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History of Life - A Very Short Introduction

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Here is the extraordinary story of the unfolding of life on Earth, told by Michael J. Benton, a world-renowned authority on biodiversity. Ranging over four billion years, Benton weaves together the latest findings on fossils, earth history, evolutionary biology, and many other fields to highlight the great leaps that enabled life to evolve from microbe to human--big breakthroughs that made whole new ways of life possible--including cell division and multicellularity, hard skeletons, the move to land, the origin of forests, the move to the air. He describes the mass extinctions, especially the Permian, which obliterated 90% of life, and he sheds light on the origins of human beings, and of the many hominids that went before us. He ends by pointing out that studying the past helps us to predict the future: what happens if the atmosphere warms by 5 degrees? What happens if we destroy much of the biodiversity on Earth? These things have happened before, Benton notes. We need only look to the distant past to know the future of life on Earth.
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The History of Life: A Very Short Introduction

VERY SHORT INTRODUCTIONS are for anyone wanting a stimulating and
accessible way in to a new subject. They are written by experts, and have been
published in more than 25 languages worldwide.
The series began in 1995, and now represents a wide variety of topics in
history, philosophy, religion, science, and the humanities. Over the next
few years it will grow to a library of around 200 volumes – a Very Short
Introduction to everything from ancient Egypt and Indian philosophy to
conceptual art and cosmology.

Very Short Introductions available now:
John Parker and Richard Rathbone
Charles O. Jones
Harry Sidebottom
Antisemitism Steven Beller
ARCHITECTURE Andrew Ballantyne
ARISTOTLE Jonathan Barnes
ART THEORY Cynthia Freeland
Michael Hoskin
ATHEISM Julian Baggini
AUGUSTINE Henry Chadwick
AUTISM Uta Frith
BARTHES Jonathan Culler
BESTSELLERS John Sutherland
THE BIBLE John Riches
THE BRAIN Michael O’Shea
BUDDHA Michael Carrithers
BUDDHISM Damien Keown
CAPITALISM James Fulcher
CATHOLICISM Gerald O’Collins
THE CELTS Barry Cunliffe
CHAOS Leonard Smith

CHOICE THEORY Michael Allingham
CHRISTIAN ART Beth Williamson
CITIZENSHIP Richard Bellamy
CLASSICS Mary Beard and
John Henderson
Helen Morales
CLAUSEWITZ Michael Howard
THE COLD WAR Robert McMahon
Julian Stallabrass
Simon Critchley
THE CRUSADES Christopher Tyerman
Fred Piper and Sean Murphy
David Hopkins
DARWIN Jonathan Howard
Timo; thy Lim
DEMOCRACY Bernard Crick
DESIGN John Heskett
DINOSAURS David Norman
Patricia Aufderheide
DREAMING J. Allan Hobson
DRUGS Leslie Iversen
THE EARTH Martin Redfern
ECONOMICS Partha Dasgupta
EGYPTIAN MYTH Geraldine Pinch
Paul Langford

EMOTION Dylan Evans
EMPIRE Stephen Howe
ENGELS Terrell Carver
ETHICS Simon Blackburn
and Simon Usherwood
Deborah Charlesworth
FASCISM Kevin Passmore
FEMINISM Margaret Walters
Michael Howard
FOSSILS Keith Thomson
FOUCAULT Gary Gutting
FREE WILL Thomas Pink
William Doyle
FREUD Anthony Storr
GALAXIES John Gribbin
GALILEO Stillman Drake
Game Theory Ken Binmore
GANDHI Bhikhu Parekh
GEOGRAPHY John A. Matthews and
David T. Herbert
THE NEW DEAL Eric Rauchway
HABERMAS James Gordon Finlayson
HEGEL Peter Singer
HEIDEGGER Michael Inwood
HIEROGLYPHS Penelope Wilson
HISTORY John H. Arnold
The History of Life
Michael Benton
William Bynum
HIV/AIDS Alan Whiteside
HOBBES Richard Tuck
HUMAN RIGHTS Andrew Clapham
HUME A. J. Ayer
IDEOLOGY Michael Freeden

Khalid Koser
Paul Wilkinson
ISLAM Malise Ruthven
JOURNALISM Ian Hargreaves
JUDAISM Norman Solomon
JUNG Anthony Stevens
KAFKA Ritchie Robertson
KANT Roger Scruton
KIERKEGAARD Patrick Gardiner
THE KORAN Michael Cook
LAW Raymond Wacks
LINGUISTICS Peter Matthews
LOCKE John Dunn
LOGIC Graham Priest
MACHIAVELLI Quentin Skinner
MARX Peter Singer
MATHEMATICS Timothy Gowers
Terry Eagleton
John Gillingham and Ralph A. Griffiths
MEMORY Jonathan K. Foster
MODERN ART David Cottington
Richard Lyman Bushman
MUSIC Nicholas Cook
MYTH Robert A. Segal
NEWTON Robert Iliffe
NIETZSCHE Michael Tanner
Christopher Harvie and
H. C. G. Matthew
Marc Mulholland
Joseph M. Siracusa
Michael D. Coogan

PAUL E. P. Sanders
Raymond Wacks
Samir Okasha
PLATO Julia Annas
David Miller
POLITICS Kenneth Minogue
POSTMODERNISM Christopher Butler
Catherine Belsey
Catherine Osborne
Gillian Butler and Freda McManus
THE QUAKERS Pink Dandelion
John Polkinghorne
RACISM Ali Rattansi
RELATIVITY Russell Stannard
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ROUSSEAU Robert Wokler
RUSSELL A. C. Grayling
S. A. Smith

Chris Frith and Eve Johnstone
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Thomas Dixon
SCOTLAND Rab Houston
SEXUALITY Véronique Mottier
SHAKESPEARE Germaine Greer
SIKHISM Eleanor Nesbitt
John Monaghan and Peter Just
SOCIALISM Michael Newman
SOCRATES C. C. W. Taylor
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SPINOZA Roger Scruton
TERRORISM Charles Townshend
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Nothing Frank Close
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For more information visit our websites

Michael J. Benton

The History
of Life
A Very Short Introduction


Great Clarendon Street, Oxford OX2 6DP
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Oxford is a registered trade mark of Oxford University Press
in the UK and in certain other countries
Published in the United States
by Oxford University Press Inc., New York
c Michael J. Benton 2008

The moral rights of the author have been asserted
Database right Oxford University Press (maker)
First Published 2008
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,
without the prior permission in writing of Oxford University Press,
or as expressly permitted by law, or under terms agreed with the appropriate
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outside the scope of the above should be sent to the Rights Department,
Oxford University Press, at the address above
You must not circulate this book in any other binding or cover
and you must impose the same condition on any acquirer
British Library Cataloguing in Publication Data
Data available
Library of Congress Cataloging in Publication Data
Data available
ISBN 978–0–19–922632–0
1 3 5 7 9 10 8 6 4 2
Typeset by SPI Publisher Services, Pondicherry, India
Printed in Great Britain by
Ashford Colour Press Ltd, Gosport, Hampshire


List of illustrations ix
Introduction 1


The origin of life 15
The origin of sex 33
The origin of skeletons 51
The origin of life on land 69
Forests and flight 87
The biggest mass extinction 101
The origin of modern ecosystems 122
The origin of humans 146
Index 167

This page intentionally left blank

List of illustrations


A selection of fossils from
a mid-Victorian textbook
(1860) 3

6 The universal tree of life 36

Mansell/Time & Life Pictures/
Getty Images

7 The endosymbiotic theory for
the origin of eukaryotes 38

Professor Norman Pace

Inspired by

2 An exceptionally well
preserved fossil from Liaoning
Province, China 6
Spencer Platt/Getty Images

3 Geological timescale 18–19
4 The formation of an RNA
protocell 28
Reprinted by permission from
Macmillan Publishers Ltd (Nature

5a Stromatolite fossils in the
Stark Formation, Mackenzie,
Canada 30
P. F. Hoffman (GSC)

5b Filamentous microfossils in a
massive sulfide from
Australia 31
Courtesy of Birger Rasmussen

8 A close up of Bangiomorpha
filaments 46
Dr Nick Butterfield

9 Life as it may have looked in
Ediacaran times 49
Smithsonian Institution

10 Fossils from the Early
Cambrian 55
M. Alan Kazlev/Dorling Kindersley

11 The Burgess Shale scene,
Middle Cambrian 58
Christian Jegou Publiphoto
Diffusion/Science Photo Library

12 Cooksonia 74
13 The Rhynie ecosystem 76
Simon Powell, Bristol University

14 Ichthyostega and Acanthostega
reconstructions 80
Mike Coates

15 A Carboniferous riverbank 88
Walter Myers

16 Life before and after the
end-Permian mass
extinction 104
John Sibbick

17 Life on land in the Late
Permian in what is now
Russia 110
John Sibbick

18 The pattern of marine
extinction through the
end-Permian crisis 114
From fig. 1, Y. G. Jin et al., Science
289: 432–36 (21 July 2000).
Reprinted with permission from

19 Reptiles from the Triassic 127
From Mike Benton, Vertebrate
Palaeontology (3rd edn., Blackwell,
Oxford, 2005)

20 Dinosaurs of the Late Jurassic
of North America 139
Ernest Unterman/Dinosaur National
Monument Museum,

The publisher and the author apologize for any errors or omissions in the
above list. If contacted they will be pleased to rectify these at the earliest


The Age of Reptiles ended because it had gone on long enough and
it was all a mistake in the first place.
Will Cuppy, How to become extinct (1941)

It is hard to make sense of the history of life on Earth. A mass of
strange and extraordinary animals and plants perhaps flits before
our eyes when we think of prehistory: Neanderthal man,
mammoths, dinosaurs, ammonites, trilobites . . . and of course a
time when there was no life at all, or at least merely microscopic
beasts of extreme simplicity floating in the primeval ocean.
These impressions come from many sources. Children today are
weaned on dinosaur books, and the images of living, breathing
dinosaurs are everywhere, in movies and television
documentaries. Then, too, as children, many people have gone to
coastal cliffs or quarries and collected their own fossil ammonites
or trilobites. These common fossils, as well as many much more
spectacular and beautiful examples, such as petrifactions of
exquisite fishes showing all their scales, still shiny after millions of
years, may be seen in fossil shops, or in lavish photographs in
coffee table books and on the web.
Most people are aware that dinosaurs, despite their ubiquity in
modern culture, lived a long time before the first humans, and

there were untold spans of time before the dinosaurs existed that
were populated by ever-more unusual and strange animals and
plants. How are we to make sense of all of this?

