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Human Growth and Development AP N. Cameron 年: 2002 语言: english 文件: PDF, 20.11 MB 你的标签: 0 / 0

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Human Evolution. An Illustrated Introduction Wiley-Blackwell Roger Lewin 年: 2004 语言: english 文件: PDF, 10.42 MB 你的标签: 0 / 0

Bernard Wood

HUMAN
EVOLUTION
A Very Short Introduction

1

3

Great Clarendon Street, Oxford o x 2 6 d p
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© Bernard Wood 2005
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First published as a Very Short Introduction 2005
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Printed in Great Britain by
TJ International Ltd., Padstow, Cornwall

Contents

Acknowledgements viii
List of illustrations ix
List of tables

1
2
3
4
5
6
7
8

xi

Introduction 1
Finding our place 7
Fossil hominins: their discovery and cont; ext 24
Fossil hominins: analysis and interpretation 37
Early hominins: possible and probable 58
Archaic and transitional hominins 71
Pre-modern Homo 84
Modern Homo 100
Timeline of thought and science relevant to
human origins and evolution
Further reading 121
Index

125

116

Acknowledgements

For an author used to the luxury of lengthy academic papers and the
occasional 500-page monograph, and to the protection afforded
by technical language and multiple qualifications, boiling down
human evolutionary history to the size constraints and style of a
VSI was a considerable challenge. That it was overcome at all is in
large measure due to the contributions of Barbara Miller, senior
co-author with me of Anthropology (Allyn & Bacon, 2006). The
clarity of the writing and many of the ideas in the VSI are the result
of our collaboration. Thanks go to Mark Weiss and Matthew
Goodrum for much valued advice about, respectively, genetics and
the history of human origins research, to Monica Ohlinger for
advice about style, my colleague, Robin Bernstein, my OUP editor,
Marsha Filion, and to an anonymous reviewer, for reading the
entire manuscript and making valuable suggestions for revisions.
Graduate students in the Hominid Paleobiology program at George
Washington University and my program assistant, Phillip Williams,
wittingly and unwittingly contributed by providing information,
helping me find ‘lost’ files and notes. I am grateful to several
publishers, notably Allyn & Bacon, for allowing me to adapt and use
previously published images and figures. This book is for my family
and my teachers, living and dead, young and old.

List of illustrations

1

The vertebrate part of the
Tree of Life
2
© Bernard Wood

2 Diagram showing how
progress can be made
in palaeoanthropology
research
4
© Bernard Wood

3 C. K. (Bob) Brain
demonstrating the
complex stratigraphy at
Swartkrans
29

5 Plot of oscillations in
oxygen isotope levels
during the past six million
years
36
http://delphi.esc.cam.ac.uk/
coredata/v677846.html

6 The two main hypotheses
for evolution: ‘phyletic
gradualism’ and
‘punctuated
equilibrium’
45
Adapted from Miller and Wood,
Anthropology (Allyn & Bacon)

© Bernard Wood

4 Some of the methods
used to date fossil
hominins

33

Adapted from C. Stanford, J. S.
Allen, and S. Antón, Biological
Anthropology p. 250
(Pearson/ Prentice Hall, 2005)

7 Comparison of the
concepts of clades and
grades as applied to
living higher primates 52
© Bernard Wood

8 ‘Lumping/simple’ (A) and
‘splitting/complex’
(B) interpretations of
the higher primate twig
of the Tree of Life
62
© Bernard Wood

9 Time chart of ‘possible’
and ‘probable’ early
hominin species
64

14

Adapted with permission from
Miller and Wood, Anthropology
p. 197 (Allyn & Bacon)

Adapted with permission from
Miller and Wood, Anthropology
p. 179 (Allyn & Bacon)

15
10

Map of Africa showing the
main early and archaic
hominin fossil sites
67
Adapted with permission from
Miller and Wood, Anthropology
p. 179 (Allyn & Bacon)

11

Reconstruction of the
skeleton of ‘Lucy’
73
(AL 288) by Peter Schmid of
the Anthropological Institute
of Zurich

12

Time chart of ‘archaic’
and ‘transitional’ hominin
species
80
Adapted with permission from
Miller and Wood, Anthropology
p. 179 (Allyn & Bacon)

13

Time chart of ‘premodern’ Homo species 91

Map of major
Neanderthal sites

94

Adapted with permission from
Miller and Wood, Anthropology
p. 209 (Allyn & Bacon)

16

The ‘strong’ and ‘weak’
versions of the
multiregional and recent
out of Africa models for
the origin of modern
Homo
102
Adapted from L. Aiello, ‘The
Fossil Evidence for Modern
Human Origins in Africa:
A Revised View’, American
Anthropologist, 95/1 (1993),
73–96

Map of the main ‘archaic’,
‘transitional’ and
‘pre-modern’ Homo
sites
88
Adapted with permission from
Miller and Wood, Anthropology
p. 197 (Allyn & Bacon)

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 opportunity.

