impact of K–T Boundary Events on Marine
Life, 229
r.d. norris
2.4.6 Impact of K–T Boundary Events on Terrestrial
Life, 232
j.a. wolfe and d.a. russell
2.4.7 Pleistocene Extinctions, 234
k. roy
3 Taphonomy
3.1 Fossilized Materials, 241
3.1.1 DNA, 241
h.n. poinar and s. pääbo
3.1.2 Proteins, 245
m.j. collins and a.m. gernaey
3.1.3 Lipids, 247
r.p. evershed and m.j. lockheart
3.1.4 Bacteria, 253
k. liebig
3.1.5 Resistant Plant Tissues
—
Cuticles and
Propagules, 256
p.f. van bergen
3.1.6 Animal Cuticles, 259
b.a. stankiewicz and d.e.g. briggs
3.1.7 Shells, 262
k.h. meldahl
3.1.8 Bones, 264
c. denys

3.2 Fossilization Processes, 270
3.2.1 Decay, 270
p.a. allison
3.2.2 Bioerosion, 273
e.n. edinger
3.2.3 Preservation by Fire, 277
a.c. scott
3.2.4 Role of Microbial Mats, 280
j c. gall
3.2.5 Bioimmuration, 285
p.d. taylor and j.a. todd
3.2.6 Transport and Spatial Fidelity, 289
l.c. anderson
3.2.7 Time-averaging, 292
k.w. flessa
3.3 Preservation in Different Ecological
Settings, 297
3.3.1 Major Biases in the Fossil Record, 297
s.m. kidwell
3.3.2 Benthic Marine Communities, 303
w.d. allmon
3.3.3 Ancient Reefs, 307
j.m. pandolfi
3.3.4 Marine Plankton, 309
r.e. martin
3.3.5 Terrestrial Plants, 312
r.a. gastaldo
3.3.6 Pollen and Spores, 315
j.m. van mourik
3.3.7 Terrestrial Vertebrates, 318

a.k. behrensmeyer
3.3.8 Sphagnum-dominated Peat Bogs, 321
t.j. painter
3.3.9 Archaeological Remains, 325
v. straker
3.4 Lagerstätten, 328
3.4.1 Exceptionally Preserved Fossils, 328
d.e.g. briggs
3.4.2 Precambrian Lagerstätten, 332
a.h. knoll and shuhai xiao
3.4.3 Chengjiang, 337
j. bergström
3.4.4 The Soom Shale, 340
r.j. aldridge, s.e. gabbott and
j.n. theron
3.4.5 The Rhynie Chert, 342
n.h. trewin
3.4.6 Hunsrück Slate, 346
r. raiswell, c. bartels and
d.e.g. briggs
vi Contents
3.4.7 La Voulte-sur-Rhône, 349
p.r. wilby
3.4.8 The Santana Formation, 351
d.m. martill
3.4.9 Las Hoyas, 356
j.l. sanz, m.a. fregenal-martínez,
n. meléndez and f. ortega
3.4.10 The Princeton Chert, 359
r.a. stockey

3.4.11 Dominican Amber, 362
g.o. poinar jr
4 Palaeoecology
4.1 Fossils as Living Organisms, 367
4.1.1 Bringing Fossil Organisms to Life, 367
p.w. skelton
4.1.2 Stromatolites, 376
m.r. walter
4.1.3 Plant Growth Forms and Biomechanics, 379
t. speck and n.p. rowe
4.1.4 Sessile Invertebrates, 384
w.i. ausich and d.j. bottjer
4.1.5 Trilobites, 386
b.d.e. chatterton
4.1.6 Trackways
—
Arthropod Locomotion, 389
s.j. braddy
4.1.7 Durophagy in Marine Organisms, 393
r.b. aronson
4.1.8 Buoyancy, Hydrodynamics, and Structure in
Chambered Cephalopods, 397
d.k. jacobs
4.1.9 Feeding in Conodonts and other Early
Vertebrates, 401
m.a. purnell
4.1.10 Locomotion in Mesozoic Marine Reptiles, 404
m.a. taylor
4.1.11 Trackways
—

Dinosaur Locomotion, 408
m.g. lockley
4.1.12 Dinosaur Ethology, 412
j.r. horner
4.1.13 Predatory Behaviour in Maniraptoran
Theropods, 414
a.d. gishlick
4.1.14 Pterosaur Locomotion, 417
d.m. unwin
4.1.15 Predation in Sabre-tooth Cats, 420
b. van valkenburgh
4.1.16 Plant–Animal Interactions: Herbivory, 424
s. ash
4.1.17 Plant–Animal Interactions: Insect
Pollination, 426
w.l. crepet
4.1.18 Plant–Animal Interactions: Dispersal, 429
j.j. hooker and m.e. collinson
4.2 Ancient Communities, 432
4.2.1 Ecological Changes through Geological
Time, 432
m.l. droser
4.2.2 Do Communities Evolve? 437
r.k. bambach
4.2.3 Palaeobiogeography of Marine
Communities, 440
g.r. shi
4.2.4 Deep-sea Communities, 444
t. oji
4.2.5 Ancient Hydrothermal Vent and Cold Seep

Faunas, 447
c.t.s. little
4.2.6 Zooplankton, 451
s. rigby and c.v. milsom
4.2.7 Terrestrial Palaeobiogeography, 454
r.s. hill
4.2.8 Epibionts, 460
h.l. lescinsky
4.2.9 Fungi in Palaeoecosystems, 464
t.n. taylor and e.l. taylor
4.3 Fossils as Environmental Indicators, 467
4.3.1 Taphonomic Evidence, 467
m.v.h. wilson
4.3.2 Oxygen in the Ocean, 470
w. oschmann
4.3.3 Carbon Isotopes in Plants, 473
d.j. beerling
4.3.4 Bathymetric Indicators, 475
p.j. orr
4.3.5 Atmospheric Carbon Dioxide
—
Stomata, 479
j.c. mcelwain
4.3.6 Climate
—
Wood and Leaves, 480
d.r. greenwood
4.3.7 Climate
—
Modelling using Fossil Plants, 483

g.r. upchurch jr
4.3.8 Climate
—
Quaternary Vegetation, 485
t. webb
5 Systematics, Phylogeny, and
Stratigraphy
5.1 Morphology and Taxonomy, 489
5.1.1 Quantifying Morphology, 489
r.e. chapman and d. rasskin-gutman
5.1.2 Morphometrics and Intraspecific
Variation, 492
n.c. hughes
5.1.3 Disparity vs. Diversity, 495
m.a. wills
5.2 Calibrating Diversity, 500
5.2.1 Estimating Completeness of the Fossil
Record, 500
m. foote
Contents vii
5.2.2 Analysis of Diversity, 504
a.b. smith
5.3 Reconstructing Phylogeny, 509
5.3.1 Phylogenetic Analysis, 509
m. wilkinson
5.3.2 Fossils in the Reconstruction of Phylogeny, 515
p.l. forey and r.a. fortey
5.3.3 Stratigraphic Tests of Cladistic
Hypotheses, 519
m.a. norell

5.3.4 Molecular Phylogenetic Analysis, 522
j.p. huelsenbeck
5.3.5 Molecules and Morphology in Phylogeny
—
the Radiation of Rodents, 529
f.m. catzeflis
5.3.6 Using Molecular Data to Estimate Divergence
Times, 532
a. cooper, n. grassly and a. rambaut
5.4 Fossils in Stratigraphy, 535
5.4.1 Stratigraphic Procedure, 535
p.f. rawson
5.4.2 Calibration of the Fossil Record, 539
s.a. bowring and m.w. martin
5.4.3 Confidence Limits in Stratigraphy, 542
c.r. marshall
5.4.4 High-resolution Biostratigraphy, 545
j. backman
5.4.5 Sequence Stratigraphy and Fossils, 548
s.m. holland
Index, 555
viii Contents
L.C. AIELLO Department of Anthropology, University
College London, Gower Street, London WC1E 6BT, UK.
R.J. ALDRIDGE Department of Geology, University of
Leicester, University Road, Leicester LE1 7RH, UK.
P.A. ALLISON T.H. Huxley School for Environment,
Earth Science & Engineering, Imperial College of Science,
Technology & Medicine, Prince Consort Road, South Kens-
ington, London SW7 5BP, UK.