The History of Life

The keys to understanding the history of life are fossils (Fig. 1).
Fossils are the remains of plants, animals, or microbes that once
existed. Fossils may be petrifactions, which means literally ‘turned
into rock’, and these are some of the commonest examples.
Petrified fossils may be of two kinds, first, those that are literally
turned to rock, and where none of the original organism remains.
The leaf or tree trunk, or shell, or worm, has completely
disappeared, and the cavity left behind has been replaced by
grains of sand or mud, or more often by minerals in solution that
have flowed through the spaces in the surrounding rock and have
then infiltrated the space and crystallized.
The second, and commoner, kind of petrifaction still retains some
of the original material of the animal, perhaps the calcium
carbonate that made up the shell, or some cuticle or carbonized
relic of the plant. Rock grains or minerals then merely fill the
cavities. So, many people might be surprised to realize that
common fossils, such as a 400-million-year-old trilobite or a
200-million-year-old ammonite, are actually largely made from
the original calcium carbonate of their external skeleton or shell,
as in life. Similarly, by far the majority of dinosaur bones are still
made of the original calcium phosphate (apatite), the main
mineralized constituent of bone then and today. If you look closely
at the outer surface of these fossils, perhaps with a magnifying
glass, you can see extremely fine features, such as pimples and
growth lines on the trilobite carapace, original multicoloured
mother-of-pearl on the ammonite shell, and muscle scars or tooth
marks on the surface of the dinosaur bone. If the fossil shells or
bones are cut across and examined under the microscope, all the
original growth layers and internal structures are still there. So, a

1. A selection of fossils from a mid-Victorian textbook, showing trilobites
(top), Coal Measure plants and brachiopods (centre) and a selection
of ammonites, fossil fishes, an ichthyosaur, and a plesiosaur (lower half )

The History of Life

section cut through dinosaur bone looks just as fresh today as a
section through a modern bone.
Every plant or animal that has ever lived has not turned into a
fossil. Indeed, if this were the case, the surface of the Earth would
be covered in avalanches of fossils everywhere, great mounds of
dinosaur bones, trilobites, giant coal forest trees, ammonites, and
the like, probably extending to the moon. No one knows what
proportion of life has ended up fossilized, but it is clearly a tiny
fraction, much less than 1 per cent. Plants or animals must at least
have hard parts such as a skeleton, a shell, or a toughened, woody
trunk to be readily preservable. Even so, the majority of animal
carcasses and dead plants enter the food chain almost
immediately, being scavenged by animals or decomposed by
bacteria. Dead organisms can only turn into fossils if
sedimentation is happening, that is, sand or mud are being
dumped on top of the remains, perhaps on the floor of a deep lake,
under a sand bar in a river, or deep in the ocean, below the zone
that is constantly churned up by currents and tides.

Worms and feathered dinosaurs: exceptional
Other fossils may be preserved in slightly unusual conditions that
may, on occasion, provide unique and unexpected insights into
ancient life, so-called exceptional preservation. Exceptionally
preserved fossils may show soft structures, such as flesh, eyes,
stomach contents, feathers, hair, and the like. Sites of exceptional
fossil preservation are sometimes called ‘windows’ on the life of
the past. They allow palaeontologists, the scientists who study
fossils, to see a snapshot of everything that existed at particular
times and in particular places. These at least allow
palaeontologists to see the soft-bodied worms, jellyfish, and other
creatures that are rarely preserved in normal circumstances.


The Burgess Shale in Canada is one of the most famous of these
sites of exceptional preservation. These rocks are 505 million years
old, so they document some of the oldest animals. Without the
Burgess Shale, and similar sites of about the same age in
Greenland and China, palaeontologists would know only about
shelled and skeletonized organisms such as brachiopods (‘lamp
shells’), trilobites, and sponges. The Burgess Shale has increased
our knowledge of life in the Cambrian many-fold: it has revealed
whole clans of worm-like creatures, some related to modern
swimming and burrowing worms, others seemingly unique and
hard to link to modern animals. The Burgess Shale also shows the
feathery legs and gills of the trilobites, their mouths, guts, and
sense organs, and it reveals strange tadpole-like swimming
animals that have primitive backbones and so are close to our own

There are dozens of other such sites of exceptional preservation
scattered pretty randomly through time and space. But why do
they exist and how are the soft structures preserved? Most of these
sites come from times and places where oxygen was limited. Deep
lakes and deep oceans sometimes lose the normal oxygen content
of the waters, if, for example, there is a dramatic growth of algae
and other floating plants at the surface, a so-called algal bloom.
These occur in warm conditions, and the lakes and oceans may
become temporarily stagnant. The stagnation of the waters may
itself kill swimming creatures, and beasts that crawl around on the
bottom muds. The lack of oxygen can also mean that the normal



Equally famous are the sites of exceptional preservation in
Liaoning Province in north-east China. These date back to 125
million years ago, and they have produced spectacular fossils of
birds (and dinosaurs) with feathers and internal organs, mammals
with hair, fishes with gills and guts, and any number of worms,
jellyfish, and other soft-bodied denizens of those ancient Chinese
lakes (Fig. 2).

The History of Life

2. An exceptionally well preserved small dinosaur specimen,
Microraptor, from the Early Cretaceous of Liaoning Province, China

scavenging creatures cannot survive, and the carcasses do not have
all their flesh stripped.
Experiments show that, in oxygen-poor, or anoxic, conditions, soft
tissues, even muscles, guts, and eyeballs, can be invaded by
minerals that come from the body fluids of the animals, or from
the surrounding sediments. These are typically flash-mineralizing
processes, where the fibres of a muscle, or the complex tissues of a
gill or a stomach, are invaded and replaced within hours or days at
most. Once mineralized, the replicas of soft tissues can then
survive to the present day.

Living blimps? Quality of the record
Like most palaeontologists, I sometimes sit bolt upright in bed at
night and worry whether the fossil record is informative or not.
Charles Darwin wrote about the ‘imperfection of the geological
record’, and he was well aware that most organisms are never
fossilized, and so palaeontologists miss so much of ancient life.

The question though is: how much is missing? Is it 50 per cent or
90 per cent or 99.99999 per cent? This can never be determined,
of course. A more sensible question might be: how adequate is the
fossil record?

Palaeontologists have no way of detecting such hypothetical
extinct beasts. Other soft-bodied creatures can be assumed to
have existed, though. For example, there are many phyla, or major
groups, of worm-like creatures today, nematodes, platyhelminths,
gastrotrichs, sipunculids, and others, that have no known fossil
representatives. And yet, because they exist today, and because we
can establish their evolutionary relationships to other organisms
with shells or skeletons, we know the length of their missing
fossil record. If a soft-bodied worm group is the closest relative
of another wormy creature with a shell, both groups must have
existed for the same length of time; their common ancestor must
have lived at a particular time, and the fossil record of the shelled
group establishes a minimum age for both groups. The known
missing record of the soft-bodied group is called a ghost range, a
part of the missing fossil record we can predict with some certainty.


Palaeontologists have speculated that there might be whole
sectors of extinct life that we know nothing about. What if there
were a diverse class of floating animals that were constructed of
extremely lightweight materials, and provided with great air
bladders that filled with gases lighter than air? These creatures
might have been many metres long, perhaps as large as dirigible
aircraft, sometimes called blimps during the Second World War.
These blimp beasts could well have dominated the Earth, if they
were so large, and yet they might have entirely escaped
fossilization. Their bodily tissues might have been so lightweight
that they rotted away when they died. Their gas bladders would
clearly burst and disappear during decay. Living in the air, in any
case, means their carcasses might have generally fallen onto the
surface of the Earth, and so they might not often have been
covered with sediment in any case.

The History of Life

What do the sites of exceptional preservation tell us? If they
preserve more or less everything that lived at the time, soft- and
hard-bodied, they can be used as a yardstick against which to test
the ‘normal’ fossil record. It seems that the ancient exceptional
sites, such as the Burgess Shales, tell us more about unknown
groups than the more recent ones, such as the Liaoning beds in
China. In fact the soft-bodied organisms from Liaoning, worms,
jellyfish, insects, and the like, are all entirely predictable from
other known fossils and from ghost ranges.
Palaeontologists have been pretty assiduous in retrieving fossils.
As time goes on, it now seems to take much more effort than it
took a century ago to find something new. Indeed, not much has
changed in our knowledge of the fossil record since the time of
Darwin. In the 1850s, palaeontologists knew about trilobites and
ammonites, fossil fishes, dinosaurs, and fossil mammals. They did
not know anything about the first life from the Precambrian, nor
did they know much about human evolution. But the fact that
neither trilobites nor humans have been found in the age of the
dinosaurs, nor have any other fossils been found in seriously
unexpected places, suggests that the record is known more or less
well. Our work now is merely to flesh out the details.
But that still says nothing about the giant blimps . . .

Molecules and the history of life
It might seem unexpected to introduce molecular biology at this
point. But, just as historians have parallel sets of evidence from
artefacts and from written records, so too do students of the
history of life. Until the 1960s, there were only fossils; after that
there were also molecules – even though most palaeontologists at
the time probably did not appreciate it.
In an extraordinary paper published in 1962 by Emil Zuckerkandl
and Linus Pauling, in a rather obscure conference volume, the

molecular clock was born. Molecular biology had arisen ten years
earlier when, in 1953, James Watson and Francis Crick announced
the structure of deoxyribose nucleic acid, DNA, the chemical that
makes up genes and is the basis of the genetic code. By 1963,
several proteins, such as haemoglobin, the protein that carries
oxygen in the blood and makes it red, had been sequenced, that is,
the detailed structure had been determined, and the new breed of
molecular biologists had noted something extraordinary. The
proteins of different species of animal were not identical, and their
structures differed more between distantly related species. In
other words, the haemoglobin molecules of humans and
chimpanzees were identical, but the haemoglobin of a shark was
very different.