List of tables

1

A traditional taxonomy (A) and a modern taxonomy (B) that
take account of the molecular and genetic evidence that
chimpanzees are more closely related to modern humans than
they are to gorillas 22

2

Two taxonomic hypotheses, one ‘splitting’ and one ‘lumping’,
for the hominin fossil record 47

3

Major differences between the skeletons of a modern human
and a living chimpanzee 60

4

The main morphological and behavioural differences between
modern humans and Neanderthals 110

Chapter 1
Introduction

Many of the important advances made by biologists in the past
150 years can be reduced to a single metaphor. All living, or extant,
organisms, that is, animals, plants, fungi, bacteria, viruses, and
all the types of organisms that lived in the past, are situated
somewhere on the branches and twigs of an arborvitae or Tree
of Life.
We are connected to all organisms that are alive today, and all the
organisms that have ever lived, via the branches of the Tree of Life
(TOL). The extinct organisms that lie on the branches that connect
us to the root of the tree are our ancestors. The rest, on branches
that connect directly with our own, are closely related to modern
humans, but they are not our ancestors.
The ‘long’ version of human evolution would be a journey that starts
approximately three billion years ago at the base of the TOL with
the simplest form of life. We would then pass up the base of the
trunk and into the relatively small part of the tree that contains all
animals, and on into the branch that contains all the animals with
backbones. Around 400 million years ago we would enter the
branch that contains vertebrates that have four limbs, then around
250 million years ago into the branch that contains the mammals,
and then into a thin branch that contains one of the subgroups
of mammals called the primates. At the base of this primate
1

Human Evolution

1. A diagram of the vertebrate part of the Tree of Life emphasizing the
branches that led to modern humans

branch we are still at least 50–60 million years away from the
present day.
The next part of this ‘long’ version of the human evolutionary
journey takes us successively into the monkey and ape, the ape and
then into the great ape branches of the Tree of Life. Sometime
between 15 and 12 million years ago we move into the small branch
that gave rise to contemporary modern humans and to the living
African apes. Between 11 and 9 million years ago the branch for the
2

gorillas split off to leave just a single slender branch consisting of
the ancestors of both extant (i.e. living) chimpanzees and modern
humans. Around 8 to 5 million years ago this very small branch split
into two twigs. One of the twigs ends on the surface of the TOL with
the living chimpanzees, the other leads to modern humans.
Palaeoanthropology is the science that tries to reconstruct the
evolutionary history of this small, exclusively human, twig.

This Very Short Introduction has three objectives. The first is to try
and explain how paleoanthropologists go about the task of
improving our understanding of human evolutionary history. The
second is to convey a sense of what we think we know about human
evolutionary history, and the third is to try to give a sense of where
the major gaps in our knowledge are.
We use two main strategies to improve our understanding of
human evolutionary history. The first is to obtain more data. You
can get more data by finding more fossils, or by extracting more
information from the existing fossil evidence. You can find more
fossils from existing sites, or you can look for new sites. You can
extract more information from the existing fossil record by using
techniques such as confocal microscopy and laser scanning to make
more precise observations about their external morphology. You can
also gather information about the internal morphology and
3

Introduction

This book focuses on the last stage of the human evolutionary
journey, the part between the most recent common ancestor shared
by chimpanzees and humans and present-day modern humans. To
understand this we need to use some scientific jargon. So instead of
referring to ‘twigs’ we need to use the proper biological term ‘clade’:
extinct side branches are called ‘subclades’. Species anywhere on the
main human twig, or on its side branches, are called ‘hominins’; the
equivalent species on the chimp twig are called ‘panins’. And
instead of writing out ‘millions of years’ and ‘millions of years ago’
(and the equivalents for thousands of years) we will use instead the
abbreviations ‘MY ’ and ‘MYA’ and ‘KY ’ and ‘KYA’.

biochemistry of fossils. This ranges from using non-invasive
medical imaging techniques such as computed tomography to
obtain information about structures like the inner ear, to using new
types of microscopes to investigate the microscopic anatomy of
teeth, and the latest molecular biology technology to detect small
amounts of DNA in fossils.

Human Evolution

The second strategy for reducing our ignorance about human
evolutionary history is to improve the ways we analyse the data we
do have. These improvements range from more effective statistical
methods to the use of novel methods of functional analysis.
Researchers also try to improve the ways they generate and test
hypotheses about the numbers of species in the hominin fossil
record, and about how those species are related to each other and
to modern humans and chimpanzees.

