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Everything we hear is an opinion not the fact.
Everything we see is a perspective not the truth.
Marcus Aurelius
The most complicated machines are made from words.
Jacques Lacan
Where does life come from? This is a question that has fascinated mankind
since the beginning of time. As soon as somebody or ‘something’ (either
life form or machine) becomes aware of itself, it also asks questions about
its world and about its own origins. It is no mystery that the oldest myths
and legends of human culture are so often centered on genesis motifs. Yet
despite tremendous progress in science during the last century, we are still
far from understanding the origin of life. Because we do not know exactly
what life is (or maybe because we cannot agree upon this issue), a wide variety
of theories and pseudo-theories have been proposed about the origin
of life. The palette of visions concerning the origin of life has reached out
in many directions: life being generated by incomprehensible-to-us mystical
forces, spontaneous generation (Buchner 1855), creationism (Bernard 1878b),
random genesis, panspermia (Arrhenius cited in Servos 1996), qualitative upgrades
of quantitative accumulations (Oparin 1924), gradualist (smooth) ‘upgrades’
from lifeless matter into life, life as an inevitable natural consequence
(Klabunovsky 2002), life as an emergent property of matter (Turian 1999),
life as an extraterrestrial manipulation or a phenomenon from another physical
dimension. Because human knowledge is a shared type of knowledge, our
understanding of the world is collective. Therefore, most theories about early
life are not fully independent of each other and show significant overlap.
The scientific field studying the physical meaning of life on Earth and in
the Universe, its origin and its fundamental properties has been given various
names such as prebiology (Rossler 1983), exobiology (Ponnamperuma 1972)
or originology (Kompanichenko 1996). Another possible name that might
better suggest the physical ambiguity of its study subject is ‘parabiology’.
Because this book is a quest and a challenge for the understanding of life
anywhere and, whatever its physical substance, I prefer to use the term exobiology.
Astrobiology, bioastronomy and artificial life are connected disciplines
which, although interested in the same major issues, have more pragmatic
purposes such as searching for life elsewhere, the quest for non-terrestrial
types of life and the creation of artificial types of life.
Among all sciences, exobiology holds an unmatched record. It has the
highest ratio between the number of hypotheses and the number of relevant
findings. The reasons are quite simple. Earth is an old planet, almost 4.6 Gyr
(giga years). Therefore the Earth we observe today is very different from the
Earth in its beginnings. Without a time machine that would allow one to
probe the early Earth, we will never be sure whether our models are an accurate
representation of the environment in which life originated. Moreover,
modern life is not simple but appears as an intricate web of large and complex
molecules that seem very unlikely to have appeared spontaneously and
that cannot exist independently of each other. For a biochemist or molecular
biologist, this ‘Gordian’ knot has no apparent beginning but only endless
loops and interdependencies. Therefore, modern life appears as a large collection
of interlocked chicken-and-egg paradoxes. Consequently, life cannot be
understood through either purely deductive logic or through purely experimental
approaches. Facing such an enormous challenge, a society predisposed
to philosophical, contemplative and intuition-based approaches is as helpless
as a super-technological, pragmatic and deductive society. An integrated and
realistic attitude seems the only sound approach toward scientific satisfaction.
Being aware that the vision presented here might contradict certain postulates
that other theories about the origin of life consider as fundamental,
I support the postulate that life emerged very early in the universe, that life
is probably present in other forms in other parts of the universe as well, that
life was ‘pushed’ into existence by understandable and foreseeable forces and
that an intelligent mind is capable of understanding life as a general concept.
Certainly, nothing is forever settled in science and no theory must be taken
for granted, irrespective of how much experimental evidence we humans may
have for it, and how precise that evidence may be. This is just a reminder
of the relativity of our ‘scientific truths’, a warning that “in science one can
proclaim a theory about reality as being the latest but never as the last theory.”
Consequently, although as an author I might be tempted to envision
my opinion as legitimate, the interpretation of the origin, properties, meaning
and definition of life presented here can offer no more than what current
scientific knowledge would allow. Aware of this unavoidable caveat, I have
tried throughout this study to be as open as possible to alternative interpretations.