In the 1960s, protein sequencing was a laborious process, and
the new data came slowly, but by 1967 the haemoglobin of the
great apes was known sufficiently that the first attempt was
made to produce an evolutionary tree. The science of molecular
phylogenetics was born. Vincent Sarich and Allan Wilson, in a
three-page paper in the American journal Science, plotted the
relationships of humans and apes, and showed that our nearest
relative was the chimpanzee, then the gorilla, and then the
orang-utan. This was not so unexpected, and it agreed with the
pattern of relationships established from studies of anatomy.
The shocking part of the paper was that the molecular clock
said humans and chimps had diverged only 5 million years


Zuckerkandl and Pauling took the brave leap of suggesting, on
rather limited evidence then, that the amount of difference was
proportional to time. The negligible difference between the
haemoglobins of humans and chimpanzees showed these two
species had diverged only a short time ago, geologically speaking,
whereas the 79 per cent difference between human and shark
haemoglobin pointed to a divergence 400 million years ago, or

The History of Life

Palaeontologists were variously bemused and horrified. Most
dismissed the new technique: after all, if it produced such
ludicrous results, it was clearly not working. Everyone knew that
humans and chimpanzees had split some 15–20 million years ago,
based on studies of Proconsul and other early human-like fossils
from the Miocene of Africa. Others took the method seriously, but
were equally unhappy about the result.
As the protein data sets grew, more mammals were added to
the tree, and the branching dates seemed quite reasonable for
most other groups. This increased the nervousness of the
palaeontologists, who then faced a conundrum: do we accept the
new molecular date, or insist on the established fossil evidence?
Slowly, they came to realize the molecular date was probably
right. Closer study of the fossils showed that they had been
over-interpreted. The supposedly ‘human’ characters of Proconsul
and its kin were not really human at all. This fossil was related to
the common ancestors of humans and the African apes, and so
said nothing about the true timing of divergence. Since the 1970s,
new finds in Africa have shown that the divergence date between
humans and chimps must be at least 6–7 million years ago.
Now, molecular biologists interested in the tree of life, the great
pattern of relationships linking all species, use DNA sequences.
Protein sequencing is slow, and the evidence limited. DNA, the
genetic code, offers much more information, and new techniques
developed in the 1980s have made sequencing almost automatic.
Computers can also crunch enormous masses of data these days,
so sequencers are happy to run lengthy segments of the genetic
code, consisting of many genes, and for dozens, or even hundreds
of species, to produce patterns of relationships for specific groups
or for large sectors of life. It is possible to assess the genome of,
say, twenty species of lizards, and draw up a tree that documents
evolution over a span of perhaps 10 million years. Equally, the
analyst can select, say, twenty species across all of life – a human, a


shark, a mollusc, a tree, a fern, a bacterium – and find a tree of
relationships that extends deeply back in time.
But where do the fossils fit into all this?


On reading around, I discovered that cladistics had been
promulgated by a German entomologist, Willi Hennig. He had
written about the technique in the 1950s, but it had only really
attracted attention when the book was translated into English and
reissued in 1966. But, from 1966 to 1980, only a rather small
group of true believers espoused the method, and it had not in any
way become mainstream. Hennig argued passionately that
systematists, the biologists and palaeontologists who were
interested in species and the tree of life, should be more objective
in their methods.
Until Hennig’s time, systematists had attempted to draw up trees
of relationships based on a judicious sifting of the character
evidence. A biological character is any observable feature of an
organism – ‘possession of feathers’, ‘possession of four fingers’,



I remember when I attended my first scientific meeting, as an
undergraduate, a session of the Society for Vertebrate
Palaeontology and Comparative Anatomy at University College,
London, in 1976, I wondered if I would ever go back. As I looked
on nervously, the big beasts of the subject were bickering and
squabbling appallingly over something called ‘cladistics’. I’d heard
nothing about this – it wasn’t taught then as part of my degree.
One person would assert with fervour that everyone should adopt
this new technique. Another would say it was all nonsense – even a
Marxist plot to overthrow the scientific method. I stumbled back
to the train, wondering whether my decision to become a
professional palaeontologist was mistaken. Were they all mad?

The History of Life

‘iridescent blue feathers on top of the head’, ‘multiple flower heads
on each stem’ – and systematists had long understood that if two
organisms share a character they might well be related. The
problem was always convergence, the well-known observation that
unrelated organisms might evolve similar features independently.
Insects, birds, and bats have wings, but no one ever suggested that
this was sufficient evidence to group these organisms together as
close relatives: in detail, their wings are anatomically quite
different in structure, and so they evolved them independently, but
for the same purpose. But how were systematists to distinguish
convergence from truly shared, evolutionarily identical,
This was Hennig’s point: objective techniques were required to
distinguish truly shared characters from convergences, but also to
distinguish inherited ‘primitive’ characters from those that truly
marked a particular branching point. So, while it is true that
humans and chimpanzees share the character ‘hand with five
fingers’, and this is not a convergence, the character is not helpful
at the level of the branching point between the two species.
In fact, all land-living vertebrates basically have a five-fingered
hand – lizards, crocodiles, dinosaurs, rats, bats, whales, and so on.
Hennig had identified the critical point, that anatomical
characters had to be evolutionarily unique (not convergent) and
they had to be assessed at the correct level in the tree before they
could be considered useful. He termed such characters
synapomorphies, sometimes rendered in English as ‘shared
derived characters’. (Hennig’s writing, in any language, is heavy
going, and he liked inventing long words – neither of which helped
gain him converts.)
Hennig’s concept of a synapomorphy is more or less the same as
the classic notion of a homology, that is, any structure that shares
a common fundamental pattern because of common ancestry –
such as the human arm, the wing of a bat, and the paddle of a
whale. These limbs may have different functions today, but they all

share the same bones and muscles inside, and we now know they
evolved from the ancestral front limb of the first mammal.

The great leap forward
Palaeontologists are aware that their field has transformed itself
immeasurably since the 1960s, but public attention has focused
elsewhere – the space race, genetic engineering, computer
technology, nanoscience, global change. But, cladistics and
molecular phylogeny have introduced new rigour into the field of
drawing up evolutionary trees. Whereas in the 1950s and 1960s a
palaeontologist did his or her best to make a tree by ‘joining the
dots’ – linking similar-looking beasts through time – today there
are many independently derived trees of the evolution of different
groups, some based on different genes, others on different
combinations of fossil and recent data on anatomy. But do they
The astonishing discovery is that molecular and palaeontological
trees agree with each other more often than not. The two
approaches are pretty well independent, so it is possible then to


Since the 1970s, systematists have increasingly switched to using
cladistics in their work. After all, there was no alternative – the
older techniques were really inspired guesswork. Acceptance came
largely for a reason Hennig could not have predicted, namely the
growth in power and ease of use of computers. The secret to
cladistics is the character matrix, a listing of all the species of
interest, and codings of their characters (1 for presence, 0 for
absence). Multiple cross-checking over the matrix, and repeated
runs of the analysis, provided statistical methods of assessing
which tree or trees explained the data best, and the probability
that synapomorphies were correctly identified or not. In practice,
there have been many problems, but cladistic methods are
ubiquitous, and repeat analyses by different analysts allow
published trees to be tested and confirmed or rejected.

The History of Life

compare, say, a tree based on molecular sequences of modern
rodents with a tree constructed by measuring the teeth and other
anatomical features of living and extinct species. Inevitably,
everyone hears about the cases where the results disagree. In the
early days of molecular sequencing, some bizarre results emerged,
but the methods were young, and mistakes were easy to make.
Such bizarre results are rare now. In some cases, palaeontologists
have humbly accepted that they have been entirely unable to
resolve certain parts of the evolutionary tree, and the molecules
give an unequivocal answer straight away. In other cases, there is
no resolution yet, and more work is required. Some parts of the
great tree of life may remain forever mysterious, perhaps because
rates of evolution were so fast that characters did not accumulate,
or the branching points are so ancient that subsequent evolution
has obliterated the clues to relationship.
The third methodological or technological advance has been in
dating the rocks. Since the 1960s, the accuracy of dating has
improved greatly, and sequences of rocks and sequences of events
can be compared more accurately than before. But we can look at
that later. Let’s begin the story.


Chapter 1
The origin of life

As a general rule, then, all testaceans grow by spontaneous generation in mud, differing from one another according to the differences
of the material; oysters growing in slime, and cockles and the other
testaceans above mentioned on sandy bottoms; and in the hollows of
the rocks the ascidian and the barnacle, and common sorts, such as
the limpet and the nerites.
Aristotle, History of Animals

From the earliest days people have wondered about the origins of
life. The ancient Greeks and Romans considered the topic, and
had many ideas. Most, like Aristotle (384–322 BC), focused on the
idea of spontaneous generation, a process that they believed
happened today, and that had presumably happened when life
first arose. As Aristotle wrote above, he believed that marine
shellfish all arose spontaneously from the mud, sand, and slime on
the seabed and among the coastal rocks. He made similar
assumptions about other forms of life: moths arose from woollen
garments, garden insects arose from the spring dew or from
decaying wood, and many fishes arose from froth on the surface of
the ocean. Such views held sway until the nineteenth century.
Louis Pasteur (1822–95) famously showed conclusively that life
could not arise spontaneously. He repeated experiments that had
been performed before, but took great pains to exclude all

possibility of contamination. Earlier workers had gone through
the process of boiling a broth of water and hay in sealed flasks so
that anything living in the water or the air within the flasks would
be killed. But, despite these precautions, they still found
microscopic organisms living in the water, and Pasteur argued
that the germs entered the vessels when they were being cooled in
a mercury trough. So he repeated the experiments, sterilizing the
glassware and the water in the flasks, but ensuring also that
laboratory air could not enter the cooling mixtures. With the air
excluded, nothing living was detected in the boiled water even
many months later.

The History of Life

The age of the Earth
The death of spontaneous generation was not the only problem for
scientists interested in studying the origin of life about 1900. They
also had no truly ancient fossils to work with, and no real idea of
the age of the Earth, nor of the major events that might have
preceded the origin of life. There was a widely held view that the
Earth was something like a huge ball of iron – iron is one of the
commonest elements – that had once been molten, and had been
cooling down. Indeed, the eminent late Victorian physicist
William Thomson, later Lord Kelvin (1824–1907), used this
assumption, and his knowledge of thermodynamics, to speculate
that the Earth formed only 20–40 million years ago.
Kelvin’s view that the Earth was relatively young influenced many
people at the turn of the twentieth century. No matter that the
biologists and geologists were quite unhappy with this estimate;
the leading physicist of the day had pronounced, and he had based
his evidence on clear calculations. Charles Darwin had long
assumed, for example, that the Earth must be hundreds or
thousands of millions of years old, although he never speculated
more closely than that. Nonetheless, he could see how the rocks of
the south coast of England had accumulated rather slowly, made
up from many millions of thin layers, each perhaps representing a

year or a century. Other geologists held similar views, whether
based on their calculations of the time taken for sedimentary
rocks, such as limestones and mudstones, to accumulate, or the
time it might have taken for the oceans to separate from the initial
molten rock, and then to become salty.
Ironically, Kelvin lived through the crucial discoveries that were to
show that his physical view of the Earth was too simplistic, but he
was reluctant to shift. The discovery of radioactivity by Henri
Becquerel (1852–1908) in 1896, the property of certain elements,
such as uranium, radium, and polonium, to emit rays and to
change their atomic number, changed everything. Radioactive
elements may decay into another element, with the emission of
rays. In radioactive decay, the parent element, such as uranium,
would decay into another element, called the daughter, such as
thorium, over a certain amount of time.