W.D. ALLMON Paleontological Research Institution,
1259 Trumansburg Road, Ithaca, New York 14850, USA.
L.C. ANDERSON Department of Geology & Geophysics,
Louisiana State University, Baton Rouge, Louisiana 70803,
USA.
R.B. ARONSON Dauphin Island Sea Lab, 101 Bienville
Boulevard, Dauphin Island, Alabama 36528, USA.
S. ASH Department of Earth & Planetary Sciences, Univer-
sity of New Mexico, Albuquerque, New Mexico 87131,
USA.
W.I. AUSICH Department of Geological Sciences, Ohio
State University, 155 South Oval Mall, Columbus, Ohio
43210-1397, USA.
J. BACKMAN Department of Geology & Geochemistry,
Stockholm University, S-106 91 Stockholm, Sweden.
R.K. BAMBACH Department of Geological Sciences,
Virginia Polytechnic Institute & State University, 4044
Derring Hall, Blacksburg, Virginia 24061-0420, USA.
C. BARTELS Deutsches Bergbau-Museum, Am Bergbau-
museum 28, D-44791 Bochum, Germany.
D.J. BEERLING Department of Animal & Plant Sciences,
University of Sheffield, Alfred Denny Building, Western
Bank, Sheffield S10 2TN, UK.
A.K. BEHRENSMEYER Department of Paleobiology,
National Museum of Natural History, Smithsonian Insti-
tution, Washington, DC 20560-0121, USA.
M.J. BENTON Department of Earth Sciences, University
of Bristol, Wills Memorial Building, Queen's Road, Bristol
BS8 1RJ, UK.
P.F. van BERGEN Shell Global Solutions International

BV, PO Box 38000, 1030 BN Amsterdam, The Netherlands.
J. BERGSTRÖM Department of Palaeozoology, Swedish
Museum of Natural History, PO Box 50007, S-104 05
Stockholm, Sweden.
D.J. BOTTJER Department of Earth Sciences, University
of Southern California, Los Angeles, California 90089,
USA.
S.A. BOWRING Department of Earth, Atmospheric &
Planetary Sciences, Massachusetts Institute of Technology,
Cambridge, Massachusetts 02139, USA.
S.J. BRADDY Department of Earth Sciences, University of
Bristol, Wills Memorial Building, Queen's Road, Bristol
BS8 1RJ, UK.
P.J. BRENCHLEY Department of Earth Sciences, Univer-
sity of Liverpool, PO Box 147, Liverpool L69 3GP, UK.
D.E.G. BRIGGS Department of Geology & Geophysics,
Yale University, New Haven, Connecticut 06511, USA.
R. BUICK Department of Earth & Space Sciences and
Astrobiology Program, University of Washington, Seattle,
Washington 98195-1310, USA.
N.J. BUTTERFIELD Department of Earth Sciences,
University of Cambridge, Downing Street, Cambridge
CB2 3EQ, UK.
F.M. CATZEFLIS Institut des Sciences de l'Evolution,
UMR 5554 CNRS & Université Montpellier 2, F-34095
Montpellier 05, France.
T.E. CERLING Department of Geology & Geophysics,
University of Utah, Salt Lake City, Utah 84103, USA.
R.E. CHAPMAN Applied Morphometrics Laboratory
(ADP) & Department of Paleobiology, National Museum of

Natural History, Smithsonian Institution, Washington,
DC 20560, USA.
B.D.E. CHATTERTON Department of Earth & Atmos-
pheric Sciences, University of Alberta, Edmonton, Alberta,
Canada T6G 2E1.
A.H. CHEETHAM Department of Paleobiology, National
Museum of Natural History, Smithsonian Institution,
Washington, DC 20560, USA.
L.M. CHIAPPE Department of Vertebrate Paleontology,
Natural History Museum of Los Angeles County, 900 Ex-
position Boulevard, Los Angeles, California, 90007, USA.
List of Contributors
ix
E.N.K. CLARKSONGrant Institute of Geology, Univer-
sity of Edinburgh, West Mains Road, Edinburgh EH9 3JW,
UK.
M.I. COATES Department of Organismal Biology and
Anatomy, University of Chicago, 1027 East 57th Street,
Chicago, Illinios 60637, USA.
M.J. COLLINS Postgraduate Institute in Fossil Fuels &
Environmental Geochemistry (NRG), Drummond Build-
ing, The University, Newcastle upon Tyne NE1 7RU, UK.
M.E. COLLINSON Department of Geology, Royal
Holloway University of London, Egham, Surrey
TW20 0EX, UK.
S. CONWAY MORRIS Department of Earth Sciences,
University of Cambridge, Downing Street, Cambridge
CB2 3EQ, UK.
A. COOPER Institute of Biological Anthropology,
University of Oxford, 58 Banbury Road, Oxford

OX2 6QS, UK.
P.R. CRANE Royal Botanic Gardens, Kew, Richmond,
Surrey TW9 3AB, UK.
W.L. CREPET L.H. Bailey Hortorium, Cornell Univer-
sity, Mann Lib 462, Ithaca, New York 14853, USA.
P.R. CROWTHER Keeper of Geology, Museums and
Galleries of Northern Ireland, Ulster Museum, Botanic
Gardens, Belfast, BT9 5AB, UK.
C. DENYS Laboratoire Mammifères et Oiseaux, Muséum
National d’Histoire Naturelle, 55 rue Buffon, F-75005
Paris, France.
W.A. DIMICHELE Department of Paleobiology,
National Museum of Natural History, Smithsonian Insti-
tution, Washington, DC 20560, USA.
M.L. DROSER Department of Earth Sciences, University
of California at Riverside, California 92521, USA.
E.N. EDINGER Departments of Geography & Biology,
Memorial University of Newfoundland, St John’s, New-
foundland A1B 3X9, Canada.
D. EDWARDS Department of Earth Sciences, Cardiff
University, Main Building, PO Box 941, Cardiff CF10
3YE, UK.
D.H. ERWIN Department of Paleobiology, National
Museum of Natural History, Smithsonian Institution,
Washington, DC 20560, USA.
R.P. EVERSHED Organic Geochemistry Unit, School of
Chemistry, University of Bristol, Cantock's Close, Bristol
BS8 1TS, UK.
J.D. FARMER Department of Geological Sciences, Arizona
State University, Box 871404, Tempe, Arizona 85254-

1404, USA.
K.W. FLESSA Department of Geosciences, University of
Arizona, Tucson, Arizona 85721, USA.
M. FOOTE Department of the Geophysical Sciences, Uni-
versity of Chicago, 5734 South Ellis Avenue, Chicago,
Illinois 60637, USA.
P.L. FOREY Department of Palaeontology, The Natural
History Museum, Cromwell Road, London SW7 5BD, UK.
R.A. FORTEY Department of Palaeontology, The Natural
History Museum, Cromwell Road, London SW7 5BD, UK.
M.A. FREGENAL-MARTÍNEZ Departamento de
Estratigrafía, Facultad de Ciencias Geológicas, Universidad
Complutense, 28040 Madrid, Spain.
E.M. FRIIS Department of Palaeobotany, Swedish Museum
of Natural History, Box 50007, S-104 05 Stockholm,
Sweden.
S.E. GABBOTT Department of Geology, University of
Leicester, University Road, Leicester LE1 7RH, UK.
J C. GALL Institut de Géologie, Université Louis Pasteur,
1 rue Blessig, F-67084 Strasbourg, France.
R.A. GASTALDO Department of Geology, Colby College,
5820 Mayflower Hill, Waterville, Maine 04901-8858,
USA.
A.M. GERNAEYBiosciences Research Institute, Cock-
croft Building, University of Salford, Greater Manchester
M5 4WT, UK.
A.D. GISHLICK
National Center for Science Education,
PO Box 9477, Berkeley, California 94709-0477, USA.
S.J. GOULD (Deceased) Formerly of Museum of

Comparative Zoology, Harvard University, Cambridge,
Massachusetts 02138, USA.
T.A. GRANTHAM Department of Philosophy, College of
Charleston, 66 George Street, Charleston, South Carolina
29424-0001, USA.
N. GRASSLY Infectious Disease Epidemiology, St Mary’s
Hospital, Norfolk Place, London W2 1PG, UK.
D.R. GREENWOOD School of Life Sciences & Technol-
ogy, Victoria University of Technology, PO Box 14428,
Melbourne City, Victoria 8001, Australia.
T.A. HANSEN Department of Geology, Western
Washington University, Bellingham, Washington 98225,
USA.
R.S. HILL Department of Environmental Biology,
University of Adelaide, Adelaide, South Australia 5005,
Australia.
S.M. HOLLAND Department of Geology, University of
Georgia, Athens, Georgia 30602-2501, USA.
J.J. HOOKER Department of Palaeontology, The Natural
History Museum, Cromwell Road, London SW7 5BD, UK.
x List of Contributors
J.A. HOPSON Department of Organismal Biology &
Anatomy, University of Chicago, 1027 East 57th Street,
Chicago, Illinois 60637, USA.
J.R. HORNER Museum of the Rockies, Montana State
University, Bozeman, Montana 59717, USA.
J.P. HUELSENBECK Department of Biology, University
of Rochester, Rochester, New York 14627, USA.
N.C. HUGHES Department of Earth Sciences, University
of California at Riverside, California 92521-0423, USA.