2. Diagram showing how progress can be made in palaeoanthropology
research
4

I begin Chapter 2 by reviewing the history of how philosophers and
then scientists came to realize that modern humans are part of the
natural world. I then explain why scientists think chimpanzees are
more closely related to modern humans than they are to gorillas,
and why they think the chimp/human common ancestor lived
between 8 and 5 MYA.

In Chapter 5 I consider ‘possible’ and ‘probable’ early hominins. The
chapter reviews four collections of fossils that represent each of the
‘candidate’ taxa that have been put forward for being at the very
base of the hominin clade. Then in Chapter 6 I look at ‘archaic’ and
‘transitional’ hominins. These are fossil taxa that almost certainly
belong to the hominin clade, but which are still a long way from
being like modern humans. Chapter 7 looks at hominins
researchers believe might be the earliest members of the genus
Homo: we call these ‘pre-modern’ Homo. I look at the earliest fossil
evidence of pre-modern Homo from Africa, and then follow Homo
as it moves out of Africa into the rest of the Old World.
Chapter 8 considers evidence about the origin and subsequent
migrations of anatomically modern humans, or Homo sapiens.
When and where do we find the earliest fossil evidence of
anatomically modern humans? Did the change from pre-modern
Homo to anatomically modern humans happen several times and in
5

Introduction

In Chapter 3 I review the lines of evidence that can be used to
investigate what the 8–5 MY-old hominin clade looks like. Is it
‘bushy’, or straight like the stem of a thin spindly plant? How
much of it can be reconstructed by looking at variation in modern
humans, and what needs to be investigated by searching for,
finding, and then interpreting fossil and archaeological evidence?
Where do researchers look for new fossil sites, and how do they date
the fossils they find? In Chapter 4 I explain how researchers decide
how many species there are within the hominin clade. I also review
the methods researchers use to determine how many hominin
subclades there are, and how they are related to one another.

several different regions of the world? Or did anatomically modern
humans emerge just once, in one place, and then spread out, either
by migration or by interbreeding, so that modern humans
eventually replaced regional populations of pre-modern Homo?

Human Evolution

Finally, what will not be in this book? This Very Short Introduction
to ‘Human Evolution’ will concentrate on the physical and not the
cultural aspects of human evolution. The latter, often referred to as
‘Prehistoric Archaeology’, is the topic of a separate Very Short
Introduction called ‘Prehistory’.

6

Chapter 2
Finding our place

Long before researchers began to accumulate material evidence
about the many ways modern humans resemble other animals,
and long before Charles Darwin and Gregor Mendel laid the
foundations of our understanding of the principles and
mechanisms that underlie the connectedness of the living world,
Greek scholars had reasoned that modern humanity was part of,
and not apart from, the natural world. When did the process of
using reason to try and understand human origins begin, and how
did it develop? When was the scientific method first applied to
the study of human evolution?
Plato and Aristotle in the 5th and 6th bce provide the earliest
recorded ideas about the origin of humanity. These early Greek
philosophers suggested that the entire natural world, including
modern humans, forms one system. This means that modern
humans must have originated in the same way as other animals.
The Roman philosopher Lucretius, writing in the 1st century bce,
proposed that the earliest humans were unlike contemporary
Romans. He suggested that human ancestors were animal-like
cave dwellers, with neither tools nor language. Both classical
Greek and Roman thinkers viewed tool and fire making and the
use of verbal language as crucial components of humanity. Thus, the
notion that modern humans had evolved from an earlier, primitive
form was established early on in Western thought.
7

Reason is replaced by faith

Human Evolution

After the collapse of the Roman Empire in the 5th century
Graeco-Roman ideas about the creation of the world and of
humanity were replaced with the narrative set out in Genesis:
reason-based explanations were replaced by faith-based ones.
The main parts of the narrative are well known. God created
humans in the form of a man, Adam, and then a woman, Eve.
Because they were the result of God’s handiwork Adam and Eve
must have come equipped with language and with rational and
cultured minds. According to this version of human origins, the
first humans were able to live together in harmony, and they
possessed all the mental and moral capacities that, according to
the biblical narrative, set humanity above and apart from other
animals.
The biblical explanation for the different races of modern humans is
that they originated when Noah’s offspring migrated to different
parts of the world after the last big biblical flood, or deluge. The
Latin for ‘flood’ is diluvium, so we call anything very old
‘antediluvial’, or dating from ‘before the flood’. Explanations for
the creation of the living world involving successive floods had
implications for the science that was to become known as
palaeontology. All the animals created after a flood must inevitably
perish at the time of the next flood. Thus ‘antediluvial’ animals
should never coexist with the animals that replaced them. We will
return to this and other implications of diluvialism later in this
chapter.
The Bible also has an explanation for the rich variety of human
languages. It suggests that God wanted to promote confusion
among the people constructing the tower of Babel, and that he did
so by creating mutually incomprehensible languages. In the Genesis
version of human origins, the Devil’s successful temptation of Adam
and Eve in the Garden of Eden forced them and their descendants
8