Rutherford’s suggestion was put into practice remarkably rapidly.
In a bravura performance, the young British geologist Arthur
Holmes (1890–1965), aged only 21 at the time, published the first

The origin of life

The discovery of radioactivity caused excitement throughout the
world of physics, and only four years later, Ernest Rutherford
(1871–1937) and Frederick Soddy (1877–1956) showed that
radioactive decay is exponential – that is, the quantity of
radioactive material halves over fixed amounts of time. In other
words, 1,000 atoms of uranium reduce to 500 in a certain span of
time, those 500 to 250 in the same amount of time, then to 125,
and so on. Three years later, and in the hearing of an ageing and
somewhat crotchety Lord Kelvin, Ernest Rutherford suggested
that radioactive decay might provide a geological clock. He argued
that, if scientists measured the time it takes for half the quantity of
the parent radioactive element to decay to the daughter element, a
span since called the half life, measurements of the proportions of
parent to daughter element in a suitable rock sample could then
give an estimate of the age of the rock.















3. Geological timescale






Apes and humans


Modern orders of animals and plants


Flowering plants

Cone-bearing plants
Mammals, dinosaurs


Reptiles flourish

Mammal-like reptiles



Seed plant forests


Amphibians, insects, plants


Colonization of land


Most modern phyla



Soft-bodied animals


The History of Life

age estimates for rocks in 1911: his estimated dates ranged from
340 million years (a Carboniferous rock), to 1,640 million years
(a Precambrian rock). These are not far off the modern age
estimates (Fig. 3). Note that the first nine-tenths of the history of
the Earth is called the Precambrian, because it precedes the
Cambrian period: this is rather an apologetic, or negative term, for
such a vast span of the Earth’s history, but the term is established
now and cannot be readily changed.
After the first very crude estimates had been made, Holmes,
and many others, worked hard to improve their understanding
of age measurements, and the chemistry and physics were much
revised, so that by 1927 Holmes was able to produce a reasonable
summary of key dates for the history of the Earth. Holmes
suggested that the age of the Earth was between 1,600 and
3,000 million years. In the same year, Rutherford suggested
3,400 million years, and by the 1950s, the age of the Earth
was estimated at 4,500–600 million years, the currently accepted
figure. It was, and still is, hard to date the exact origin of the Earth
because rocks were presumably molten then, and so there are no
solidified crystals that may be dated.

Making the Earth habitable
There is some debate about when the Earth became habitable:
did it take 200 or 600 million years? Most geologists have
favoured the latter view: after all the initially molten surface
had to cool to below 100 ◦ C, or any organic compounds would
have been burnt off. Life is based on carbon, hydrogen, and
oxygen, and these all remain in a gaseous state at high
temperatures. Of course water boils at 100 ◦ C, and life is
essentially water (H2 O) with carbon.
The Sun and its accompanying planets formed some 4.6 billion
years ago from gas into which earlier generations of stars had
spewed not only hydrogen and helium but small amounts of

As the Earth’s surface cooled, the lithosphere, the rocky crust and
outer mantle, began to differentiate as a cooler upper layer above
the underlying asthenosphere. As the rocky lithosphere formed,
and the upper crust divided into plates that were moved by mantle
convection, slow-moving gyres of heat rising from the depths of
the mantle moved laterally as they came close to the base of the
cooler solid crust, and began the stately journey of the Earth’s
tectonic plates.
Geologists keep searching for the oldest rocks on Earth, and they
are at all times pushing the limits of what might be possible
(molten rocks cannot be dated, and error bars on dates become
quite large when such ancient dates are attempted).
The oldest rock unit on Earth is said to be the Acasta Gneiss from
the Northwest Territories, Canada, dated at up to 4.0 billion years
old. This is a metamorphic rock, and the date is assumed to reflect

The origin of life

carbon, oxygen, and other elements forged in their cores. At first,
the Earth was a molten mass, but it cooled, separating into an
outer cool crust and an inner molten mantle and core. The heavier
iron sank to the core, while lighter elements such as silicon rose to
the surface. It took some 50 million years for the separation to
occur, and the Moon may have spun off at this time, the result, it is
thought, of a collision with an enormous planetoid. Massive
volcanic eruptions rent the semi-molten silicon-rich rocks at the
Earth’s surface, and produced great volumes of gases: carbon
dioxide, nitrogen, water vapour, and hydrogen sulphide.
Temperatures on the Earth’s surface were too high, and the crust
was too unstable, for any form of carbon-based life to exist. At this
time, the record of craters on the Moon suggests that there were a
few huge impacts on Earth, impacts from large comets or
asteroids that would have provided enough energy to turn the
ocean into steam. Thus, if life had got started before 4 billion
years ago, it would probably have been wiped out, only to start

the age of the older granite from which the gneiss was formed.
Even older are zircons, isolated mineral grains, from the Jack Hills
in Australia, which have yielded a date of 4.4 billion years. Could
these minerals really have been solid, and even accumulating
under water, at that point? Their discoverers claim this is the case,
while others are sceptical that the Earth could have been cool
enough for water to exist so soon after its formation.

The History of Life

The oldest sedimentary rocks have been reported from the
Isua Group in Greenland, dated at 3.8–3.7 billion years ago.
There is no doubt that water existed on the Earth by this point,
and that some of the Isua Group rocks really are formed from
accumulated sand, laid down under water, and deriving from
older rock sources. It has even been claimed that these oldest
sedimentary rocks also contain traces of life, but this claim is
still much debated.

Traces of early life
In 1996, Stephen Mojzsis, then a graduate student at the Scripps
Institution of Oceanography at La Jolla, California, made a
startling announcement in the journal Nature. He claimed to have
identified a clear chemical signature for life in carbon compounds
from Isua Group rocks. He had analysed minute grains of
graphite, a form of carbon, in the rocks, and found an unusually
high proportion of carbon-12. The carbon atom has two stable
isotopes, carbon-12 and carbon-13. The ratio of these two forms of
carbon can indicate the presence or absence of organic residues of
previously living organisms: enrichment in carbon-12 relative to
carbon-13 is characteristic of photosynthesizing organisms, and
the organisms that eat them. Mojzsis was confident he had
identified life: ‘Our evidence establishes beyond reasonable doubt
that life emerged on Earth at least 3.85 billion years ago, and this
is not the end of the story. We may well find that life existed even

If the interpretation is correct, then the grains of graphite in the
Isua rocks prove that photosynthesis was happening 3.85 billion
years ago. Photosynthesis is the process by which green plants
convert energy from sunlight into food. Carbon dioxide and water
combine, and produce oxygen, usually given off as a gas, and
sugars, which form the building blocks of the plant. Now, in the
early part of the history of the Earth, these photosynthesizing
organisms were not trees or flowers, but presumably simple
microbes known as cyanobacteria.

The Isua graphites are still held as evidence for early life, and the
debates continue to rage. But how does this chime with current
theoretical views about the origin of life?

The biochemical theory for the origin of life
There are many models for the origin of life, all based on an
understanding of how the simplest living organisms today operate.
The first ‘modern’ model for the origin of life was presented in the
1920s independently by two remarkable scientists, the Russian
biochemist A. I. Oparin (1894–1980) and the British evolutionary
biologist J. B. S. Haldane (1892–1964). Oparin and Haldane share
the distinction of being independent co-founders of the so-called

The origin of life

Other researchers have argued strongly against this interpretation.
They noted, for example, that the Isua graphite was not in the
sedimentary rocks of the area, but in the metamorphic rocks.
Indeed, the Isua sedimentary rocks contained relatively low
proportions of graphite. The alternative argument was then that
the Isua graphites were of secondary, inorganic origin and might
have formed by heating of iron carbonate. One of the critics, Roger
Buick of the University of Washington, Seattle, said that ‘These
rocks have been buried and cooked at least three times. They’ve
been severely squashed and strained and tied in knots at least
three times too.’

biochemical theory for the origin of life, as well as being known
normally only by their initials.

The History of Life

According to the Oparin–Haldane model, life could have arisen
through a series of organic chemical reactions that produced ever
more complex biochemical structures. They proposed that
common gases in the early Earth atmosphere combined to form
simple organic chemicals, and that these in turn combined to form
more complex molecules. Then, the complex molecules became
separated from the surrounding medium, and acquired some of
the characters of living organisms. They became able to absorb
nutrients, to grow, to divide (reproduce), and so on. The
Oparin–Haldane model was not tested until the 1950s.
In 1953, Stanley Miller (1920–2007), then a student of Harold
Urey (1893–1981) at the University of Chicago, made a model of
the Precambrian atmosphere and ocean in a laboratory glass
vessel. He exposed a mixture of water, nitrogen, carbon monoxide,
and nitrogen to electrical sparks, to mimic lightning, and found a
brownish sludge in the bottle after a few days. This contained
sugars, amino acids, and nucleotides. So Miller had apparently
recreated the first two steps in the Oparin–Haldane model, mixing
the basic elements to produce simple organic compounds, and
then combining these to produce the building blocks of proteins
and nucleic acids.
It should be noted that critics have said that the mixture of gases
that Miller used (with high percentage concentrations of hydrogen
and methane) was rather different from the likely atmosphere of
the early Earth. Atmospheric hydrogen is ultimately replenished
from the mixture of gases released from the solid Earth; but the
geochemistry of the subsurface means that the mixture generally
should contain the oxidized form of hydrogen, namely water
vapour, H2 O, rather than the large proportion of free hydrogen gas
in Miller’s model atmosphere.


Further experiments in the 1950s and 1960s led to the production
of polypeptides, polysaccharides, and other larger organic
molecules, the next step in the hypothetical sequence. Sidney Fox
at Florida State University even succeeded in creating cell-like
structures, in which a soup of organic molecules became enclosed
in a membrane. His ‘protocells’ seemed to feed and divide, but
they did not survive for long, so they were not living, despite the
hype made by the press at the time.