D. JABLONSKI Department of Geophysical Sciences,
University of Chicago, 5734 South Ellis Avenue, Chicago,
Illinois 60637, USA.
D.K. JACOBS Department of Organismic Biology, Ecology
& Evolution, University of California at Los Angeles, 621
Charles Young Drive South, Box 951606, Los Angeles,
California 90095-1606, USA.
C.M. JANIS Department of Ecology & Evolutionary
Biology, Box G-B207, Brown University, Providence,
Rhode Island 02912, USA.
P.H. KELLEY Department of Earth Sciences, University of
North Carolina at Wilmington, 601 South College Road,
Wilmington, North Carolina 28403-3297, USA.
S.M. KIDWELL Department of Geophysical Sciences,
University of Chicago, 5734 South Ellis Avenue, Chicago,
Illinois 60637, USA.
A.H. KNOLL Botanical Museum, Harvard University, 26
Oxford Street, Cambridge, Massachusetts 02138, USA.
C.C. LABANDEIRA Department of Paleobiology,
National Museum of Natural History, Smithsonian Insti-
tution, Washington, DC 20560, USA.
D.B. LAZARUS Institut für Paläontologie, Museum
für Naturkunde, Zentralinstitut der Humboldt-Universität
zu Berlin, Invalidenstrasse 43, D-10115 Berlin, Germany.
A. LAZCANO Facultad de Ciencias, Universidad
Nacional Autónoma de México, Apdo. Postal 70-407, Cd.
Universitaria, Mexico City, 04510 DF, Mexico.
H.L. LESCINSKY Department of Life & Earth Sciences,
Otterbein College, Westerville, Ohio 43081, USA.
K. LIEBIG Fakultät für Biologie, Ruprecht-Karls-

Universität Heidelberg, Im Nevenheimer Feld 234, D-
69120 Heidelberg, Germany.
D.R. LINDBERG Department of Integrative Biology,
University of California at Berkeley, Berkeley,
California 94720, USA.
C.T.S. LITTLE School of Earth Sciences, University of
Leeds, Leeds LS2 9JT, UK.
M.J. LOCKHEART Organic Geochemistry Unit, School
of Chemistry, University of Bristol, Cantock's Close, Bristol
BS8 1TS, UK.
M.G. LOCKLEY Department of Geology, University of
Colorado at Denver, Campus Box 172, PO Box 173364,
Denver, Colorado 80217-3364, USA.
G.A. LOGAN Geoscience Australia, GPO Box 378,
Constitution Avenue, Canberra, ACT 2601, Australia.
J.A. LONG Department of Earth & Planetary Sciences,
Western Australian Museum, Francis Street, Perth,
Western Australia 6000, Australia.
C.R. MARSHALL Departments of Earth & Planetary
Sciences, and Organismic & Evolutionary Biology, Har-
vard University, Cambridge, Massachusetts 02138, USA.
D.M. MARTILL School of Earth & Environmental Sci-
ences, Burnaby Building, University of Portsmouth,
Burnaby Road, Portsmouth PO1 3QL, UK.
M.W. MARTIN Shell International Exploration and
Production Inc., Woodcreek-Rm-7516, 200 North Dairy
Ashford, Houston, Texas 77001-2121, USA.
R.E. MARTIN Department of Geology, University of
Delaware, Newark, Delaware 19716, USA.
A.R. McCUNE Department of Ecology & Evolutionary

Biology, Corson Hall, Cornell University, Ithaca, New York
14853, USA.
J.C. McELWAIN Department of Geology, Field Museum
of Natural History, 1400S Lake Shore Drive, Chicago,
Illinois 60605-2496, USA.
G.R. McGHEE Department of Geological Sciences,
Rutgers University, Wright-Rieman Laboratories, Busch
Campus, Piscataway, New Jersey 08854-8066, USA.
M.L. McKINNEY Department of Geological Sciences,
University of Tennessee, Knoxville, Tennessee 37996-1410,
USA.
K.J. McNAMARA Department of Earth & Planetary
Sciences, Western Australian Museum, Francis Street,
Perth, Western Australia 6000, Australia.
D.W. McSHEA Department of Biology, Box 90338, Duke
University, Durham, North Carolina 27708-0338, USA.
K.H. MELDAHL Physical Sciences Department,
MiraCosta College, 1 Barnard Drive, Oceanside, California
92056, USA.
N. MELÉNDEZ Departamento de Estratigrafía, Facultad
de Ciencias Geológicas, Universidad Complutense, 28040
Madrid, Spain.
A.I. MILLER Department of Geology, PO Box 210013,
University of Cincinnati, Cincinnati, Ohio 45221-0013,
USA.
C.V. MILSOM School of Biological & Earth Sciences, Liv-
erpool John Moores University, James Parsons Building,
Byrom Street, Liverpool L3 3AF, UK.
List of Contributors xi
M.A. NORELL Department of Vertebrate Paleontology,

American Museum of Natural History, Central Park West
at 79th Street, New York, New York 10024-5192, USA.
R.D. NORRIS Scripps Institution of Oceanograpy,
University of California, La Jolla, California 92093-0244,
USA.
T. OJI Department of Earth & Planetary Science, University
of Tokyo, Hongo, Tokyo 113-0033, Japan.
P.J. ORR Department of Geology, National University of
Ireland, Galway, Ireland.
F. ORTEGA Unidad de Paleontología, Departamento
de Biología, Facultad de Ciencias, Universidad Autónoma,
Cantoblanco, 28049 Madrid, Spain.
W. OSCHMANN Geologisch-Paläontologisches Institut,
Senckenberganlage 32-34, Universität Frankfurt, D-60054
Frankfurt-am-Main, Germany.
S. PÄÄBO Max-Planck Institute for Evolutionary Anthro-
pology, Inselstrasse 22, D-04103 Leipzig, Germany.
T.J. PAINTER Institute of Biotechnology, Norwegian
University of Science & Technology, N-7491 Trondheim,
Norway.
J.M. PANDOLFI Department of Paleobiology, National
Museum of Natural History, Smithsonian Institution,
Washington, DC 20560, USA.
P.N. PEARSON Department of Earth Sciences, Univer-
sity of Bristol, Wills Memorial Building, Queen's Road,
Bristol BS8 1RJ, UK.
K.R. PEDERSEN Department of Geology, University of
Aarhus, DK-8000 Aarhus C, Denmark.
G.O. POINAR Jr Department of Entomology, Oregon
State University, 2046 Cordley Hall, Corvallis, Oregon

97331-2907, USA.
H.N. POINAR Max-Planck Institute for Evolutionary
Anthropology, Inselstrasse 22, D-04103 Leipzig, Germany.
M.A. PURNELL Department of Geology, University of
Leicester, University Road, Leicester LE1 7RH, UK.
R. RAISWELL School of Earth Sciences, University of
Leeds, Leeds LS2 9JT, UK.
A. RAMBAUT Department of Zoology, University of
Oxford, Oxford OX1 3PS, UK.
D. RASSKIN-GUTMAN The Salk Institute for Biologi-
cal Studies, PO Box 85800, San Diego, California 92186-
5800, USA.
P.F. RAWSON Department of Geological Sciences, Uni-
versity College London, Gower Street, London WC1E 6BT,
UK.
S. RIGBY Grant Institute of Geology, University of
Edinburgh, West Mains Road, Edinburgh EH9 3JW, UK.
K.D. ROSE Department of Cell Biology & Anatomy, School
of Medicine, The Johns Hopkins University, Baltimore,
Maryland 21205, USA.
N.P. ROWE Institut des Sciences de l'Evolution, UMR
5554 CNRS & Université Montpellier 2, F-34095 Mont-
pellier 05, France.
K. ROY Department of Biology, University of California,
San Diego, 9500 Gilman Drive, La Jolla, California 92093-
0116, USA.
D.A. RUSSELL North Carolina State Museum of Natural
Science, Raleigh, North Carolina 27695, USA.
I.J. SANSOM School of Earth Sciences, University of
Birmingham, Edgbaston, Birmingham B15 2TT, UK.

J.L. SANZ Unidad de Paleontología, Departamento de
Biología, Facultad de Ciencias, Universidad Autónoma,
Cantoblanco, 28049 Madrid, Spain.
S.E. SCHECKLER Department of Biology, Virginia
Polytechnic Institute & State University, Blacksburg,
Virginia 24061-0406, USA.
A.C. SCOTT Department of Geology, Royal Holloway Uni-
versity of London, Egham, Surrey TW20 0EX, UK.
P.A. SELDEN Department of Earth Sciences, University of
Manchester, Oxford Road, Manchester M13 9PL, UK.
J.J. SEPKOSKI Jr [Deceased] Formerly of Department of
Geophysical Sciences, University of Chicago, 5734 South
Ellis Avenue, Chicago, Illinois 60637, USA.
G.R. SHI
School of Ecology & Environment, Deakin
University, Rusden Campus, Clayton, Victoria 3168,
Australia.
P.W. SKELTON Department of Earth Sciences, Open Uni-
versity, Walton Hall, Milton Keynes MK7 6AA, UK.
A.B. SMITH Department of Palaeontology, Natural
History Museum, Cromwell Road, London SW7 5BD, UK.
M.P. SMITH Lapworth Museum, School of Earth Sciences,
University of Birmingham, Edgbaston, Birmingham
B15 2TT, UK.
T. SPECK Botanischer Garten, Albert-Ludwigs-
Universität Freiburg, Schänzlestrasse 1, D-79104
Freiburg, Germany.
B.A. STANKIEWICZ Shell International Exploration
and Production BV, Volmerlaan 8, Postbus 60, 2280 AB
Rijswijk, The Netherlands.

