The Cosmic Zoo: Complex Life on Many Worlds

238 Pages·2017·5.371 MB·English

by  Dirk Schulze-Makuch, William Bains (auth.)

Dirk Schulze-Makuch
William Bains
Complex Life
on Many Worlds
The Cosmic Zoo
Dirk Schulze-Makuch (cid:129) William Bains
The Cosmic Zoo
Complex Life on Many Worlds
DirkSchulze-Makuch WilliamBains
CenterforAstronomyand RufusScientificLtd.
Astrophysics(ZAA) Melbourn,Royston
TechnicalUniversityBerlin UnitedKingdom
Berlin,Germany
ISBN978-3-319-62044-2 ISBN978-3-319-62045-9 (eBook)
DOI10.1007/978-3-319-62045-9
LibraryofCongressControlNumber:2017951850
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Preface
OneofourmostpersistentquestionsiswhetherwearealoneintheUniverse.
Wenowknowthattheskyisfullofplanets,butaretheyempty,sterileworlds,
worlds harbouring only simple, primitive life, or is there a good chance that
there are thinking, talking, machine-making creatures out there for us to talk
to? Science is starting to provide the tools to answer that question. In this
book,wewilldiscusswhatweknowaboutthestepsthatleadfromtheorigin
of life on Earth to ourselves, using an approach that allows us to ask whether
humanity is a galactic oddity, or whether complex, smart, and potentially
technological beings are highly likely to evolve on an Earth-like planet in
the10billionyearsavailabletoitbetweenthecondensationsoftheoceanson
’
the planets surface and their evaporation by the increasing radiation from
the Sun.
Thereareprobablyasmanyopinionsonthissubjectastherearescientists.
This book represents our synthesis of the facts and opinions and our expla-
nationwhywethinkcomplexlifemaybecommon.Here,welayouthowwe
understand the evolutionary pathway from the origin of life to us humans,
and what this means for life that we think exists elsewhere in the Universe,
and why we think that smart, tool-making creatures may arise on other
worlds, maybe on many other worlds. There are some steps on the ladder
fi
tocomplexitywhichwefeelmorecon dentaboutthanothers.Wewillpoint
outtheweakerstepsaswediscussthem,butoverallwefeelthattheargument
is compelling.
WethankFrancesWestall,CharlesCockell,andananonymousreviewerfor
their constructive comments, and Louis Irwin for frequent and fruitful
v
vi Preface
fi
discussions, which improved the book signi cantly, and our editor Christian
Caron for guiding us along this book project. We are also grateful to our
families, particularly our wives, Joanna Schulze-Makuch and Jane Bains, who
hadto putupwithuswhilewewereeven morepreoccupiedthan usualwhile
writing this book.
Berlin, Germany Dirk Schulze-Makuch
Melbourn, Royston, UK William Bains
Introduction
Is there intelligent life other than humans in the Universe? It is an ancient
’ fi
question. We still cant answer this question, but based on new scienti c
insights we can speculate on what the answer may be. This book is based on
whatweknowoflifeonEarth,andwhatthatcantellusabouthowlikelyitis
that complex, active, tool-using life could evolve elsewhere.
Unlike many books on life in the Universe, we are not much concerned
withbacteriaoralgaeexceptinthattheyilluminatethepathtous.Wehappily
admitthatthisisahighlyanthropocentricapproach,butwetakeitdeliberately
because we are interested in the evolution of complex, smart organisms. We
‘
lookintothenightskyandseenoaliens,andsoweask,withEnricoFermi, If
’
lifeissocommon,wherearethey? Thisisaparadox.Itiswidelybelievedthat
lifewillariseonanysuitableplanet(althoughwewillquestionthisinPartII).
Yet, we see no examples of intelligent, radio-transmitting, spaceship-making
‘ ’
life in the sky. So there must be what Robin Hanson calls The Great Filter
between the existence of planets and the occurrence of a technological civili-
fi
sation.That ltercould,inprinciple,beanyofthemanystepsthathaveledto
modern humanity over roughly the last 4 billion years. So which of those
major steps or transitions are highly likely and which are unlikely? What
implicationsdoesthishaveforthelikelyfrequencyoflife,particularlycomplex
and technologically advanced life?