RNA world
Biologists have long been unhappy with aspects of the
Oparin–Haldane model. They have pointed out, for example, that
the two fundamental functions of any living thing are that it must
have some form of genetic code, the ability to pass on information
from one generation to the next, and it must be able to perform
chemical reactions, to break down food, for example. These are,
respectively, the functions of genes and enzymes. Genes are the
segments of the genetic code, written in the sequence of bases in
the DNA (deoxyribose nucleic acid), that specify particular
functions. Enzymes are chemicals that stimulate, or catalyse,


The origin of life

In a recent twist to the classic Oparin–Haldane biochemical
model, Euan Nisbet (University of London) and Norman Sleep
(Stanford University) proposed the hydrothermal model for the
origin of life in 2001. In this model, the ancestor of all living things
was a hyperthermophile, a simple organism that lived in unusually
hot conditions. The transition from isolated amino acids to DNA
may then have happened in a hot-water system associated with
active volcanoes, rather than in some primeval soup at the ocean
surface. There are two main kinds of hot-water systems on Earth
today, ‘black smokers’ found in the deep oceans above mid-ocean
ridges where magma meets sea water, and hot pools and fumaroles
fed by rainwater that are found around active volcanoes.

chemical reactions. The conundrum was to determine whether life
originated according to a ‘genes first’ or ‘enzymes first’ model.

The History of Life

The solution seems to be that perhaps both functions arose at the
same time. In 1968, Francis Crick (1916–2004) suggested that
RNA was the first genetic molecule. He argued that RNA could
have the unique property of acting both as a gene and an enzyme,
so RNA on its own could be a precursor of life. RNA (ribonucleic
acid) is one of the nucleic acids and it has key roles in protein
synthesis within the cells. The genetic code, the basic instructions
that contain all the information to construct a living organism, is
encoded in the DNA strands that make up the chromosomes.
Different forms of RNA act as the template for translation of genes
into proteins, transfer amino acids to the ribosome (the cell
organelle where protein synthesis takes place) to form proteins,
and also translate the transcript into proteins.
When Walter Gilbert from Harvard University first used the term
‘RNA world’ in 1986, the concept was controversial. But the first
evidence came soon after when Sidney Altman of Yale University
and Thomas Cech of the University of Colorado independently
discovered a kind of RNA that could edit out unnecessary parts of
the message it carried before delivering it to the ribosome.
Because RNA was acting like an enzyme, Cech called his discovery
a ribozyme. This was such a major finding that the two were
awarded the Nobel Prize for Chemistry in 1989; Altman and Cech
had confirmed part of Crick’s prediction.
But how could naked RNA molecules exist, and how could they
act as a foundation for life? The argument was that the simple
RNA molecules may have assembled themselves by chance in rock
pools, more or less following the assumptions made by Oparin and
Haldane, and as shown in the Stanley Miller experiment. These
simple naked RNA molecules mainly existed and then
disappeared, but perhaps one or two were able to copy themselves,
and they could have become dominant.

To take this forward to create a living cell, there might have been
two stages, the production of a protocell by combination of two
components, an RNA enzyme and a self-replicating vesicle
(Fig. 4). This satisfies the minimum requirement that two RNA
molecules should interact, one to act as the enzyme to bring
together the components, and the other to act as the gene/
template. Together the template and the enzyme RNA combine as
an RNA replicase. But these components have to be kept together
inside some form of compartment or cell, or they would only
occasionally come into contact to work together. This is the second
pre-life structure, termed a self-replicating vesicle, a
membrane-bound structure composed mainly of lipids (organic
compounds that are not soluble in water, including fats) that
grows and divides from time to time. The RNA replicase at some
point entered a self-replicating vesicle, and this allowed the RNA
replicase to function efficiently (Fig. 4).

Some aspects of the RNA world hypothesis have been tested, but
much remains to be done. And in any case, the model remains
hypothetical, because none of these stages would be likely to be
fossilized. If the RNA world existed, it had to pre-date the oldest
fossils, and the Earth had to be cool enough for the organic
elements to survive being burned off. Some estimate that this
might have been a time of 100–400 million years, somewhere
between 4.0 and 3.5 billion years ago.

The origin of life

This is a protocell, but it is not yet living. It is just a self-replicating
membrane bag with an independent self-replicating molecule
inside. To make the protocell function both components have to
interact, the vesicle protecting the RNA replicase, and the RNA
replicase perhaps producing lipids for the vesicle. If the
interaction works, the protocell has become a living cell. The cell is
alive because it has the ability to feed itself, to grow, and to
replicate. Evolution can happen because the cells show differential
survival (‘survival of the fittest’), and the genetic information for
replication is coded in the RNA.



The History of Life

Linking function
(e.g. ribozyme)


4. The formation of an RNA protocell

The first fossils
The oldest fossils appear to date from about 3.5 million years ago.
Fossils of this age have always been controversial, but there are
two kinds, microfossils and stromatolites. The first truly ancient
fossils were reported in the 1950s, and the pressure to find
ever-older specimens is intense. Mistakes have often been made,
and that is no surprise because the oldest fossils are bound to be
from extremely simple organisms, and microscopic ones at that.
So it’s no wonder that great experts have often been caught out

over-interpreting a chance bubble or mineral fragment in a
microscope slide, even a bit of fluff or a modern plant spore.
It is probably unexpected that the most convincing truly ancient
fossils are large structures called stromatolites. These are mounds
made partly from living organisms and partly from sediment, and
they still exist today. Stromatolites (Fig. 5a) are made from many
thin layers that apparently build up over many years or hundreds
of years to form irregular mushroom- or cabbage-shaped
structures. They are built from microbial mats composed of some
of the simplest of living organisms called cyanobacteria, and these
have sometimes been called, rather misleadingly, blue-green algae.
Algae, like seaweeds, have advanced cells with nuclei, whereas
cyanobacteria, like ordinary bacteria, are made from the simplest
of cells, without a nucleus.

Perhaps the oldest currently accepted microfossils other than
stromatolites date from 3.2 billion years ago. They were reported
in 2000, from a massive sulphide deposit in Western Australia.
The fossils are thread-like filaments (Fig. 5b) that may be straight,
sinuous, or sharply curved, and even tightly intertwined in some
areas. The overall shape, uniform width, and lack of orientation all
tend to confirm that these might really be fossils, and not merely

The origin of life

Typical cyanobacteria photosynthesize, so they live in shallow
water, near the water’s edge. Today, they are found generally in
highly saline waters, often in tropical regions, where pools of
seawater have partly evaporated. In less saline waters, herbivorous
animals eat them up. The thin microbial mat may sometimes then
be swamped by fine grains of mud, and the cyanobacteria grow up
through the sediment to keep in touch with the sunlight. Over
time, extensive layered structures may build up. In most fossil
examples, the constructing microbes are not preserved, but the
layered structure remains. Many early examples have proved
controversial, but the oldest that are generally accepted come from
Australia, and are dated as 3.43 billion years old.

The History of Life

inorganic structures. If so, they confirm that some of the earliest
life may have been thermophilic (‘heat-loving’) bacteria that lived
near a hot, sulphur-producing structure under the sea, as
predicted by Euan Nisbet and Norman Sleep’s model for the origin
of life.
There is a long gap in time after the 3.4-billion-year-old
stromatolites and microfossils before more convincing fossils are
found. There are some specimens from rocks dated at 2.5 billion
years old in South Africa, and then the famous Gunflint Chert of
Canada, dated at 1.9 billion years ago. The Gunflint microfossils
include six distinctive forms, some shaped like filaments, others
spherical, and some branched, or bearing an umbrella-like
structure. These Precambrian cells resemble in shape various
modern bacteria, and some were found within stromatolites. Most
unusual is Kakabekia, the umbrella-shaped microfossil; it is most
like rare micro-organisms found today at the foot of the walls of
Harlech Castle in Wales. These modern forms are tolerant of
ammonia (NH3 ), produced by ancient Britons urinating against

5a. Stromatolite fossils in the Stark Formation, Mackenzie, Canada

5b. Filamentous microfossils in a 3,235-million-year-old massive
sulfide from Australia

the castle walls. So were conditions in Gunflint Chert times also
rich in ammonia?

The History of Life

Strange things were happening on the Earth 2 billion years ago,
apart from the ammonia-loving Kakabekia. The atmosphere
suddenly seemed to carry oxygen, there are organic traces of quite
diverse life, and new kinds of microfossils appear, some of them
with nuclei. If this is true, these mark the origin of the eukaryotes,
and so the origin of sex.


Chapter 2
The origin of sex

What use is sex?
John Maynard Smith, ‘The origin and maintenance of sex’ (1971)

It has often been noted that sex is a ludicrous and messy business.
Simple organisms seem to be able to reproduce perfectly
successfully by splitting or budding: amoebas go on feeding until
they are quite large, and then one individual splits into two; a
yeast or a sponge buds off side shoots that eventually break free as
separate little organisms. So what’s the point of sex?
In his book The evolution of sex, the noted British evolutionary
thinker John Maynard Smith (1920–2004) wrote in 1978 about
the twofold cost of sex. He pointed out that asexual organisms,
those that have only one gender and that reproduce by splitting or
budding, can increase their population sizes rapidly. Because each
individual is effectively a female, each of the offspring is capable of
reproducing independently. Sexual organisms, those that
reproduce following exchange of genetic material, have two sexes,
female and male, and it’s the males (of course) that are the
problem. So if each female produces two offspring, and there is
1 : 1 sex ratio, then on average the two offspring will consist of one
female and one male. The rate of doubling of the population size is
half that of an equivalent asexual organism.


The History of Life

Technically speaking, the sexual female has half the fitness of the
asexual female. Fitness, genetically speaking, is a measure of
reproductive success. So the ‘twofold cost of sex’ is that a sexual
organism has half the fitness of its asexual counterpart.
So what is it about sex that has made it such a worthwhile pursuit?
Maynard Smith suggested that the advantage was a long-term
one, that sex shuffles genes more effectively than parthenogenesis
(the production of live young from unfertilized eggs), introducing
more genetic variability, and hence adaptability, into a population.
He showed that sexual populations can evolve more rapidly than
asexual ones, an ability that makes species which reproduce
sexually much more resilient when the population is attacked by
disease or parasites. The balance of advantage can go both ways.
Normally asexual organisms such as aphids may pass through
occasional sexual generations. Equally, parthenogenesis has
evolved many times among lizards and snakes, groups that are
typically sexual, of course.
Sex requires the transfer of genetic material between the male
and female, and it is a feature unique to eukaryotes, the more
complex organisms. So when, in the rather obscure history of
Precambrian life, did eukaryotes arise, and then when did sex first
happen? The evidence comes partly from the study of modern
organisms, partly from geochemical studies of biomarkers, partly
from investigations of ancient atmospheres, and partly from fossil

The universal tree of life
In the popular mind, and probably in many older biology
textbooks, all of life can be divided comfortably into plants,
animals, and microbes. Plants are green and they don’t move,
animals are usually not green and they usually move, and
microbes are just small.