S.M. STANLEY Department of Earth & Planetary Sci-
ences, The Johns Hopkins University, Baltimore, Maryland
21218, USA.
xii List of Contributors
R.A. STOCKEY Department of Biological Sciences, Uni-
versity of Alberta, Edmonton, Alberta, Canada T6G 2E9.
V. STRAKER School of Geographical Sciences, University
of Bristol, University Road, Bristol BS8 1SS, UK.
R.E. SUMMONS Department of Earth, Atmospheric
and Planetary Sciences, Massachusetts Institute of Tech-
nology, 77 Massachusetts Avenus E34-246, Cambridge,
Massachusetts 02139-4307, USA.
A.R.H. SWAN School of Geological Sciences, Kingston
University, Penrhyn Road, Kingston-upon-Thames,
Surrey KT1 2EE, UK.
E.L. TAYLOR Department of Ecology & Evolutionary
Biology, University of Kansas, Lawrence, Kansas 66045,
USA.
M.A. TAYLOR Department of Geology & Zoology,
National Museums of Scotland, Chambers Street,
Edinburgh EH1 1JF, UK.
P.D. TAYLOR Department of Palaeontology, The Natural
History Museum, Cromwell Road, London SW7 5BD,
UK.
T.N. TAYLOR Department of Ecology & Evolutionary
Biology, University of Kansas, Lawrence, Kansas 66045,
USA.
J.N. THERON Department of Geology, University of Stel-
lenbosch, Stellenbosch, Republic of South Africa.
J.A. TODD Department of Palaeontology, The Natural

History Museum, Cromwell Road, London SW7 5BD,
UK.
N.H. TREWIN Department of Geology & Petroleum
Geology, University of Aberdeen, Meston Building, King's
College, Aberdeen AB24 3UE, UK.
D.M. UNWIN Institüt für Paläontologie, Museum für
Naturkunde, Zentralinstitut der Humboldt-Universität zu
Berlin, Invalidenstrasse 43, D-10115 Berlin, Germany.
G.R. UPCHURCH Jr Department of Biology, Southwest
Texas State University, 601 University Drive, San Marcos,
Texas 78666-4616, USA.
J.M. VAN MOURIK Institute for Biodiversity & Ecosys-
tem Dynamics, University of Amsterdam, Niewe Achter-
gracht 166, 1018-WV Amsterdam, The Netherlands.
B. VAN VALKENBURGH Department of Organismic
Biology, Ecology & Evolution, University of California at
Los Angeles, 621 Charles E. Young Drive South, PO Box
951606, Los Angeles, California 90095-1606, USA.
P.J. WAGNER Department of Geology, Field Museum of
Natural History, Roosevelt Road at Lake Shore Drive,
Chicago, Illinois 60615-2496, USA.
M.R. WALTER Department of Earth & Planetary
Sciences, Macquarie University, New South Wales 2109,
Australia.
T. WEBB Department of Geology, Brown University, Provi-
dence, Rhode Island 02912, USA.
P.B. WIGNALL School of Earth Sciences, University of
Leeds, Leeds LS2 9JT, UK.
P.R. WILBY Kingsley Dunham Centre, British Geological
Survey, Keyworth, Nottingham NG12 5GG, UK.

M. WILKINSON Department of Zoology, Natural
History Museum, Cromwell Road, London SW7 5BD,
UK.
M.A. WILLS Department of Biology & Biochemistry, Uni-
versity of Bath, South Building, Claverton Down, Bath
BA2 7AY, UK.
M.V.H. WILSON Department of Biological Sciences,
University of Alberta, Edmonton, Alberta, Canada T6G
2E9.
J.A. WOLFE Department of Geosciences, University of
Arizona, Tucson, Arizona 85721-0077, USA.
B.A. WOOD Department of Anthropology, George Wash-
ington University, 2110 G Street NW, Washington, DC
20052, USA.
R.A. WOOD Schlumberger Cambridge Research, High
Cross, Madingley Road, Cambridge CB3 0EL, UK.
G.A. WRAY Department of Biology, Duke University,
Durham, North Carolina 27708-0325, USA.
SHUHAI XIAO Department of Geology, Tulane Univer-
sity, New Orleans, Louisiana 70118, USA.
List of Contributors xiii
When Palaeobiology
—
a synthesis appeared in 1990, it was
immediately recognized as an invaluable compilation
which no palaeobiologist should be without. Each of the
articles had been commissioned to provide authoritative
and up-to-date information in as concise a form as poss-
ible, and only essential references were included. While
almost any of these articles could be read by non-experts,

their value for advanced students was unquestioned.
Where else between two covers could such appropriate
and easily mastered source material be found for essays
and presentations?
In the decade since the publication of Palaeobiology
—
a
synthesis, new data have accumulated, new expertise has
arisen, concepts have evolved, and emphases have
changed. It is now time for a new synthesis, and here it
is
—
Palaeobiology II. Readers familiar with the first book
will recognize the main divisions here, but Palaeobiology
II is by no means a second edition
—
it is an entirely new
book. The great majority of the 137 articles deal with new
topics (all are new treatments), and over 100 authors are
new. The basic concept that proved so successful in the
first book has nevertheless been retained: the articles
have been written by recognized authorities in each
field; the content is concise but informative; and the
accompanying reference lists are brief and up to date.
In all respects this volume is timely, and it will be
widely used. The new generation of articles reflects not
only the vigorous and exciting developments that are
taking place in palaeontology at the opening of the
twenty-first century, but also the many links with other
scientific disciplines. Palaeontologists today must

know about developmental genes and fossil proteins,
sequence stratigraphy and fossils, and how to test
cladistic analyses against the fossil record. All these
topics, and many more, are to be found here. But the sci-
entific themes that have been developed within palaeo-
biology are also necessary for other sciences, and the
book will prove of great value outside its own specific
area.
Derek Briggs and Peter Crowther commissioned and
edited the articles for Palaeobiology
—
a synthesis and saw
the whole gigantic project through to completion. They
have again been active; Palaeobiology II is a testament to
their vision, to the many hours of labour required to
realize it, and to the fruitful partnership that they have
developed with Blackwell Science.
On behalf of the Palaeontological Association, I wish
Palaeobiology II the success it deserves. It is very wel-
come, and the topics covered here cannot fail to interest
biologists and palaeontologists of all kinds. Perhaps in
another ten years there will be a further version, but this
one is unlikely to be quickly superseded.
Foreword
EUAN N.K. CLARKSON
President of the
Palaeontological Association
1998–2000
The
Palaeontological

Association
xv
1
MAJOR EVENTS IN THE
HISTORY OF LIFE
Cranium of Neandertal (Guattari 1) from Monte Circeo, Italy, approximately two-thirds natural size. (Photograph
courtesy of R. Macchiarelli, Museo Nazionale Preistorico Etnografico, Rome.)
Palaeobiology II
Edited by Derek E.G. Briggs, Peter R. Crowther
Copyright © 2001, 2003 by Blackwell Publishing Ltd
1.1 Early Life
development of proteins and DNA genomes, during
which alternative life forms based on ribozymes existed
(Gesteland et al. 1999). This has led many to argue that
the starting point for the history of life on Earth was the
de novo emergence of the RNA world from a nucleotide-
rich prebiotic soup. Others are more sceptical and
believe that it lies in the origin of cryptic and largely
unknown pre-RNA worlds. There is even a third group
that favours the possibility that life began with the
appearance of chemoautotrophic autocatalytic meta-
bolic networks, lacking genetic material.
Despite the seemingly insurmountable obstacles sur-
rounding the understanding of the origin of life (or
perhaps because of them), there has been no shortage
of discussion about how it took place. Not surprisingly,
several alternative and even opposing suggestions have
been made regarding how life emerged and what were
the defining characteristics of the first organisms. While
the classical version of the hypothesis of chemical evolu-

tion and primordial heterotrophy needs to be updated, it
still provides the most useful framework for addressing
the issue of emergence of life.
How can the origin of life be studied?
Of necessity, work on the origin of life should be
regarded as enquiring and explanatory rather than
definitive and conclusive. This does not imply that our
theories and explanations can be dismissed as pure
speculation, but rather that the issue should be
addressed conjecturally, in an attempt to construct a
coherent, non-teleological historical narrative (Kam-
minga 1991). It is unlikely that the origin of life will ever
be described in full detail; at best a sketchy outline, con-
sistent with conditions on the prebiotic Earth (such as its
anoxic environment) and the physicochemical proper-
ties of the likely molecular precursors of living systems,
will be constructed.
The attributes of the first living organisms are
unknown. They were probably simpler than any cell
now alive, and may have lacked not only protein-based
catalysis, but perhaps even the familiar genetic macro-
molecules, with their ribose-phosphate backbones. It is
possible that the only property they shared with extant
organisms was the structural complementarity between
monomeric subunits of replicative informational poly-
mers, e.g. joining together a growing chain of residues
in a sequence directed by preformed polymers. How-
ever, such ancestral polymers may not have involved
nucleotides. Hence caution must be exercised in extrapo-
3

