Table Of ContentYoung Sun, Early Earth
and the Origins of Life
M. Gargaud · H. Martin · P. López-García
T. Montmerle · R. Pascal
Young Sun, Early Earth
and the Origins of Life
Lessons for Astrobiology
Translated by Storm Dunlop
123
Muriel Gargaud Thierry Montmerle
CNRS-Université Bordeaux 1 Institut d’Astrophysique de Paris,
Laboratoire d’Astrophysique de Bordeaux Paris, France
Bordeaux, France
Robert Pascal
Hervé Martin CNRS–Université de Montpellier 2
Université Blaise Pascal Institut des Biomolécules Max Mousseron
Laboratoire Magmas et Volcans Montpellier, France
Clermont-Ferrand, France
Purificación López-García
CNRS–Université Paris-Sud
Unité d’Ecologie, Systématique et Evolution
Paris, France
ISBN 978-3-642-22551-2 e-ISBN 978-3-642-22552-9 (eBook)
DOI 10.1007/978-3-642-22552-9
Springer Heidelberg Dordrecht London New York
Library of Congress Control Number: 2012955750
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Introduction
Questions relating to the origin of life on Earth, and its possible presence elsewhere in
the Universe, have fascinated Man since antiquity, whether as a man of science, a phi-
losopher, or quite simply as a man-in-the-street. Understanding how life appeared on Earth
would be the culmination of a form of a very ancient quest for our origins. Such a culmina-
tion would also be a decisive advance in our knowledge of an extremely complex natural
process, the sequence of which raises numerous questions. It is these questions, which are
fascinating in themselves, and frequently still without any reply, that are at the heart of the
current work.
To this day, we know of one single example of life: that of life on Earth. Defining life
and its essential properties is not a simple task either for a biologist, who studies life, or for
an epistemologist, as someone who seeks to understand the way in which Mankind forms
a concept of the world around it. We are able, however, to recognize life as a state of com-
plex matter that is evolving in a dynamical context. And as such, life does not, therefore,
escape the laws of physics and chemistry. Far from it. It is based on physical and chemical
mechanisms that take place in a specific geological and atmospheric environment: that of
our planet. So, life is the result of a natural process, and that implies that it is possible that
other life forms have appeared elsewhere in the universe, based on analogous physical and
chemical foundations, yet must always be constrained by the universal laws of nature.
Exploring When and How Life Emerged
Is it really possible that the life that we know today on Earth resulted from a unique combi-
nation of circumstances in the universe? What was the relative significance of determinism
(predictable events obeying natural laws), chance, and contingency (not predictable, his-
torical sequences of events) in the emergence of living beings? Is life the result of a gradual
and continual increase in complexity, or was there, at one specific moment, a sudden jump
in complexity leading to the emergence of new properties – for example, through a combi-
nation of several elements that interacted? All these questions currently remain open, but it
is very probable that examination of the scenarios capable of explaining the emergence of
life on Earth could progressively provide us with some of the factors in the answer. But for
all that, both the spirit and the approach of scientists who are interested in this problem are
extremely varied, depending on the disciplines involved.
So, for example, astrophysicists seek to know if other objects in the Solar System could
shelter life, or even if there exist, beyond the Solar System, “other Earths” or, at least, other
“habitable” planets. They expect that the study of terrestrial life and its origins will reveal
the conditions that were necessary for its development, and they focus their research on
extraterrestrial objects where similar conditions are likely to be combined – such objects
then being considered as potentially inhabitable. Chemists, however, try to understand
the process of self-organization and the establishment of sequences of reactions leading to
systems capable of evolving in the same way as living beings. They thus try to determine,
VI Introduction
how, on Earth, the passage from abiotic organic chemistry to biochemistry could have taken
place. Geologists are interested in the history of the planet and the impact of life on its evolu-
tion, but, above all, they try to define as precisely as possible the environmental conditions
that prevailed when life emerged and that favored its development. Finally, biologists seek to
know how biological evolution started, such that it gave birth to an extraordinary diversity of
organisms, with very different forms, sizes, and abilities, but which all possess common prop-
erties. Exploring how and when – two crucial questions – life emerged in this small corner of
the universe that is the planet Earth, concerns any one of these just as much as any other, but
the scientists are only able to answer within the limits of the field in which they are involved.