This rather unsophisticated classification has been supplemented
and revised substantially. First, there is clearly a deep division
between the prokaryotes and the eukaryotes. Prokaryotes are all
single cells, they have no nucleus, and they have merely a single
strand of DNA that carries all their genetic material. They
generally reproduce asexually, although many forms have
processes for sharing genetic material. Eukaryotes include many
single-celled forms, but also many multicelled plants and animals
also. Their cells include organelles, specialized structures such as
the nucleus, energy-transmitting structures called mitochondria,
and photosynthesizing chloroplasts in green plants. Their DNA is
typically in many strands, forming chromosomes within the
nucleus of each cell.

So, all living things fall into these three great domains. The
Domain Bacteria includes Cyanobacteria and most groups
commonly called bacteria. The Domain Archaea (‘ancient ones’)
comprises the Halobacteria (salt-digesters), Methanobacteria
(methane-producers), Eocytes (heat-loving sulphur-metabolizing
bacteria), and others. The Domain Eucarya includes an array of
single-celled forms that are often lumped together as ‘algae’, as

The origin of sex

A five-kingdom classification of life was popular for a while,
with plants and animals supplemented by fungi among the larger
forms, and two major groups of microscopic organisms, the
eukaryotic protoctists and the prokaryotic monerans. The
five-kingdom model was demolished after 1977 in a remarkable
series of papers by Carl Woese and colleagues from the University
of Illinois. Their molecular trees showed a deep split into three
fundamental divisions, the domains Bacteria (or Eubacteria),
Archaea (or Archaebacteria), and Eucarya (or Eukaryota). So the
prokaryotes are no more, forming the domains Bacteria and
Archaea, and it is still not clear whether Archaea and Bacteria
split first, or Archaea and Eucarya. Despite this uncertainty
at the root, Woese had produced the first universal tree of life
(Fig. 6).

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The History of Life


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6. The universal tree of life

well as the multicellular organisms. Perhaps the most startling
observation is that, within Eucarya, the fungi are more closely
related to the animals than to the plants, and this has been
confirmed in several analyses. This poses a moral dilemma for
vegetarians: should they eat mushrooms or not?


The origin of eukaryotes
Until recently, it seemed clear that prokaryotes had dominated the
Earth for a billion years or more, before the first eukaryotes
appeared. However the evidence is far from clear now. First, as we
have seen, molecular reconstructions of the universal tree of life
do not confirm that Eucarya arose later than Bacteria or Archaea,
as had been expected. In fact, all three domains might have arisen
at about the same time for all we know. Geochemical data from
biomarkers has also given surprising evidence.

But how could eukaryotes, with their complex internal structure of
nucleus and other organelles, have arisen from simpler
prokaryotes? The most popular idea has been the endosymbiotic
theory, proposed by Lynn Margulis, then a young faculty member
at Boston University, in 1967. According to her theory (Fig. 7), a
prokaryote consumed, or was invaded by, some smaller
energy-producing prokaryotes, and the two species evolved to live
together in a mutually beneficial way. The small invader was
protected by its large host, and the larger organism received


The origin of sex

Biomarkers are organic chemical indicators of life. Most
biomarkers are lipids, fatty and waxy compounds found in living
cells. Some biomarkers are indicative of life in general, but others
can be associated with particular domains or kingdoms. In 1999,
Jochen Brocks, a research fellow at Harvard, and colleagues,
announced new biomarker evidence from organic-rich shales in
Australia dated at 2.7 billion years ago. As expected, some of the
biomarkers were indicators of cyanobacteria, but the investigators
also unexpectedly identified C28–C30 steranes, which are
sedimentary molecules derived from sterols. Such large-ring
sterols are synthesized only by eukaryotes, and not by prokaryotes.
So, this biomarker evidence confirms the existence of
cyanobacteria at least 2.7 billion years ago, but it is also the oldest
hint of the occurrence of eukaryotes, long before any fossils.

eukaryotic cell in
an animal, a fungus,
or certain protists

aerobic bacteria

bacteria turned
into mitochondria


eukaryotic cell
in a plant or in
certain protists

The History of Life

host cell

(folding in)

…turned into chloroplasts

7. The endosymbiotic theory for the origin of eukaryotes

supplies of sugars. These invaders became the mitochondria of
modern eukaryote cells. Other invaders may have included
worm-like swimming prokaryotes (spirochaetes) that became
motile flagella (the whip-like appendages used by some
micro-organisms to get around), and photosynthesizing
prokaryotes that became the chloroplasts of plants.
The endosymbiotic model is immensely attractive, and some
aspects have been confirmed spectacularly. Most notable is that
the mitochondria and chloroplasts in modern eukaryotes are
confirmed as prokaryotes, the mitochondria being closely related
to alpha-proteobacteria and the chloroplasts to cyanobacteria.
So the amazing thing is that a modern eukaryote cell has proven
prokaryotic invaders that possess their own DNA and that
coordinate their cell divisions with the divisions of the larger
host cell.

Many experts reject the endosymbiotic theory, or at least most of
it. They point out that the only real evidence for engulfment is for
the mitochondria. There is no evidence to support the idea that
the nucleus was engulfed, nor is it clear what kind of prokaryote
did the engulfing, and in fact engulfment is seen today only among
eukaryotes, and not among prokaryotes. So the alternative view,
termed the protoeukaryotic host theory, is that an ancestral
eukaryote, the so-called protoeukaryote, already equipped with a
nucleus, indeed did engulf an energy-transferring prokaryote that
became the mitochondrion. But this does not tell us where the
protoeukaryote itself came from. Further doubt is cast on the
classic endosymbiotic theory by the suggestion that neither
Archaea nor Bacteria appear to be ancestral to Eucarya, and that
biomarker evidence indicates an unexpectedly ancient origin for
eukaryotes. Back to the drawing board!

The atmosphere of the earliest Earth was devoid of oxygen, and
life originated in the absence of oxygen. Then, about 2.4 billion
years ago, perhaps a billion years after life had first appeared,
atmospheric oxygen levels rose to 1 or 2 per cent of modern levels.
This may not sound much, but geologists have termed this grandly
the Great Oxygenation Event. The world would never be the same
again. But what caused this rather dramatic change in the
The first organisms had anaerobic metabolisms, that is, they
operated in the absence of oxygen. Indeed the first prokaryotes
would have been killed by oxygen. This is a shocking fact that is
confirmed by living microbes: some can switch from anaerobic to
aerobic respiration depending on oxygen levels. Others, though,
are obligate anaerobes that have to respire anaerobically and
cannot survive in the presence of even the smallest amount of

The origin of sex


The History of Life

The simplistic view is that organisms produced the atmospheric
oxygen, and that may be partly true. Gaseous oxygen is a major
product of photosynthesis, and it is likely that the earliest
cyanobacteria could photosynthesize. But it is unlikely that all the
early atmospheric oxygen came from photosynthesis because
cyanobacteria had been around since 3.5 billion years ago (see
p. 29), and they had not oxygenated the atmosphere at all during
the subsequent billion years. Probably all the oxygen produced by
photosynthesis was mopped up by combining with gases produced
by volcanoes and with soluble metals in hot springs and seafloor
vents to produce water and oxides. This left little or no oxygen to
enter the earliest atmosphere as a gas.
So where did the first atmospheric oxygen come from? David
Catling at the University of Bristol argues that the source, initially
at least, was inorganic. He suggests that the key is in methane.
Methane, a compound of carbon and hydrogen, is a potent
greenhouse gas produced largely by anaerobic microbes. Before
life existed in abundance, there was not much methane, but levels
rose as more and more was generated by the early microbes.
Today, methane is consumed by oxygen in the atmosphere, but in
the absence of oxygen, early Precambrian methane levels might
have been 100 to 1,500 times as high as today. This created a
burning hot greenhouse climate worldwide.
The methane greenhouse collapsed 2.4 billion years ago. As
methane levels rose, hydrogen atoms were transferred out of the
Earth’s atmosphere into space, so there was no longer enough
hydrogen to combine with free oxygen to form water molecules
(H2 O), and so the surplus oxygen flooded out as an atmospheric
gas. The rise of oxygen in the atmosphere had a profound effect on
life and the planet. New aerobic organisms arose that exploited the
atmospheric oxygen molecules in their chemical activity. The
oxygen also built up an ozone layer high in the atmosphere that
blocks out solar ultraviolet radiation.


There was a second rise in atmospheric oxygen, to 10 per cent of
modern levels, around 0.8–0.6 billion years ago, and this might
indicate further changes in global chemical cycles, and further
expansions in the diversity of life on Earth. Such are the indicators
from the rocks.

First eukaryote fossils
At one time, there was a clear story of prokaryotes-first,
eukaryotes-second. As we have seen, however, the waters are
considerably muddied by new molecular and chemical evidence.
There are clear-cut biomarkers for eukaryotes dating from
2.7 billion years ago, and the universal tree of life resolutely
refuses to resolve itself in a clear way to show that Eucarya is a
younger branch than either Bacteria or Archaea.

The oldest supposed eukaryote fossil is impressive in some ways,
disappointing in others. Later Precambrian rocks of various ages
have yielded examples of a strange fossil, consisting of great
spaghetti-like coils of tubes about 5 millimetres wide, preserved as
thin carbon films, which have been called Grypania. The oldest
Grypania fossils date from 1.85 billion years ago. The fossil looks


The origin of sex

The fossils are equally ambiguous. Textbooks used to illustrate nice
cells with clear nuclei, from the Bitter Springs Chert of Australia,
dated at 800 million years ago. Some of the Bitter Springs fossils
even seemed to show cell division: a sensational discovery! In the
way of things, of course, these were too good to be true, and they
are now reinterpreted as clusters of cyanobacteria. The supposed
nuclei, dark splodges on the cell, are interpreted now as folds and
irregularities in the cell membranes. Further, basic cell division,
called technically mitosis, where one cell splits more or less
equally into two, is seen in both eukaryotes and prokaryotes. As
Thomas Henry Huxley once said, it is terrible to see ‘the slaying of
a beautiful theory by an ugly fact’; in this case, several ugly facts.

most like a coiled seaweed and, if that’s what it was, then it is a
eukaryote. This interpretation is disputed, and other researchers
say it is some kind of giant bacterium. They claim that the oldest
eukaryote fossils are actually microscopic fossils called acritarchs,
marine plant-like planktonic organisms that were roughly
spherical and carried tiny whiskers and hooks. The oldest
acritarchs are 1.45 billion years old.
These early supposed eukaryotes are still disputed, but by about
1 billion years ago, there are several contenders, both microscopic
acritarchs and relatives, as well as numerous seaweeds, and other
rather more complex fossils. The current view is that
multicellularity and sex may be linked.