1.1.1 Origin of Life
A. LAZCANO
Introduction
‘All the organic beings which have ever lived on this
Earth’, wrote Charles Darwin in On the Origin of Species
by Means of Natural Selection, ‘may be descended from
some one primordial form’. It is not known how this
first ancestor came into being nor what its nature was.
However, the presence of cyanobacteria-like microfos-
sils in the 3.5 billion years old (Ga) Australian Apex sedi-
ments (Schopf 1993), deposited only a few million years
after the end of the intense bombardment caused by the
late accretion of planetesimals left over from the forma-
tion of the Solar System, demonstrates that the emer-
gence and early diversification of life on Earth required
no more than 500 million years. Together with the inclu-
sions enriched in light isotopic carbon in 3.86Ga samples
from south-west Greenland (Mojzsis et al. 1996), these
results show that a widespread, complex, and highly
diversified Archaean microbiota was thriving soon after
the Earth had cooled down and the influx of myriads of
comets and asteroids had ceased.
It is unlikely that data on how life originated will be
provided by the palaeontological record. There is no
geological evidence of the environmental conditions on
Earth at the time of the origin of life, nor any fossil reg-
ister of the evolutionary processes that preceded the
appearance of the first cells. Direct information is lacking
not only on the composition of the terrestrial atmosphere
during the period of the origin of life, but also on the

temperature, ocean pH values, and other general and
local environmental conditions which may or may not
have been important for the emergence of living
systems.
The lack of an all-embracing, generally agreed
definition of life sometimes gives the impression that
what is meant by its origin is defined in somewhat
imprecise terms, and that several entirely different ques-
tions are often confused. For instance, until a few years
ago the origin of the genetic code and of protein synthe-
sis were considered synonymous with the appearance of
life itself. This is no longer a dominant point of view; the
discovery and development of the catalytic activity of
RNA molecules has given considerable support to the
idea of an ‘RNAworld’
—
a hypothetical stage, before the
lating deep molecular phylogenies back into primordial
times. Genome sequencing and analysis is becoming
critical for understanding early cellular evolution, but it
cannot be applied to events prior to the evolution of
protein biosynthesis. Older stages are not yet amenable
to this type of analysis, and the organisms at the base of
universal phylogenies are cladistically ancient species,
not primitive unmodified microbes.
Given the huge gap between the abiotic synthesis of
biochemical monomers and the DNA/protein-based last
common ancestor of all living systems, it is naive to
attempt to describe the origin of life on the basis of avail-
able phylogenetic trees. Like a mangrove, the roots of

universal evolutionary trees may be submerged in the
muddy waters of a prebiotic broth
—
but how the transi-
tion from the non-living to the living took place is still
unknown.
Heterotrophic or autotrophic origins of life?
Although the idea of life as an emergent feature of nature
has been widespread since the nineteenth century, a
major methodological breakthrough by A.I. Oparin and
J.B.S. Haldane in the 1920s transformed the origin of life
from a purely speculative issue to a workable research
programme. This was based on the idea that the first life
forms were the outcome of a slow, multistep process that
began with the abiotic synthesis of organic compounds
and the formation of a ‘primitive soup’. There followed
the formation of colloidal gel-like systems, from which
anaerobic heterotrophs evolved that could take up sur-
rounding organic compounds and use them directly for
growth and reproduction.
Many of Oparin’s original ideas have been super-
seded, but his hypothesis provided a conceptual frame-
work for the development of this field. His proposal
became widely accepted, not only because it is simpler to
envision a heterotrophic organism originating from
organic molecules of abiotic origin rather than from an
autotroph, but also because laboratory experiments have
shown how easy it is to produce a number of biochem-
ical monomers under reducing conditions.
The first successful synthesis of organic compounds

under plausible primordial conditions was accom-
plished by the action of electrical discharges acting for a
week over a mixture of CH
4
, NH
3
, H
2
, and H
2
O; racemic
mixtures of several proteinic amino acids were pro-
duced, as well as hydroxy acids, urea, and other organic
molecules (Miller 1993). This was followed a few years
later by the demonstration of rapid adenine synthesis by
the aqueous polymerization of HCN. The potential role
of HCN as a precursor in prebiotic chemistry is further
supported by the discovery that the hydrolytic products
of its polymers include amino acids, purines, and orotic
acid (a biosynthetic precursor of uracil). A potential pre-
biotic route for the synthesis of cytosine in high yields
is provided by the reaction of cyanoacetylene with
urea, especially when the concentration of the latter is
increased by simulating the conditions of an evaporating
pond.
The ease with which amino acids, purines, and
pyrimidines can form by reactions in a simple vessel
strongly suggests that these molecules were components
of the prebiotic broth. They would have been associated
with many other compounds, such as urea and carb-

oxylic acids, sugars formed by the non-enzymatic con-
densation of formaldehyde, a wide variety of aliphatic
and aromatic hydrocarbons, alcohols, and branched and
straight fatty acids, including some which are mem-
brane-forming compounds. The list also includes several
highly reactive derivatives of HCN, such as cyanamide
(H
2
NCN) and its dimer (H
2
NC(NH)NH–CN), di-
cyanamide (NC–NH–CN), and cyanogen (NC–CN),
which are known to catalyse polymerization reactions.
Additional aspects of prebiotic chemistry have been
reviewed by Miller (1993), Deamer and Fleischaker
(1994), Chyba and McDonald (1995), and Brack (1998).
The synthesis of chemical constituents of contempor-
ary organisms by non-enzymatic processes under labor-
atory conditions does not necessarily imply that they
were either essential for the origin of life or available in
the primitive environment. However, the significance of
prebiotic simulation experiments is supported by the
occurrence of a large array of protein and non-protein
amino acids, carboxylic acids, purines, pyrimidines,
hydrocarbons, and other molecules in the 4.6Ga Murch-
ison meteorite (a carbonaceous chondrite which also
yields evidence of liquid water) (Miller 1993; Chyba and
McDonald 1995). The presence of these compounds in
the meteorite makes it plausible, but does not prove, that
a similar synthesis took place on the primitive Earth

—
or
is it simply a coincidence?
The evolutionary framework provided by Oparin’s
theory and methodology has allowed further develop-
ment and refinement without losing the overall structure
and internal coherence of his approach (Kamminga
1991). Several competing approaches to the study of the
origin of life coexist today, including proposals for RNA
or thioester worlds, for an extraterrestrial origin of the
primitive soup’s components, and for the role of sub-
marine hot springs as sites for prebiotic chemistry
(ChybaandMcDonald1995;deDuve1995).All,however,
are based on the assumption that abiotic organic com-
pounds were a necessary precursor to the appearance
of life.
Pyrite formation and the emergence of life
So far, the only serious rival to the heterotrophic theory
stems from the work of Wächtershäuser (1988). Accord-
4 1 Major Events in the History of Life
ing to this hypothesis, life began with the appearance
of an autocatalytic two-dimensional chemolithotrophic
metabolic system based on the formation of the highly
insoluble mineral pyrite. Synthesis and polymeriza-
tion of organic compounds took place on the surface
of FeS and FeS
2
in environments that resemble those
of deep-sea hydrothermal vents. Replication followed
the appearance of non-organismal iron sulphide-based

two-dimensional life, in which chemoautotrophic
carbon fixation took place by a reductive citric acid cycle,
or reverse Krebs cycle, of the type originally described
for the photosynthetic green sulphur bacterium Chloro-
bium limicola. Molecular phylogenetic trees show that
this mode of carbon fixation and its modifications (such
as the reductive acetyl-CoA or the reductive malonyl-
CoA pathways) are found in anaerobic archaebacteria
and the most deeply divergent eubacteria, which has
been interpreted as evidence of its primitive character
(Maden 1995). But is the reverse Krebs cycle truly
primordial?
The reaction FeS + H
2
S = FeS
2
+ H
2
is a very favourable
one. It has an irreversible, highly exergonic (energy liber-
ating) character with a standard free energy change
DG° =-9.23 kcal/mol, which corresponds to a reduction
potential E° =-620mV. Thus, the FeS/H
2
S combination
is a strong reducing agent, and has been shown to
provide an efficient source of electrons for the reduction
of organic compounds under mild conditions. Pyrite for-
mation can produce molecular hydrogen, and reduce
nitrate to ammonia, acetylene to ethylene, thioacetic acid

to acetic acid, as well as more complex synthesis (Maden
1995), including peptide-bonds that result from the
activation of amino acids with carbon monoxide and
(Ni, Fe)S (Huber and Wächtershäuser 1998). Although
pyrite-mediated CO
2
reduction to organic compounds
has not been achieved, the fixation under plausible pre-
biotic conditions of carbon monoxide into activated
acetic acid by a mixture of coprecipitated NiS/FeS has
been reported (cf. Huber and Wächtershäuser 1998).
However, in these experiments the reactions occur in an
aqueous environment to which powdered pyrite has
been added; they do not form a dense monolayer of ion-
ically bound molecules or take place on the surface of
pyrite.
None of the above experiments itself proves that both
enzymes and nucleic acids are the evolutionary outcome
of surface-bounded metabolism. In fact, the results are
also compatible with a more general, modified model of
the primitive soup in which pyrite formation is recog-
nized as an important source of electrons for the reduc-
tion of organic compounds. It is thus possible that under
certain geological conditions the FeS/H
2
S combination
could have reduced not only CO but also CO
2
released
from molten magma in deep-sea vents, leading to bio-