If not deliberate, it might sound ironical that quotes given at the
beginning of some chapters are actually contradictory to what the chapters
have to say.
Probably every scientific generation before us believed that the end of
their quest was in sight. Yet to this day they have all failed to resolve their
questions. Over and over, research has demonstrated that the intrinsic complexity
of the living state is too overwhelming to resolve. Because later generations
will scrutinize life and its origin with more experience and better
insight than us, and with an appropriate respect for the magnificence of this
subject, the purpose of any sound theory about life cannot be to clarify the
problem, but to point questions in the right direction, to identify a plausible
and not an ultimate answer and to navigate on a likeliest path through the
fog of often conflicting experimental observations and alternative hypotheses.
In exobiology one can never expect ultimate answers, only illumination.
Acknowledgements
This book is dedicated to my family for their support. I would like to acknowledge
all my professors, mentors and friends who cultivated in me the
passion for this fascinating discipline. Special thanks to Cezar Radu, Arthur
Marx and Kenneth Nealson for their inspiring guidance.
University of Southern California Radu Popa
Los Angeles
September 2003
Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
2 The Early History of Bioenergy . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.1 Energy-Related Phase Transitions Toward Life . . . . . . . . . . . . . 15
2.1.1 Catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.1.2 Reflexive Activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.1.3 Energy Transduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.2 Energy Control Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
2.3 Quantitative Consequences of Energy Control Mechanisms . . . 33
2.4 Distribution of Energy Input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
3 The Origin of Cell Boundaries and Metabolism . . . . . . . . . . . 39
3.1 The Make-up of Life’s Boundaries . . . . . . . . . . . . . . . . . . . . . . . . 42
3.2 The Early Evolution of Cellular Boundaries . . . . . . . . . . . . . . . . 44
3.3 Outcomes of an Early Boundary . . . . . . . . . . . . . . . . . . . . . . . . . . 50
3.4 Systems Without Defined Boundaries . . . . . . . . . . . . . . . . . . . . . 51
3.5 Systems with Non-Specialized Boundaries . . . . . . . . . . . . . . . . . 51
3.6 Self-Assembly of Specialized Boundaries . . . . . . . . . . . . . . . . . . . 52
3.7 Boundary-Derived Properties of Life . . . . . . . . . . . . . . . . . . . . . . 53
3.8 Coupling of Spatial Seclusion with a Reflexive Activity . . . . . . 55
3.9 The Origin of Metabolism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
4 The Origin of Early Specificity.
The Order, Complexity and Diversity of Life . . . . . . . . . . . . . 63
4.1 Order . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
4.2 Complexity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
4.3 Diversity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
4.4 Specificity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
4.5 Specificity of Polymer-Based Life . . . . . . . . . . . . . . . . . . . . . . . . . 70
4.6 The Origin of Specificity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
4.7 Transition from External to Internal Control . . . . . . . . . . . . . . . 75
4.8 Major Events in the Early History of Specificity . . . . . . . . . . . . 76
4.9 The Origin of Feedback Mechanisms
as a Source of Internal Stability . . . . . . . . . . . . . . . . . . . . . . . . . . 76
4.10 The Origin of Forward Regulation . . . . . . . . . . . . . . . . . . . . . . . . 78
4.11 Consequences of Internal Regulation . . . . . . . . . . . . . . . . . . . . . . 78
4.12 Forced Oscillations and Periodic Clocks . . . . . . . . . . . . . . . . . . . 79
4.13 Specificity-Related Phase Transitions Toward Life . . . . . . . . . . 79
4.14 Specificity-Related Minimal Requirements of Life . . . . . . . . . . . 80
5 The Origin of Handedness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
5.1 Chirality and Life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
5.2 Natural Sources of Chirality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
5.3 Evolutionary Steps Toward Biological Chirality. . . . . . . . . . . . . 89