The History of Life

Multicellularity . . .
There are no truly multicellular organisms among the Bacteria or
Archaea. Admittedly, some prokaryotes form filaments and loose
aggregations of cells, but these associated cells do not exchange
messages and their functions are not coordinated. So all truly
multicellular organisms are eukaryotes, so far as we know. The
simplest multicellular organisms are microscopic, and consist of
little more than a string of identical cells, but some modern algae
give clues about how multicellularity might have arisen.
The slime mould Dictyostelium generally operates as a single cell,
but at certain times, notably when food is limited, numerous
individual cells aggregate together and the whole colony moves to
a new location. Other simple eukaryotes today, such as the
protozoan Volvox, can form colonies of up to 10,000 individual
cells, and these may show some cell differentiation. Volvox has
fascinated scientists for years; when Anton van Leeuwenhoek
(1632–1723), the famous inventor of the first microscope, first
viewed Volvox he could not believe what he saw. The colony
formed a hollow ball, and moved through the water seemingly
by rolling (the name Volvox means ‘fierce roller’). Most of

the 10,000 cells act as feeding and swimming organs, beating
furiously with their flagella, and causing the whole colony to
spin. But small numbers of cells in the colony can take on a
reproductive function, and Volvox colonies/individuals can mate
and produce dormant offspring. In nature, Volvox reproduces
asexually, and the sexual offspring seem to be an insurance against
particularly bad conditions.

But what are the advantages of multicellularity? These must be
many, because multicellularity has arisen independently many
times, and it is still evolving in certain algae such as Volvox.
Advantages of multicellularity include greater efficiency in
feeding, movement, reproduction, and defence by having
specialized cells. A specialized cell that only has to feed or provide
a virulent defensive capability can perhaps evolve much further
and specialize to a much greater extent than a single cell ever
could if it has to provide all the normal services and functions of
life. There are also clearly advantages in being larger than
microscopic, not necessarily because big is always best, but if you
are the only large organism in a sea of midgets. These advantages
include access to new food sources, including larger prey, and the
possibility of moving faster and further.

The origin of sex

This example illustrates all kinds of extraordinary biological
principles. First, where do you draw the line between an individual
and a colony? The Volvox ball seems to act as an individual, in that
the cells all stick together and work together to make it swim. But
each cell is still essentially an individual, acting on its own to feed
and split from time to time. Other examples of colonies today are
found on coral reefs, where numerous individual corals of one
species grow together as a single structure, or an ant’s nest, where
numerous specialized kinds of ants work together. The individual
components of the colony (the coral, the ant) can live on their own
and perhaps found a new colony, although that is not really true
for most individual colonial ants – they rely on others to
reproduce, find food, protect the nest, or keep the nest cool.

. . . and sex

The History of Life

But where does sex come in? After all, some prokaryotes and
single-celled eukaryotes reproduce sexually from time to time. But
Nick Butterfield from the University of Cambridge argues that
true sexual reproduction enabled multicellularity to arise, and the
two appear to be intimately linked. Asexual reproduction, or
budding as it is sometimes called, is really just a form of growth:
cells feed and grow in size, and when they are big enough they split
by mitosis to form two organisms. The DNA splits at the same
time and is shared by the two new cells. The products of asexual
reproduction are clones, being genetically identical replicas.
Sexual reproduction, on the other hand, involves the exchange
of gametes (sperms and eggs) between organisms. Typically, the
male provides sperm that fertilize the egg from the female.
Gametes have half the normal DNA complement, and the two half
DNA sets zip together to produce a different genome in the
offspring, but clearly sharing features of father and mother. In
eukaryotes, the DNA exists as two copies, each strand forming one
half of the double helix structure. Cell divisions in sexual
reproduction are called meiosis, where the DNA unzips to form
two single copies, one going into each gamete, prior to fusion after
Butterfield’s argument is that the advantages of multicellularity
are so clear that this property would have arisen as soon as sexual
reproduction had appeared. No asexual organism can evolve true
multicellularity because asexual organisms do not evolve in the
normal way. As clones, there is little opportunity for change and
for natural selection. Evolution is possible, of course, but there is
no speciation, the formation of new species, in the sense we see
among multicellular animals and plants.
How do we date the origin of sex? Butterfield argues that there are
two lines of evidence, one phylogenetic, and one based on the tight

link of sex and multicellularity. The phylogenetic argument is
based on the tree of life. If we can draw a tree of relationships, we
can then map certain characters onto the tree on the basis of living
organisms, and then track them down to the root. We must be
certain that the characters in question are true homologues, that is,
features that arose once only and are not convergences . The
argument is that sexual reproduction, as seen in modern
eukaryotes, is so complex that it arose only once, so the point of
origin of sex can be marked on the tree of eukaryote evolution
near the base.
The other argument is based on fossils. Find a multicellular fossil,
says Butterfield, and you have found sex. At present, the oldest
accepted multicellular eukaryote fossil is an extraordinary
organism called Bangiomorpha from the Hunting Formation of
Canada, dated as 1.2 billion years old.

Red algae (rhodophytes) are relatively common forms of seaweed
today, seen on shorelines around the world, and forming a staple
part of some cuisines, such as Japanese nori. Red algae range from
single cells to large ornate structures, and they may be tolerant of
a wide variety of conditions. The modern red alga Bangia, for
example, can survive in a full range of salinities, from the sea to
freshwater lakes. The oldest red alga was reported in 1990, and
named Bangiomorpha because it resembled the modern Bangia in
certain ways, but also perhaps for other reasons.
When he named Bangiomorpha in 2000, Nick Butterfield
employed all the smutty medieval humour of England in
explaining why he had chosen the name. Its full name is
Bangiomorpha pubescens, the species name pubescens chosen
‘with reference to its pubescent or hairlike form, as well as the
connotations of having achieved sexual maturity’. The name
Bangiomorpha pubescens has even made it into the dictionaries of

The origin of sex

Bangiomorpha: what’s in a name?

The History of Life

8. A close-up of Bangiomorpha filaments, showing cell division in the
terminal structure

bizarre and cheeky names; one website notes: ‘The fossil shows
the first recorded sex act, 1.2 billion years ago. The “bang” in the
name was intended as a euphemism for sex.’ The fossils do not
show sex acts, and the commentators surely exaggerate, but the
name is a useful mnemonic.
Bangiomorpha grew in tufts of whiskery strands attached to
shoreline rocks by holdfast structures made from several cells

(Fig. 8). The individual filaments are up to 2 millimetres long, and
the cells are less than 50 microns (thousandths of a millimetre)
wide. The cell walls are dark and enclose circular to disc-like cells,
and filaments may be composed of a single series of cells, or of
several series running side by side.

The Neoproterozoic and Snowball Earth
The last phase of Precambrian time is called the Neoproterozoic, a
term applied to rocks dated from 1,000 to 542 million years ago.
During this time, the diversity of fossils increases. This might
reflect a real burst of new life forms following the invention of sex
and multicellularity, or it might simply reflect the fact that it is
perhaps easier for palaeontologists to find larger fossils that are
visible to the naked eye. Some quite remarkable multicelled
animals appeared about 575 to 565 million years ago.
The world was also changing rapidly. Oxygen had appeared and
then increased in the atmosphere in two bursts, as we have seen.
The Earth might also have gone through a period of freezing,

The origin of sex

Many dozens of specimens of Bangiomorpha have been found,
and these show how the filaments developed. Starting with a
single cell, the filament grew by division of cells (mitosis) along the
filament axis. One cell divided into two, then two into four, and so
on. Along the filaments, disc-shaped cells occur in clusters of two,
four, or eight, and these reflect further cell divisions within the
filament. Some broader filaments show clusters of spherical
spore-like structures at the top end; if correctly identified, these
prove that sexual reproduction and meiosis were taking place.
Close study of the filaments, and of series of developmental stages,
shows that Bangiomorpha was not only multicellular, but it
showed differentiation of cells (holdfast cells vs. filament cells),
multiple cycles of cell division, differentiated spores, and sexually
differentiated whole plants.

called the Cryogenian, but more graphically termed ‘Snowball

The History of Life

The concept of Snowball Earth is highly controversial. There is no
doubt that much of the Earth was cold for a long time in the
Neoproterozoic: geologists had long noted evidence for glaciation
such as glacial tills (rocks ground to dust by glaciers), scratches
produced by the passage of glaciers carrying boulders, and
dropstones, rocks dropped from the bases of icebergs into marine
sediments. For many geologists, this simply showed that there had
been ice caps at the Neoproterozoic poles, but for others it meant
something quite different.
Joseph Kirschvink, a professor at the California Institute of
Technology, coined the term ‘Snowball Earth’ in 1992, and
envisaged a world that was almost completely covered with snow
from the poles to the equator. He invoked the evidence of glacial
sediments, including some examples from regions that apparently
lay near the Neoproterozoic equator, and his work was extended
and promoted by Paul Hoffman, from Harvard University, based
on his studies of Neoproterozoic successions in Namibia.
Hoffman and others have presented extensive evidence from
Neoproterozoic sediments that the Earth was entirely icebound
for millions of years, and then the ice melted during a subsequent
greenhouse phase as a result of massive volcanic eruptions with
the production of copious amounts of carbon dioxide. Advocates
of the Snowball Earth suggest that life survived under the ice,
and did not diversify greatly until melting ensued. Critics suggest
that it is impossible for the Earth to freeze over completely, and
that at least there must have been habitable oceans around the
equator. Whether the Earth was entirely or largely covered
in ice, there certainly were major glacial episodes in the
Neoproterozoic, and complex multicellular organisms appeared
only after the glacial episodes had ended. These were the Ediacara

The fossils of the Ediacara Hills
Palaeontologists had occasionally found strange frond-like
structures in rather ancient sandstones, perhaps Precambrian,
perhaps Cambrian in age, but they had been unable to interpret
them. One such finding happened in 1946, when Reginald Sprigg,
a young mining geologist, was prospecting through the Ediacara
Hills, north of Adelaide, Australia. He found round impressions
that looked like jellyfish, branching fronds, and worm-like

9. Life as it may have looked in Ediacaran times

The origin of sex

When Sprigg reported his findings, the Ediacara Hills became
famous, and the particular fossil assemblage has been called
Ediacaran; this is also the name for the time interval marking the
last part of the Neoproterozoic. Ediacaran organisms have been
reported from more than thirty localities, from Australia, Africa,
Europe, and elsewhere. The Ediacaran fossils are mostly about the
same age, some 575 to 542 million years old, and they are the first
true fauna, that is, life assemblage, of diverse complex organisms
on Earth.