chemical monomers. Peptide synthesis, for instance,
could have taken place in an iron and nickel sulphide
system (Huber and Wächtershäuser 1998) involving
amino acids formed by electrical discharges via a Miller-
type synthesis. If the compounds synthesized by this
process do not remain bound to the pyrite surface, but
drift away into the surrounding aqueous environment,
then they would become part of the prebiotic soup, not
of a two-dimensional organism. Thus, the experimental
results achieved so far with the FeS/H
2
S combination
are consistent with a heterotrophic origin of life.
The essential question in deciding between these two
different theories is not whether pyrite-mediated
organic synthesis can occur, but whether direct CO
2
reduction and synthesis of organic compounds can be
achieved by a hypothetical two-dimensional living
systemthatlacks genetic information. Proof of Wächters-
häuser’s hypothesis requires the demonstration of not
only the tight coupling of the reactions necessary to
drive autocatalytic CO
2
assimilitation via a reductive
citric acid cycle, but also the interweaving of a network
of homologous cycles which, it is assumed, led to all the
anabolic pathways (Maden 1995).
Many original assumptions of the heterotrophic
theory have been challenged by our current understand-

ing of genetics, biochemistry, cell biology, and the basic
molecular processes of living organisms. The view advo-
cated here assumes that, even if the first living systems
were endowed with minimum synthetic abilities, their
maintenance and replication depended primarily on
prebiotically synthesized organic compounds. An
updated heterotrophic hypothesis assumes that the raw
material for assembling the first self-maintaining,
replicative chemical systems was the outcome of abiotic
synthesis, while the energy required to drive the chem-
ical reactions involved in growth and reproduction may
have been provided by cyanamide, thioesters, glycine
nitrile, or other high energy compounds (de Duve 1995;
Lazcano and Miller 1996). This modified version of the
classical theory of chemical evolution and primordial
heterotrophy can be examined experimentally, and can
be expected to generate additional lines of research.
Prebiotic chemistry and the ‘primitive soup’
Although it is generally agreed that free oxygen was
absent from the primitive Earth, there is no agreement on
the composition of the primitive atmosphere; opinions
vary from strongly reducing (CH
4
+ N
2
, NH
3
+ H
2
O, or

CO
2
+ H
2
+ N
2
) to neutral (CO
2
+ N
2
+ H
2
O). In general,
non-reducing atmospheric models are favoured by
atmospheric chemists, while prebiotic chemists lean
towards more reducing conditions, under which the
abiotic syntheses of amino acids, purines, pyrimidines,
and other compounds are very efficient.
The possibility that the primitive atmosphere was
1.1 Early Life 5
non-reducing does not create insurmountable problems,
since the primitive soup could still form. For instance,
geological sources of hydrogen, such as pyrite, may have
been available; in the presence of ferrous iron, a sulphide
ion (SH
-
) would have been converted to a disulphide ion
(S
2-
), thereby releasing molecular hydrogen (Maden

1995). It is also possible that the impacts of iron-rich
asteroids enhanced the reducing conditions, and that
cometary collisions created localized environments
favouring organic synthesis. Based on what is known
about prebiotic chemistry and meteorite composition, if
the primitive Earth was non-reducing, then the organic
compounds required must have been brought in by
interplanetary dust particles, comets, and meteorites.
Recent measurements suggest that a significant percent-
age of meteoritic amino acids and nucleobases could
survive the high temperatures associated with frictional
heating during atmospheric entry, and become part of
the primitive broth (Glavin and Bada 1999).
This eclectic view, in which the prebiotic soup is
formed by contributions from endogenous syntheses,
extraterrestrial organic compounds delivered by comets
and meteorites, and pyrite-mediated CO reduction, does
not contradict the heterotrophic theory. Even if the ulti-
mate source of the organic molecules required for the
origin of life turns out to be comets and meteorites,
recognition of their extraterrestrial origin is not a
rehabilitation of panspermia (the hypothesis that life
existed elsewhere in the universe and had been trans-
ferred from planet to planet, eventually gaining a
foothold on Earth), but an acknowledgement of the
role of collisions in shaping the primitive terrestrial
environment.
The search for the primordial genetic polymers
There is no evidence of abiotically produced oligopep-
tides or oligonucleotides in the Murchison meteorite, but

condensation reactions clearly took place in the primi-
tive Earth. Synonymous terms like ‘primitive soup’, ‘pri-
mordial broth’, or ‘Darwin’s warm little pond’ have led
in some cases to major misunderstandings, including
the simplistic image of a worldwide ocean, rich in self-
replicating molecules and accompanied by all sorts of
biochemical monomers. The term ‘warm little pond’,
which has long been used for convenience, refers not
necessarily to the entire ocean, but to parts of the hydro-
sphere where the accumulation and interaction of the
products of prebiotic synthesis may have taken place.
These include not only membrane-bound systems,
but also oceanic sediments, intertidal zones, shallow
ponds, freshwater lakes, lagoons undergoing wet-and-
dry cycles, and eutectic environments (e.g. glacial
ponds), where evaporation or other physicochemical
mechanisms (such as the adherence of biochemical
monomers to active surfaces) could have raised local
concentrations and promoted polymerization.
It is difficult to estimate the rate of self-organization of
these polymers into replicating systems, because the
chemical steps are unknown. Whatever the time scale
required for the appearance of an informational
polymer, once formed it must have persisted at least long
enough to allow its replication. If polymers formed by a
slow addition of monomers, this process must have been
rapid compared to rates of hydrolysis, especially if a con-
siderable amount of genetic information was contained
in the polymer. Self-replicating systems capable of
undergoing Darwinian evolution must have emerged in

a period shorter than the destruction rates of their com-
ponents; even if the backbone of primitive genetic poly-
mers was highly stable, the nitrogen bases themselves
would decompose over long periods of time. In fact, the
accumulation of all components of the primitive soup
will be limited by destructive processes, including the
pyrolysis of organic compounds in submarine vents.
Large amounts of the entire Earth’s oceans circulate
through the ridge crests every 10 million years, facing
temperatures of 350°C or more, and placing an upper
limit to the time available for the origin and early
diversification of life (Lazcano and Miller 1996).
The popular idea of a hot origin of life is founded
largely on the basal position in molecular phylogenies of
hyperthermophiles, which exhibit optimal growth tem-
peratures of 80–110°C. Although this hypothesis is also
consistent with the emergence of life on a turbulent, hot
primitive Earth (which may or may not be true), it is
merely an extrapolation of the growth temperature of
extant thermophilic prokaryotes. Since most biochem-
icals decomposerather rapidly at temperatures of 100°C,
prebiotic chemistry clearly supports a low-temperature
origin of life. A high-temperature origin, under con-
ditions such as those found in deep-sea vents, may
be possible, but chemical stability arguments rule out
any involvement of the purines, pyrimidines, sugar-
phosphate backbone, or even most of the 20 amino acids
used by life today: under such extreme conditions, their
half-lives are a few seconds. Any theory arguing other-
wise must explain not only how life originated under

such conditions, but also how the evolutionary trans-
ition occurred from the hypothetical high temperature-
resistant origin to extant biochemistry.
Bridge(s) to the RNA world
The primitive broth must have been a bewildering
organic chemical wonderland in which a wide array of
different molecules were constantly synthesized,
destroyed, or incorporated into cycles of chemical trans-
formations. Regardless of the complexity of the prebiotic
environment, life could not have evolved in the absence
6 1 Major Events in the History of Life
of a genetic replicating mechanism to guarantee the
maintenance, stability, and diversification of its basic
components under the action of natural selection.
The nature of the system that preceded the ubiquitous
DNA-based genetic machinery of extant living systems
is unknown, but it must have been endowed with some
capacity for self-replication. There is experimental evid-
ence for self-replication in some chemical systems which
lack the familiar nucleic acid-like structure. These
include replicative micelles and vesicles, and self-
complementary molecules which result from the chem-
ical reaction between an amino-adenosine derivative
and a complex aromatic ester (cf. Orgel 1992). There are
also prions, the infamous infectious agents associated
with bovine spongiform encephalopathy (BSE, or ‘mad
cow disease’) and several human neurodegenerative
maladies, which may represent a case of phenotypic in-
heritance that propagate by changing the harmless
conformation of a normal protein into an infectious

isoform.
Although the above examples suggest that replication
may be a widespread phenomenon, these systems do not
exhibit heritability, i.e. they are considered autocatalytic
but non-informational (Orgel 1992). Hence, they are
probably not related to the origin of life. On the other
hand, although the properties of RNA molecules make
them an extremely attractive model for the origin of life,
their existence in the prebiotic environment is unlikely. It
is not clear that phosphate esters could have been
involved in the first genetic material, and the self-
condensation of formaldehyde (i.e. the formose reaction,
which appears to be the only plausible route for the pre-
biotic synthesis of sugars) leads to a complex array of
carbohydrates, of which ribose is a minor unstable com-
ponent. Without phosphate and ribose, RNA molecules
could not have formed in the primitive soup. Thus, it is
possible that the RNA world itself was the end product
of ancient metabolic pathways that evolved in unknown
pre-RNA worlds, in which informational macromolec-
ules with different backbones may have been endowed
with catalytic activity, i.e. with phenotype and genotype
also residing in the same molecules, so that the synthesis
of neither protein nor related catalysts is necessary
(Lazcano and Miller 1996).
The chemical nature of the first genetic polymers and
the catalytic agents that may have formed the pre-RNA
worlds are completely unknown and can only be sur-
mised. Modified nucleic acid backbones have been syn-
thesized, which either incorporate a different version of