As regards the question “When?”, we shall see that the first difficulty is linked with the
question of time, that fourth dimension which is difficult to grasp, and where scientists of
the different disciplines need to establish common conventions if they wish to speak of the
same thing. In fact, time is measured differently by astrophysicists, for whom time advances
in an absolute manner from an initial instant of reference (t) – which corresponds in this
0
book to the start of the formation of the Sun, 4.57 billion years ago (4.57 Ga) – and by geolo-
gists and biologists, who measure time backwards, relative to the present. This characteristic
is very real in this book where, for a given event, we progress from an absolute time scale
expressed in billions of years running towards the present day from the reference point, t
0
(as will be the case in the early chapters, relating to the formation of the Solar System), to
a relative scale expressed in billions of years, towards the past (or “before present”, BP, and
thus, by convention, before the year 1950) when, in our story, geology and then biology take
over from astrophysics. Chemists are disoriented and worried by these notions of long time-
scales, whether relative or absolute. In fact, for them, rather than the chronological moment
when they occur, the important factor is the kinetics and thus the relative duration of chemi-
cal reactions which, to add to the difficulty, cannot be understood except in statistical terms
(that is to say, over populations of molecules, in contrast to the individual random fate of a
single molecule). Be that as it may, all these disciplines need to know to which common time
scale they are referring when they attempt to reply to the question of “when did life appear?”.
We shall see in this book that, although this question remains without any precise answer,
it is still possible to define a range of time during which the transition from inert to living
matter occurred.
Replying to the question “How?” is more difficult and controversial. We shall never be
able to obtain a definitive answer, because life is a historical (contingent) process, in other
words it has evolved in an irreversible fashion over the course of time. At best we may hope
to reconstruct a plausible scenario, compatible with the laws of physics as well as with the ex-
perimental data, and present-day and future observations. To this day there is no consensus
about this problem – far from it! – and we sorely lack reliable, realistic data on the physical
and chemical conditions that prevailed on the primitive Earth where life emerged. Conse-
quently, numerous hypotheses, often mutually exclusive, have been suggested by researchers.
Some of these, even if they do allow an explanation of the observations, could never be tested.
Others, in contrast, are susceptible to being refuted one day if they do not agree with the con-
stantly increasing body of observable data. Apart from these “structural” difficulties, there is a
human factor, as shown by the fact that there exist opposing schools of thought, which some-
times rather dogmatically refuse to consider and analyze in detail any arguments that are not
their own. However, it is essential to remain optimistic: such a situation tends to disappear as
and when new, more reliable, data are acquired, and research into alternative pathways, often
intermediate ones, enables more concrete scenarios to be proposed. We have chosen to give a
broad and as neutral as possible view of these different models, preferring to put the emphasis
on the existing data rather than to interpret them in a partisan manner.
Introduction VII
A Novel Challenge: Getting Several Disciplines to Talk to One Another
The aim of this book is to present, in a chronological manner – or, at least, logically as a relative succession of
events – the history of the origins of life on Earth and the conditions that allowed it to appear on our planet.
The novel challenge is that for each of the time periods that form this chronology, we, as different specialists,
will speak together to lift a corner of the veil; with the approach and questions appropriate for each original
discipline. The image of the questions that it presents is therefore firmly multi-disciplinary. Astrophysics and
geology will thus allow us to reconstruct the history of the formation of the Sun, of the Solar System, and of
the Earth. Geology and chemistry, subject to certain constraints derived from observation that biology will
impose, will deal with the occurrence of conditions required for complex chemistry and life to appear. Finally,
biology will enable us to sketch the main features of evolution, and in particular to discuss the emergence of
eukaryotic cells and their diversification, until the appearance of animals and terrestrial plants, which form
the greatest part of the world visible to the human eye.
We have decided to stop this great tale at the Cambrian explosion, 540 million years ago, when the ances-
tors of the major animal lines that we see today made their appearance. At that time, biological evolution had
been in progress for over 2 or even 3 billion years, and the multiplicity of directions that it was to follow subse-
quently – including the appearance of humans within a small phylogenetic line of descent among hundreds of
others – is of lesser importance for our understanding of the origins and evolution of primordial life on Earth.
Lessons for Astrobiology?
The present book is based on a translation of the French original “Le Soleil, la Terre... la Vie” (the Sun, the
Earth... Life), published in 2009 by Editions Belin (Paris). At the request of Springer, we have completed the
original version by a new chapter on “Extrasolar planets”, i.e., the hundreds of planets and planetary systems
discovered since 1995 around normal stars other than the Sun. To stick to the original spirit of the book, we
have put emphasis on the fascinating question of a particular sub-class called “habitable planets”.
As the reader will see, the state-of-the-art in this field is still entirely astronomical, with no indication
whatsoever of any “biological” evidence. In this context, is it really justified to use the terms “exobiology”, or
“astrobiology”, which stricto sensu should etymologically mean “extraterrestrial biology” and “biology applied
to astronomy”, respectively? In his Preface to the Springer “Encyclopedia of Astrobiology” (2011), C. De Duve
(awarded the Nobel-Prize for Physiology and Medicine in 1974), speaks of “the new discipline of exobiol-
ogy-cum-bioastronomy-cum-astrobiology”, implying that these three commonly found denominations are
equivalent. In the context of the present book, however, we conclude that so far no evidence for life has been
found elsewhere than on the Earth: neither in the Solar System, nor on planets around other stars – even if
we suspect, and hope, that this evidence will come in the future. At this stage, astronomers, for their part,
tend to support the term “bioastronomy”* (meaning, astronomy applied to the search for life in the universe,
as “biophysics” or “biochemistry” etymologically are the fields of physics or chemistry applied to biological
phenomena) as appropriate term to use for now.