To answer this, we can identify features of our current biology that are
central to our existence and nature, such as having bones or brains, and those
fi
thatareperipheral,suchaspossessionofearlobes,andseekto ndthepointin
our evolution where the key features were acquired. We are not arguing for
similarity in appearance, or what we will here parody as The Star Trek
vii
viii Introduction
—
Fallacy that it is inevitable that complex, intelligent aliens will all have
pentadactyllimbs,circularirises,andmale-restrictedfacialhair.WithStephen
J. Gould, we agree that rewinding and replaying the tape of life on Earth
(orplayingitagainonacomparableplanetarybody)willnotresultinhumans.
function fi
Here, we are concerned with the processes that result in , not speci c
fi
anatomy. For example, the vertebrate eye is speci c and perhaps unique, but
vision has evolved many times, producing the completely different eyes of
insects, spiders, molluscs, cephalopods, trilobites, and, of course mammals.
We are less concerned with how a creature is smart and more whether it is
smart.
Progression oflifefromtheearliestformstoourselvesisoftencharacterised
as a series of major steps, transitions, or key innovations, each providing a
fi
signi cant new capability to the newly evolved organisms that was lacking in
more primitive forms. Evolution does not occur linearly with a goal in mind
‘ ’
(there is no mind!). Each evolutionary step adds a new toolset to solve
environmental challenges that each life form stumbles on, based on anatom-
ical, biochemical, and genetic abilities that are already there. So what are the
key steps or innovations and where do they occur on the path from the Last
UniversalCommonAncestortohumans?Whichofthemajorkeyinnovations
fromtheoriginoflifetoatechnologicallyadvancedsocietylikeoursarehighly
‘ ’
likely and which are unlikely? Where does the Great Filter lie?
This book is trying to provide some answers. The title gives our answer
—
away we think that once life has arisen on a world, complex life is highly
likely to evolve. If life is common, we believe that we live in a Cosmic Zoo.
Contents
Part I The Cosmic Zoo Hypothesis 1
1 The Cosmic Zoo Hypothesis and the Evolutionary Tool Set 3
1.1 Modeling the Transitions of Life 5
Further Reading 12
2 Pre-conditions for Complex Life 13
2.1 A Very Brief History of Earth and Life Through Time 13
2.2 Astronomical Constraints on Habitability 17
2.3 Planetary Requirements on Habitability and Life 21
2.4 Habitability Requirements for Complex, Macroscopic Life 26
2.5 How Often Are the Preconditions for Complex,
Macroscopic Life Met in the Universe? 28
Further Reading 31
’
Part II Major Transitions in Earths Life History 33
3 The First Cell and the Origin of Life Challenge 35
3.1 Early Attempts to Understand the Origin of Life on Earth 35
3.2 When Did the Origin of Life on Earth Occur? 35
3.3 How Life Arose 36
3.4 The Puzzling Path to the Origin of Life 38
ix
x Contents
3.5 In What Environment Did Life Originate? 42
3.6 Are There Other Planetary Locations Where Life Could
Have Originated? 48
3.7 Looking for Life 49
3.8 Are We Living in an Empty Universe? 50
Further Reading 51
4 Using Light: The Invention of Photosynthesis 53
4.1 Finding Energy for Life 53
4.2 What Energy Can Life Use? 54
4.3 Using Light 57
4.4 Other Paths to Using Light 61
4.5 Using Energy 66
Further Reading 67
5 Oxygen: From Poison to Photosystem II 69
...
5.1 All I Need Is the Air That I Breath 69
5.2 Why Make Oxygen? 70
5.3 The Great Oxidation Event 71
5.4 How to Make Oxygen 72
Further Reading 75
6 Endosymbiosis and the First Eukaryotes 77
6.1 What Is a Eukaryote? 77
6.2 Origin of the Eukaryotic Cell 80
6.3 Endosymbiosis and the Origin of Eukaryotes 81
6.4 Origin of Mitochondria 82
6.5 Origin of Chloroplasts 84
6.6 Origin of the Nucleus 85
6.7 Endosymbiosis Is a Common Phenomenon 86
6.8 The Next Step 91
6.9 Eukaryotic Organisms on Other Worlds? 92
Further Reading 93
7 Sex: A New Way of Reproduction 95
7.1 The Geography of Genes 95
7.2 The Alternation of Generations 97
7.3 So Why Have Sex? 98