ribose or lack it altogether. Experiments on nucleic acid
with hexoses instead of pentoses, and on pyranoses
instead of furanose (Eschenmoser 1994), suggest that a
wide variety of informational polymers is possible, even
when restricted to sugar-phosphate backbones.
One possibility that has not been explored is that the
backbone of the original informational macromolecules
may have been atactic (e.g. disordered) kerogen-like
polymers such as those formed in some prebiotic simula-
tions. There are other possible substitutes for ribose,
including open chain, flexible molecules that lack asym-
metric carbons. One of the most interesting chemical
models for a possible precursor to RNA involves the
so-called peptide nucleic acids (PNAs), which have a
protein-like backbone of achiral 2-amino-ethyl-glycine,
to which nucleic acid bases are attached by an acetic acid
(Nielsen 1993). Such molecules form very stable com-
plementary duplexes, both with themselves and with
nucleic acids. Although they lack ribose, their functional
groups are basically the same as in RNA, so they may
also be endowed with catalytic activity.
Although the identification of adenine, guanine, and
uracil in the Murchison meteorite supports the idea that
these bases were present in the primitive environment
(Miller 1993; Chyba and McDonald 1995), it is prob-
able that other heterocycles capable of forming non-
standard hydrogen bonding were also available.
The Watson–Crick base-pair geometry permits more
than the four usual nucleobases, and simpler genetic
polymers may not only have lacked the sugar-phosphate

backbones, but may also have depended on alternative
non-standard hydrogen bonding patterns. The search
for experimental models of pre-RNA polymers will be
rewarding but difficult; it requires the identification of
potentially prebiotic components and the demonstration
of their non-enzymatic template-dependent polymeriza-
tion, as well as coherent descriptions of how they may
have catalysed the transition to an RNAworld.
Questions for future research
Even though considerable progress has been made in
understanding the emergence and early evolution of life,
major uncertainties remain. The chemistry of some pre-
biotic simulations is robust and supported by meteorite
analyses, but the gap between these rudimentary experi-
ments and the simplest extant cell is enormous. There is
a range of issues relevant to the origin of life, many of
which cut across different scientific fields. The geo-
chemical environments under which prebiotic syntheses
of biochemical monomers and their polymers could
have taken place need to be characterized. Experimental
systems to study polymer replication, sequestration of
organic compounds, the energy sources that may
have been employed by the first replicating systems, and
the appearance of metabolic pathways all need to be
developed.
The origin of the main features of the genetic code
is not understood, but the discovery of the catalytic
activity of RNAmolecules and the development of novel
RNA enzymes through in vitro evolution has given
1.1 Early Life 7

considerable support to the idea that the primitive
translation apparatus may have been shaped, at least in
part, by interactions between amino acids of prebiotic
origin and polyribonucleotides (Gesteland et al. 1999).
If the current interpretation of the evolutionary
significance of these and other properties of RNA mol-
ecules is correct, then one of the central issues that
origin-of-life research must confront is the understand-
ing of the processes that led from the primitive soup into
RNA-based life forms. The search for simple organic
replicating polymers will play a central role in this
inquiry. Even if the appearance of life remains an elusive
issue, redefining the questions that need to be addressed
to understand how it took place is, in itself, an encourag-
ing scientific achievement.
References
Brack, A., ed. (1998) The molecular origins of life: assembling
pieces of the puzzle. Cambridge University Press, New
York.
Chyba, C.F. and McDonald, G.D. (1995) The origin of life in
the Solar System: current issues. Annual Review of Earth and
Planetary Sciences 23, 215–249.
Deamer, D.W. and Fleischaker, G.R., eds. (1994) Origins of life:
the central concepts. Jones and Bartlett, Boston.
de Duve, C. (1995) Vital dust: life as a cosmic imperative. Basic
Books, New York.
Eschenmoser, A. (1994) Chemistry of potentially prebiological
natural products. Origins of Life and Evolution of the Biosphere
24, 389–423.
Gesteland, R.F., Cech, T. and Atkins, J.F., eds. (1999) The RNA

World II. CSHLPress, Cold Spring Harbor.
Glavin, D.P. and Bada, J.L. (1999) The sublimation and sur-
vival of amino acids and nucleobases in the Murchison
meteorite during a simulated atmospheric entry heating
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the Origin of Life (11–16 July 1999), p. 108. San Diego,
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Huber, C. and Wächtershäuser, G. (1998) Peptides by activation
of amino acids with CO on (Ni, Fe) S surfaces and implica-
tions for the origin of life. Science 281, 670–672.
Kamminga, H. (1991) The origin of life on Earth: theory, history,
and method. Uroboros 1, 95–110.
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tion of life: prebiotic chemistry, the pre-RNAworld, and time.
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Maden, B.E.H. (1995) No soup for starters? Autotrophy and
origins of metabolism. Trends in Biochemical Sciences 20,
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Miller, S.L. (1993) The prebiotic synthesis of organic
compounds on the early Earth. In: M.H. Engel and S.A.
Macko, eds. Organic geochemistry, pp. 625–637. Plenum Press,
New York.
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Nutman, A.P. and Friend, C.R.L. (1996) Evidence for life
before 3,800 million years ago. Nature 384, 55–59.
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452–484.
1.1.2 Exploring for a Fossil Record of
Extraterrestrial Life
J.D. FARMER
Introduction
While speculation about the possibility of life elsewhere
in the Cosmos has been a persistent theme throughout
the history of humankind, the last decade of the twen-
tieth century has witnessed a number of important
advances in our understanding of the nature and evolu-
tion of terrestrial life. These developments have opened
up important new possibilities for the existence of living
systems elsewhere in the Solar System (or beyond) and
have spawned a new interdisciplinary science called
‘astrobiology’
—
the study of the origin, evolution, distri-
bution, and destiny of life in the Cosmos. This new disci-
pline embraces the traditional field of exobiology, which
focuses on the origin of life and early biosphere evolu-
tion, along with a newer sister discipline, exopalaeontol-
ogy, which seeks evidence for a fossil record of ancient
life or prebiotic chemistry in extraterrestrial materials, or
from other planets in the Solar System.
An important legacy of the Apollo space missions
was the development of a detailed cratering history for

the moon. This led to the view that during early accre-
tion, prior to ª4.4Ga, surface conditions on Earth were
unfavourable for the origin of life (Chang 1994). As a
consequence of frequent giant impacts, magma oceans
could have been widespread over the Earth’s surface,
and volatile compounds, including water and the bio-
genic elements needed for life’s origin, would have
been lost to space. Models of early accretion suggest
that during the interval 4.4–4.2Ga impact rates and
object sizes declined to a point where the water (and
associated organics) delivered to the Earth by volatile-
rich impactors (e.g. comets) was retained. A stable
atmosphere and oceans probably developed during
this time, providing the first suitable environments for
prebiotic chemical evolution and the origin of life.
8 1 Major Events in the History of Life
Models also suggest that early biosphere development
overlapped with one or more late, giant impacts that
were large enough to volatilize the oceans and perhaps
sterilize surface environments (Sleep et al. 1989). Such
events would have frustrated the development of the
early biosphere and may have even required that life
originate more than once. The most protected habitat
during this early period would have been the deep
subsurface.
Discoveries of ª3.45Ga cellular microfossils from
cherts in volcanic sequences in Western Australia
(Schopf 1993), and possible 3.86Ga chemofossils (carbon
isotopic signatures) from phosphate-rich metasediments
in Greenland (Mojzsis et al. 1996) indicate that once the

conditions necessary for life’s origin were in place, life
arose very quickly, perhaps in a few hundred million
years or less. This observation significantly improves the
possibility that life originated on Mars, or elsewhere in
the Solar System where habitable zones of liquid surface
water were more ephemeral features of early planetary
evolution.
While recent discoveries in Precambrian palaeontol-
ogy have pushed back the dates for the oldest fossils,
molecular phylogenies have also provided important
clues about the origin and early evolution of life on
Earth, based on the historical record preserved in the
genomes of living organisms. Comparisons of genetic
sequences in 16S ribosomal RNA indicate that terrestrial
life is subdivided into three major domains: the Archaea,
the Bacteria, and the Eukarya. It is also apparent that
the vast proportion of biodiversity on Earth is microbial.
Higher forms of multicellular life appeared quite late
in Earth history and make up only a tiny fraction of
the total number of species. The deepest branching
lineages in the RNA tree are high-temperature forms
that utilize reduced inorganic substrates, like sulphur
or hydrogen. This suggests that the last common
ancestor of life on Earth was a high-temperature (‘ther-
mophilic’) chemotroph, a view that is consistent with the
higher rates of heat flow, volcanism, and frequent
impacts that prevailed on the early Earth. However, the
RNA tree may reveal little about life’s origin (see Section
1.1.1). The thermophilic properties of the most deeply
rooted lineages may simply be a legacy of late giant