The History of Life

More than a hundred species of Ediacaran animals have been
named (Fig. 9). Most of them have been classified in modern
groups, such as jellyfish, worms, and sea pens, but this is very hard
to confirm. Others have argued that the Ediacaran animals
represent a completely independent radiation of organisms that
do not link with later, Cambrian, faunas. One researcher has
identified all Ediacaran organisms as fungi, whereas Dolf
Seilacher from the University of Tübingen has argued that they are
unique structures that represent an independent diversification of
animals that resolved structural problems in ways quite unlike
anything now living. He argued that the skin must have been
flexible, although it could crease and fracture, and it must have
allowed oxygen and waste materials to diffuse in and out. The
vendobionts, as he termed them, were interpreted as unique
pneumatic structures, like car tyres or blow-up mattresses. Their
outer surfaces enclosed a gas-filled interior, and their radial and
segmented structures are like the divisions of a modern bouncy
castle or air mattress, designed to maintain strength and flexibility.
Whatever they were, whether early jellyfishes and worms, or
proto-bouncy castles, the Ediacaran faunas worldwide died out
about 540 million years ago. But their demise did not leave the
Earth devoid of life. Indeed, one of the greatest events in the
history of life was about to happen, the Cambrian Explosion.


Chapter 3
The origin of skeletons

The fossil record had caused Darwin more grief than joy. Nothing
distressed him more than the Cambrian explosion, the coincident
appearance of almost all complex organic designs.
Stephen Jay Gould, The Panda’s Thumb (1980)

The appearance of skeletons in the fossil record some 540 million
years ago has long been a puzzle. It is not perhaps such a puzzle
that scientists throw in the towel, as creationist critics gleefully
report on their websites, but a real problem to be resolved. The
fact is that, shortly after the beginning of the Cambrian period,
currently dated at 542 million years ago, and some time after the
extinction of the Ediacaran organisms, a broad diversity of
animals with skeletons appeared in the sea. A skeleton to a
biologist is any kind of mineralized, or partly mineralized,
structure that acts as a support or framework for an organism.
So our internal skeleton of bones fits the bill, but so too do the
calcareous shells of molluscs and corals, the outer cuticles of
insects and crabs, and even arguably the woody stems of trees.
The Ediacaran fossils of the Neoproterozoic did not have shells or
skeletons of any kind we would recognize today. Perhaps, as Dolf
Seilacher suggests, they had a quilted pneumatic structure that
stiffened their bodies and allowed them to reach reasonable body
size. Then, in Lower Cambrian rocks around the world, a diversity

The History of Life

of shelly fossils appears. It is the fact that skeletonized organisms
seem to appear suddenly, geologically speaking, and all at the
same time, that is the puzzle. Why, for example, don’t we first find
sponges with skeletons of spicules, then corals with their tube-like
houses, then perhaps shellfish with their encapsulating valves, and
so on? Of course, when looking back over half a billion years, it’s
not easy to date every rock formation precisely, but every study
seems to suggest a rather coordinated appearance of animals with
skeletons about 542 million years ago. This dramatic event has
been called the Cambrian Explosion.
The debate revolves around the reality of this event. Most
palaeontologists and evolutionists, including Darwin, have
suggested that the Cambrian Explosion was real and that what you
see actually happened. Others, however, urge caution and suggest
that we might be seeing something artificial, the result perhaps of
incomplete preservation of the fossils. It could be, for example,
that there are major gaps in the rock record at the end of the
Neoproterozoic, or that the sediments that were deposited
through that interval were not the right ones to preserve
mineralized skeletons. In this chapter we will explore what
skeletons are, what the fossil and rock record shows, new
molecular evidence, and the rather heated debates about whether
the Cambrian Explosion is real or not.

Skeletons are not just for physical support, although that is a
major, often the major, function. They also provide sites for the
attachment of muscles and a mineral store. So, for example, in
humans, we rely on the framework of our skeleton to be able to
walk and eat. The muscles attach at both ends to bones in the
skeleton, and muscle contractions make the arms and legs work.
In feeding, jaw muscles pull the lower jaw up and down against
the skull, and the jawbones carry the teeth, all essential in

Bone is composed of two main components, the protein collagen
and spicules of apatite, a form of calcium phosphate. Collagen is
the primary component of cartilage. We have cartilage in our
noses and ears, and it is a bendy kind of unmineralized bone.
Among living vertebrates, the backboned animals, sharks have
almost entirely cartilaginous skeletons that only occasionally
become mineralized (and of course their teeth are mineralized),
and it seems that the Cambrian predecessors of modern fishes also
mostly had cartilage skeletons.

Other animals have different kinds of skeletons. Skeletons may be
composed from inorganic mineralized materials, such as forms of
calcium carbonate, silica, phosphates, and iron oxides. Calcium
carbonate makes up the shells of microscopic foraminifera, some
sponges, corals, bryozoans (colonial creatures), brachiopods
(‘lamp shells’), molluscs, many arthropods (trilobites, crabs,
insects), and echinoderms (sea urchins, sea lilies). Silica forms the
skeletons of radiolarians (planktonic organisms) and most

The origin of skeletons

Our bones also act as mineral stores. When we are young and
growing, the body has to scavenge large amounts of calcium and
phosphorus from our food and it passes through the blood vessels
to the bones. If a person is starved at a young age, their bones
cannot grow properly, and they become stunted. Later in life,
calcium and phosphorus may be mobilized from within the bones
when they are needed. Bone is living, laced through with blood
vessels, and other tissues. If food is short, calcium and phosphorus
are absorbed from the bone back into the blood supply and passed
to the cells where it is needed. The minerals can be replaced later
when food is abundant. So if you were to cut through any of your
bones, you would see evidence for how it grew to its present size
during your childhood. You would also see evidence for episodic
extraction and replacement of calcium and phosphorus in the
form of channels that are widened as minerals are extracted, and
that fill up in layers as minerals are replaced, rather like a water
pipe furring up in an area of hard water.

The History of Life

sponges, while phosphate, usually in the form of apatite, is typical
of vertebrate bone, as we have seen, and the shells of certain
brachiopods and the tiny toothed jaw structures of certain worms.
There are also organic hard tissues, such as lignin, cellulose,
sporopollenin, and others in plants, and chitin, collagen, and
keratin in animals, which may exist in isolation or in association
with mineralized tissues.
The simplest skeletons are seen in the sponges, which are
composed of loose aggregates of spicules, pointed microscopic
structures made from calcium carbonate or silica. Most other
animals have an external skeleton, or exoskeleton. (Humans, and
other vertebrates, have an internal skeleton, or endoskeleton.) In
corals, brachiopods, and molluscs, the exoskeleton is a layered
structure, built up year by year, or month by month, with growth
lines often visible on the outer surface and in cross-sections.
Other animals shed their exoskeletons – animals such as
arthropods, nematode worms, and some rarer groups. Indeed,
skeleton-shedding may be a unique feature of this particular
The diversity of skeleton types, and the fact they are constructed in
so many different ways – some are internal, some external, some
are shed, and others are not, they may be made of different
mineral constituents – makes it hard to understand how skeletons
seemingly evolved at the same time in all these animal groups, and
everywhere in the world. What does the fossil record show us, if
we follow it step by step through the transition from the latest
Precambrian into the Cambrian?

Small Shelly Fauna
The first step is represented by the time of the ‘Small Shelly
Fauna’, so called, perhaps not surprisingly, because it is a fauna
that is composed of small shells. The term ‘small shells’, however,

hides a great deal of ignorance: small shells they may be, but the
affinities of many of them are unclear.
The Small Shelly Fauna (SSF) has been identified in the latest
Precambrian, but is best known in Lower Cambrian rocks, dating
from perhaps 542 to 530 million years ago. The importance of the
SSF is that it comes before the appearance of larger fossils with
skeletons, and so marks the first phase of the Cambrian Explosion.
It has proved very hard to understand the biology of the SSF
animals, and they are generally named simply according to their

The origin of skeletons


10. Fossils from the Early Cambrian. A: A selection of Small Shelly
Fossils from the Siberian Precambrian-Cambrian boundary strata;
B: Microdictyon

The History of Life

shapes (Fig. 10A). Two major groups are the hyolithelminthids
with phosphatic tubes, open at both ends, and the tomotiids with
phosphatic cone-shaped shells, usually occurring in pairs. Other
animals were tube-builders that secreted carbonate walls,
organic-walled tubes possibly of an unsegmented worm, and
phosphatic plates, or sclerites, from larger but unknown animals.
The sclerites give clues to a whole array of animals we barely
understand. Mostly their bodies have gone, and all we have are the
minute, microscopic leaf-shaped sclerites. It is assumed that these
fitted together as some kind of flexible armour over animals that
may have looked roughly like pine cones. Some exceptionally
preserved specimens from China, called Microdictyon, suggest
that some of the sclerite-bearers at least were worm-like animals
(Fig. 10B), which carried oval plates arranged in pairs along the
length of the body which may have provided a base for muscle
attachment associated with locomotion. What is intriguing is that
some of the sclerites might have come from quite large animals
that are otherwise entirely unknown, and may never be known
other than by these intriguing exuviae.

The Cambrian Explosion
The Small Shelly Fauna of the Early Cambrian was a precursor of
the Cambrian Explosion proper. Towards the end of the Early
Cambrian, and overlapping in time with the SSF, a dozen or more
major animal groups appeared. At one time, it was thought that
they all appeared at once, but more careful study suggests a rather
more orderly procession, with one group appearing after another.
Some of the evidence comes from fossils of the organisms
themselves, and other steps along the way are indicated at present
only by trace fossils, tracks, and trails. This may seem rather
uncertain evidence, but many tracks and trails can be quite
diagnostic of their makers, especially if they show foot or leg
marks, for example.

So, in sequence, the first evidence for the radiation of animals
in the sea, and the first step of the Cambrian Explosion, is
represented by tracks and trails dating from 555 million years
ago, at the end of the Neoproterozoic. These tracks were made by
elongate bilaterally symmetrical animals, mostly worms of one s