5.4 Handedness-Related Steps Toward Life . . . . . . . . . . . . . . . . . . . . 93
6 The Early History of Bio-Information. . . . . . . . . . . . . . . . . . . . . 95
6.1 Early Sources of Bio-Information . . . . . . . . . . . . . . . . . . . . . . . . . 100
6.2 Contextual vs. Nominative Information
and Explicit vs. Cryptic Information . . . . . . . . . . . . . . . . . . . . . . 102
6.3 Postulates of the Early Evolution of Bio-Information . . . . . . . . 109
6.3.1 The Contextual Information Era . . . . . . . . . . . . . . . . . . . 110
6.3.2 The Mineral-to-Organic Era . . . . . . . . . . . . . . . . . . . . . . . 110
6.3.3 The Organic-to-Organic Era . . . . . . . . . . . . . . . . . . . . . . . 112
6.3.4 The Emergence of Encryption. . . . . . . . . . . . . . . . . . . . . . 115
6.3.5 The Rise of the DNA World . . . . . . . . . . . . . . . . . . . . . . . 117
6.4 Information-Related Fundamental Phase Transitions
Toward Life. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
6.5 Minimal Requirements for the Emergence of Bio-Information. 120
7 The Purpose-Like Nature of Life . . . . . . . . . . . . . . . . . . . . . . . . . . 121
8 Assembling the Early Puzzle of Life . . . . . . . . . . . . . . . . . . . . . . 139
8.1 The First Step Toward Life:
Coupling Catalysis with Reflexive Activity. . . . . . . . . . . . . . . . . 142
8.2 Self-Assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
8.3 Seclusion Within Specialized Boundaries
and the Origin of Metabolism . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144
8.4 Probabilistic Jumps Toward Catalytic Specificity . . . . . . . . . . . 145
8.5 Feedback Regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145
8.6 Internalization of Minimal Specificity . . . . . . . . . . . . . . . . . . . . . 145
8.7 Control over Chirality. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146
8.8 Inheritable Variability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
8.9 Replication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148
8.10 The Last Step Toward Life: The Emergence of Encryption . . . 150
8.11 The Non-Life-to-Life Transition . . . . . . . . . . . . . . . . . . . . . . . . . . 150
8.12 Cosmochemical and Geochemical Requirements
for the Origin of Life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
8.13 Major Trends During the Early History of Life . . . . . . . . . . . . . 154
8.14 Differences Between Early Life and Modern Life . . . . . . . . . . . . 155
8.15 Early Life and Artificial Life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157
8.16 The Definition of Life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
9 The Material-Independent Signatures of Life.
Forensic Tools of Astrobiology . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
9.1 Rules of Thumb in Astrobiology . . . . . . . . . . . . . . . . . . . . . . . . . . 159
9.2 The Main Questions in Astrobiology . . . . . . . . . . . . . . . . . . . . . . 161
9.2.1 How Many Parameters are Required to Identify Life? . 162
9.2.2 False Premises and Misguided Fingerprints
in Astrobiology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162
9.3 The Material-Independent Signatures of Life . . . . . . . . . . . . . . . 170
Appendix A Models and Theories of Life . . . . . . . . . . . . . . . . . . . . 173
A.1 Major Steps Toward Life. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173
A.2 The (M,R)-System Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174
A.3 The Two-Polymerase System . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177
A.4 The Hypercycle Model. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178
A.5 The Autocatalytic Network Model . . . . . . . . . . . . . . . . . . . . . . . 180
A.6 The Chemoton Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183
A.7 Pargellis’s Model of Artificial Life . . . . . . . . . . . . . . . . . . . . . . . . 188
A.8 The Autopoietic Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189
A.9 The Algorithmic Chemistry Model . . . . . . . . . . . . . . . . . . . . . . . 193
A.10 Chemical Reaction Automata . . . . . . . . . . . . . . . . . . . . . . . . . . . 194
Appendix B Chronology of Definitions
and Interpretations of Life . . . . . . . . . . . . . . . . . . . . . 197
Appendix C Dictionary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207
Appendix D Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227
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