impacts that eliminated all but the highest temperature
species.
Possible extant life on Mars and Europa
The discovery of an extensive subsurface biosphere on
the Earth opened up exciting new possibilities for the
existence of habitable zones elsewhere in the Solar
System. On Earth, subsurface habitats harbour many
species that are capable of synthesizing organic mol-
ecules from simple inorganic substrates. The subsurface
is the most compelling environment for extant Martian
life because of the possibility that a deep subsurface
ground water system may exist at several kilometres
depth (Carr 1996). In addition, results from the Galileo
mission provide support for the existence of a subsurface
ocean beneath the crust of Europa, one of Jupiter’s
moons. It is postulated that heating of the moon’s inter-
ior by tidal friction could sustain a subcrustal ocean of
liquid water, and sea floor hydrothermal systems (Belton
et al. 1996). Indeed, the complexly fractured and largely
uncratered surface of Europa (Fig. 1.1.2.1) indicates an
active ice ‘tectonics’ involving the periodic upflow of ice-
brines from beneath the Europan crust. It is possible that
where water welled up from below, it carried life forms
or prebiotic chemistry from the underlying ocean and
incorporated these materials into surface ices. Terrestrial
microbes are known to retain viability at subzero tem-
peratures by exploiting thin films of brine on grain
surfaces in permafrost soils. Could viable organisms
be present within similar ice-brine environments on
Europa? Viability arguments aside, ice could also

provide a means for the prolonged cryopreservation of
organic materials, accessible to robotic landers.
Exploring for an ancient Martian biosphere
The Viking lander missions showed the present surface
environment of Mars to be unfavourable for life due to
the absence of liquid water, intense UV radiation, and
oxidizing soils. At the same time, images obtained from
Mars orbit revealed the early planet to be more Earth-
like, with a broad range of surface environments suitable
for life. It is likely that habitable environments disap-
peared from the surface ª3.8Ga as Mars began to lose
its atmosphere (Farmer and Des Marais 1999). If extant
life exists on Mars today, it is likely to be in deep subsur-
face environments that will be inaccessible to robotic
platforms. Deep subsurface drilling will likely require
a human presence. However, if life once existed in
surface environments, it is likely to have left behind a
fossil record in ancient sediments now exposed at the
surface. Such deposits could be accessed during the
robotic phase of exploration. This simple concept under-
lies the basic rationale of the present Mars exploration
programme.
Studies of the Precambrian fossil record on Earth, and
of modern microbial systems that are analogues for
those on the early Earth and Mars, provide a conceptual
framework for guiding the search for a fossil record on
Mars. An understanding of how preservation varies
between different groups of microorganisms over
extremes of the environment, and how postdepositional,
diagenetic changes affect the long-term preservation of

microbial biosignatures in rocks, is crucial (Farmer and
Des Marais 1999). Such studies allow the formulation of
1.1 Early Life 9
‘rules’ of preservation that help optimize strategies to
explore for past life on Mars and other planetary bodies,
such as Europa.
As with Earth-based palaeontology, site selection is
crucial for the successful implementation of Mars mis-
sions designed to explore for past life. Preservation is
a selective process that is strongly dependent upon
the biogeological environment. Studies of microbial
fossilization reveal that the rapid entombment of
microorganisms and their by-products by fine-grained,
clay-rich sediments and/or chemical precipitates is
of singular importance in enhancing preservation.
Favourable geological environments are those where
microbial systems coexist with high rates of fine-grained
detrital sedimentation, and/or aqueous mineral pre-
cipitation. Examples include rapidly mineralizing
hydrothermal systems (below the upper temperature
limit for life), terminal lake basins (where chemical
sediments such as evaporites, fine-grained lacustrine
sediments, and sublacustrine cold spring tufas are
deposited), and mineralizing soils (e.g. hard-pans,
including calcretes, ferracretes, and silcretes). Even if life
did not develop on Mars, this exploration strategy is still
important because the same sedimentary environments
could preserve a record of prebiotic chemistry similar to
that which spawned the development of life on Earth.
This early prebiotic history has been lost from the terres-

trial record.
Mars may preserve the most complete record of early
events of planetary evolution anywhere in the Solar
System. The 4.56Ga age of Martian meteorite ALH 84001
(McKay et al. 1996) indicates that the ancient, heavily
cratered highlands of Mars contain a crustal record
extending back to the earliest period of planetary evolu-
tion. On Earth, comparably aged crustal sequences have
been destroyed by tectonic cycling, metamorphism,
weathering, and erosion. In contrast, Mars never
developed a plate tectonic cycle and extensive water-
mediated weathering and erosion was probably limited
to the first billion years or so of the planet’s history.
Geomorphic features suggest that surface hydrological
systems were active until near the end of heavy bom-
bardment (ª3.8Ga), after which time liquid water
quickly disappeared from the surface, presumably as a
result of the loss of the Martian atmosphere (Carr 1996).
The preservation of fossil biosignatures is favoured
when organisms or their by-products are incorporated
into low permeability sedimentary deposits (producing
a closed chemical system during diagenesis) of stable
mineralogy (promoting a prolonged residence time in
the crust). Chemical sediments composed of silica, phos-
phate, and carbonate, along with fine-grained, clay-rich
detrital sediments and water-deposited volcanic ash,
are especially favourable lithologies for long-term
preservation. This is illustrated by the fact that on Earth
most of the Precambrian record is preserved in such
lithologies.

Many potential sites for a fossil record have been
10 1 Major Events in the History of Life
Fig. 1.1.2.1 (a) Galileo orbiter image of the surface of Europa,
one of the moons of Jupiter. The surface crust is composed of
water ice that has been fractured into irregular blocks. The
fracture patterns suggest that the crust was mobilized by a
layer of subsurface water which flowed up from below, filling
fractures between blocks as they separated. Such observations
support the view that Europa once had, and perhaps still has, a
subcrustal ocean of liquid water that could sustain life or
prebiotic chemistry. The smallest features visible in this image
are about 20m across. (b) Close-up of the surface of Europa
showing a complex network of ridged fractures originally
formed when plates of ice crust pulled apart. Many ridge
segments were later offset along strike–slip faults. The large
ridge in the lower right corner of the image is about 1km
across. (Photographs by courtesy of NASA.)
identified on Mars using orbital photographs obtained
by Viking (e.g. Fig. 1.1.2.2). However, information about
the mineralogical composition of the Martian surface is
still lacking. Mineralogy provides important clues about
the palaeoenvironment
,
information needed to deter-
mine the best sites for detailed surface exploration. An
important exploration goal is to identify aqueous
mineral assemblages (of the types that commonly
capture and preserve fossil biosignatures) from orbit
using spectral mapping methods prior to landed mis-
sions. In targeting sites for sample return, evaporative

lake basins and hydrothermal sites are given a high pri-
ority. In terrestrial settings, the deposits formed in these
environments frequently provide optimal conditions for
preservation.
Putative signs of life in a Martian meteorite
The report of possible fossil signatures in Martian met-
eorite Allan Hills 84001 (McKay et al. 1996) generated an
intense, ongoing debate over the usefulness of a variety
of morphological, mineralogical, and geochemical data
for detecting biosignatures in ancient rocks. Subsequent
work by the broader scientific community indicates that
the major lines of evidence used to support the biological
hypothesis for ALH 84001 are more easily explained by
inorganic processes.
Polycyclic aromatic hydrocarbons (PAHs), such as
those found in ALH 84001, are not generally regarded as
being diagnostic of life. In addition, it has been shown
that a major fraction of the organic matter present in the
meteorite exhibits radiocarbon activity, indicating that it
originated through terrestrial contamination after reach-
ing the Earth (Jull et al. 1998). Although a small fraction
of remaining organic matter could be Martian, it has not
yet been characterized.
A key test of the biological hypothesis for ALH 84001
is the formation temperature of the carbonates that
1.1 Early Life 11
Fig. 1.1.2.2 (a) Gusev Crater, Mars. Alarge river canyon to the
south (Ma’adim Vallis) drained into this ª150km diameter
crater, depositing a delta where it entered the crater. Geological
studies suggest a prolonged hydrological history for this

region of Mars, with the Gusev Crater being the site of an
ancient palaeolake system. (b) The slopes of Hadriaca Patera,
an ancient Martian volcano, show channels radiating
downslope, away from the caldera rim (caldera ª75km
across). These small channels are interpreted to be the result of
pyroclastic flows, the channels being subsequently enlarged by
sapping flow. The basal slope of Hadriaca Patera was later
eroded by outfloods of subsurface water which carved Dao
Vallis, a large channel located near the bottom of the
photograph (channel ª45km wide). The association of
subsurface water and a heat source (the subsurface magma
that produced the volcano) suggests the potential for sustained
hydrothermal activity in this region. (Photographs by courtesy
of NASA.)