But of course life is all about biology (the science of life!), so it is fair to say that all the disciplines com-
bined in this book, to describe how we think life emerged on Earth, indeed can be considered as providing
“Lessons for astrobiology” (the subtitle of this book), in the sense that the authors hope it can contribute to
laying ground for the future – if, and when, a “biology” will be discovered in another world than the Earth.
* Indeed, a commission of the International Astronomical Union is called by this name (http://www.iau.org/science/scientific_bodies/
commissions/51/).
Acknowledgements
This book is the result of a vast collective collaboration, in which more researchers have
participated than the five authors mentioned on the cover.
The story started with a specialized CNRS* school organized in 2003 at Propriano (in
Corsica, France) by Muriel Gargaud, and which was followed by two workshops organized in
2004 at Château Monlot-Capet at Saint-Émilion (in the Gironde) and at Château d’Abbadia
(Académie des Sciences) at Hendaye (in the Pyrénées-Atlantiques). The aim of these meet-
ings – financially underwritten by the Centre national de la recherche scientifique (CNRS),
the Centre national d’études spatiales (CNES)**, the Conseil régional d’Aquitaine, the Uni-
versité Bordeaux 1 and the Laboratoire d’astrophysique de Bordeaux – was to establish and
discuss the chronology of the events that led to the appearance of life on Earth, between
the formation of the Solar System, 4.57 billion years ago and the Cambrian explosion of
lifeforms, 540 million years ago.
The work was put into concrete form in nine scientific articles in a special issue of the
journal Earth, Moon and Planets (No. 98), entitled “From Suns to Life: a chronological ap-
proach to the origins of life on Earth”, which appeared in 2006. It is those articles that form
the basis for this work.
Twenty authors participated in writing the articles, and without them, this book would
not exist. This is why we owe our warmest thanks to: Francis Albarède (École normale su-
périeure de Lyon), Jean-Charles Augereau (Laboratoire d’astrophysique de Grenoble), Lau-
rent Boiteau (Département de chimie, Université de Montpellier), Marc Chaussidon (Centre
de recherches pétrographiques et géochimiques, Nancy), Philippe Claeys (Vrije Universiteit
Brussel, Belgium), Didier Despois (Laboratoire d’astrophysique de Bordeaux), Emmanuel
Douzery (Institut des science d’évolution, Université de Montpellier), Patrick Forterre,
(Institut de génétique et microbiologie, Université Paris-Sud, Orsay), Matthieu Gounelle
(Muséum national d’histoire naturelle, Paris), Antonion Lazcano (Universidad Nacional
Autónoma de México, Mexico), Bernard Marty (École nationale supérieure de géologie,
Nancy), Marie-Christine Maurel (Institut Jacques-Monod, Université Paris 6), Alessandro
Morbidelli (Observatoire de la Côte d’Azur, Nice), David Moreira (Unité d’écologie, systé-
matique et évolution, Université Paris-Sud, Orsay), Juli Peretó (Insitut Cavanilles de Bio-
diversiat i Biologia Evolutiva, Universitat de València, Spain), Daniele Pinti (Université du
Québec à Montréal, Canada), Daniel Prieur (Laboratoire de microbiologie des environne-
ments extrêmes, Université de Bretagne occidentale, Brest), Jacques Reisse (Université libre
de Bruxelles, Belgium), Franck Selsis (Laboratoire d’astrophysique de Bordeaux), et Mark
Van Zuilen (Centre for Geobiology, Bergen University, Norway).
Finally, from its inception to the present, this project has benefited from the unfailing
and enthusiastic support of Michel Viso, Director of the “Exobiology” programme at the
Centre national d’études spatiales. We give him our warmest thanks.
Our deepest gratitude also goes to our editor at Editions Belin, whose role has been es-
sential and who, by bringing the various sections together has been the true orchestrator of
this work.
* CNRS = “Centre National de la Recherche Scientifique”, the French national research agency.
** CNES is the French space agency.
Contents
Chapter 1 The Formation of the Sun and Planets . . . . . . . . . . . . . . . . . . . . 1
The Sun’s Protoplanetary Infancy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
From Disks to Planets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Chapter 2 Formation and Early Infancy of the Earth . . . . . . . . . . . . . . . . 37
The Rapid Differentiation of a Metallic Nucleus: The Core . . . . . . . . . . . . . . . . . . . . . . . . 40
Opening a Protective Umbrella:
the Birth of the Earth’s Magnetic Field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
A Partially Molten Earth:
the Magma Ocean Assumption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
The Birth of the Outer Shells: The Atmosphere and the Hydrosphere . . . . . . . . . . . . . 53
The Conclusion: a Planet That Was Undoubtedly Uninhabitable . . . . . . . . . . . . . . . . . . 59
Chapter 3 Water, Continents, and Organic Matter... . . . . . . . . . . . . . . . . 61
The Two Faces of the Earth’s Crust . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
The Fabulous Story Told by the Jack Hills Zircons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
The Atmosphere Between 4 .4 and 4 .0 Ga: An Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
From the Atmosphere to the Bottom of the Oceans:
Was the Earth Rich in Organic Matter? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
Was a Niche for Life Available as Early as This? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
Chapter 4 Intermezzo: The Gestation of Life and its First Steps . . 93
From Chemistry to Biology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
The Unavoidable Question: What is Life? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
The Origins of Metabolism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
The Origin of Genetic Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
The Origin of Compartments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
A Final Word on the Gestation of Life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
The Last Common Ancestor of All Existing Organisms: a Portrait . . . . . . . . . . . . . . . . 129
The Earliest Diversification of Life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
Chapter 5 The Late Heavy Bombardment . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
In Search of the Lost impacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
Late, or Continuous, Bombardment? The Two Competing Scenarios . . . . . . . . . . . . 160
The Late Heavy Bombardment: a Cataclysmic Scenario . . . . . . . . . . . . . . . . . . . . . . . . . . 161
XII Contents
A Rain of Meteorites: The Consequences of the Late Heavy Bombardment . . . . . . 164
Chapter 6 The Messages from the Oldest Terrestrial Rocks . . . . . . 167
The Scattered Remnants of One of the Oldest Continents . . . . . . . . . . . . . . . . . . . . . . . 169
3 .4 Billion Years Ago, in the Heart of a Vast Archaean Continent . . . . . . . . . . . . . . . . . . 172
The Saga of the Oldest Archaean Continents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174
Modeling the Terrestrial Atmosphere at 3 .8 Ga . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186
The Archaean Oceans: Saline and Hot? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190
The Earth’s Machinery During the Archaean:
Plate Tectonics Between 3 .8 and 2 .5 Ga . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193
A Newly Habitable and Already Inhabited Planet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198
Traces of Ancient Life: Data and Controversies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202
Chapter 7 A Planet Where Life Diversifies . . . . . . . . . . . . . . . . . . . . . . . . . . . 211
From the Primitive Earth to the Modern Earth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212
Birth, Life and Death of an Ocean: the Wilson Cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213
Ephemeral Giants: the Supercontinents and their Cycle . . . . . . . . . . . . . . . . . . . . . . . . . 215
The Crucial Consequences of the Supercontinent Cycle for the Earth’s
Environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216
The Appearance of Atmospheric Oxygen: a Revolutionary Event! . . . . . . . . . . . . . . . 218
Disruptive Events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222
The Evolution of the Prokaryotes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229
The Origin and Diversification of the Eukaryotes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 230
Chapter 8 Other Planets, Other Living Worlds ? . . . . . . . . . . . . . . . . . . . . 241
Life Elsewhere in the Solar System? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242
Unexpected Worlds Beyond the Solar System: Exoplanets . . . . . . . . . . . . . . . . . . . . . . . 245
From “Hot Jupiters” to “Super-Earths” . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247
“Habitable” Exoplanets: Other Earths? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249
The “Habitable Zone” . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251
“Biomarkers” . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254
Epilogue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263
The Main Principles for Rock Classification . . . . . . . . . . . . . . . . . . . . . . . . . . . 267
The 14 Chronological Stages in the Origin of the Earth and Life . . 273
Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283
Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297
Figure Credits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299
The Formation of the Sun
Chapter 1
and Planets
In the Beginning, There Was the Sun ...
Age: 4.57 billion years
Place of birth: Unknown, but probably in a nebula similar to that of Orion.
Father: A molecular cloud, nowadays lost
Mother: Universal gravitation.
Gestation period: Ten thousand years.
Childhood: Very turbulent, even subject to tantrums, with the ejection of material and numerous consecutive, eruptive
episodes in the presence of magnetic fields.
Descendants: Planets in the Solar System (giant planets in a few million years; terrestrial planets, including the Earth, in a
few tens of millions of years).
. The Orion Nebula seen in the near infrared, illuminated by the luminous “Trapezium cluster” of massive stars (seen in the center of the
picture). It was probably within a nebula of this type, inside a stellar association, that the Sun was born. (Credit ESO)
M. Gargaud et al., Young Sun, Early Earth and the Origins of Life,
DOI 10.1007/978-3-642-22552-9_1, © Springer-Verlag Berlin Heidelberg 2012