to learn afresh about agriculture and animal husbandry. They had
to reinvent all the tools needed for civilized life.
With very few exceptions Western philosophers living in and
immediately after the Dark Ages (5th to 12th centuries) supported a
biblical explanation for human origins. This changed with the
rediscovery and rapid growth of natural philosophy that was only
later called science. But, paradoxically, not long after the scientific
method began to be applied to the study of human origins in the
19th and 20th centuries some religious groups responded to
attempts by scientists to interpret the Bible less literally by being
even stricter about their biblical literalism. This reaction was the
origin of creationism, and of what, erroneously, is called ‘Creation
Science’.

Science re-emerges
The move away from reliance on biblical dogma was especially
important for those who were interested in what we now call the
natural sciences, such as biology and the earth sciences. An
Englishman, Francis Bacon, was a major influence on the way
scientific investigations developed. Theologians use the deductive
9

Finding our place

During the Dark Ages very few Greek classical texts survived in
Europe. The few that did survive were read and valued by Muslim
philosophers and scholars, and some of them were translated into
Arabic. When the Muslims were driven out of Spain in the 12th
century, a few medieval Christian scholars were curious enough to
translate these manuscripts from Arabic into Latin. Some of these
translated texts dealt with the natural world, including human
origins. For example, the 13th-century Italian Christian
philosopher, Thomas Aquinas, integrated Greek ideas about nature
and modern humans with some of the Christian interpretations
based on the Bible. The work of Thomas Aquinas and his
contemporaries laid the foundations of the Renaissance, when
science and rational learning were reintroduced into Europe.

Human Evolution

method: beginning with a belief, they then deduce the
consequences of that belief. Bacon suggested that scientists should
work in a different way he called the ‘inductive’ method. Induction
begins with observations, also called evidence or ‘data’. Scientists
devise an explanation, called a ‘hypothesis’, to explain those
observations. Then they test the hypothesis by making more
observations, or in sciences like chemistry, physics and biology, by
conducting experiments. This inductive way of doing things is the
way the sciences involved in human evolution research are meant to
work.
Bacon summarized his suggestions about how the world should be
investigated in aphorisms, and set these out in his book called the
Novum Organum or True Suggestions for the Interpretation of
Nature, published in 1620. His message was a simple one. Do not be
content with reading about an explanation in a book. Go out, make
observations, investigate the phenomenon for yourself, then devise
and test your own hypotheses.

Anatomy starts to become scientific
Nearly three-quarters of a century before Bacon published this
advice, a major change had already occurred in anatomy, the
natural science closest to the study of human evolution. That
change was the work of Andreas Vesalius. Born in 1514 in what is
now Belgium, Vesalius finished his medical studies in 1537. In the
same year he was appointed to teach anatomy and surgery in Padua,
Italy.
Vesalius’ own anatomy education was typical for the time. The
professor sat in his chair (hence professorships are called ‘chairs’)
and read out loud from the only locally available textbook. He sat at
a safe distance from a human body that was being dissected by his
assistant. It did not take long for Vesalius to realize that he and his
fellow students were being told one thing by their professor, and
were being shown something else by the professor’s assistant. In
10

1540 Vesalius visited Bologna where, for the first time, he was able
to compare the skeletons of a monkey and a human. He realized the
textbooks used by his professors were based on a confusing mixture
of human, monkey, and dog anatomy, so he resolved to write his
own, accurate, human anatomy book. The result, the seven-volume
De Humani Corporis Fabrica Libri Septem, or ‘On the Fabric of the
Human Body’, was published in 1543. Vesalius performed the
dissections and sketched the drafts of the illustrations: the Fabrica
is one of the great achievements in the history of biology. Vesalius’
successful efforts to make anatomy more rigorous ensured that
scientists would have access to reliable information about the
structure of the human body.

Geology emerges

The development of geology was substantially influenced by the
Industrial Revolution. The excavations involved in making
‘cuttings’ for canals and railroads gave amateur geologists the
11

Finding our place

Another field of science relevant to the eventual study of human
origins, geology (now usually referred to as ‘earth science’),
developed more gradually than anatomical science. One of the
implications of interpreting the Genesis narrative literally is that
the world, and therefore humanity, cannot have had a long history.
There is a long tradition of biblically based chronologies, beginning
with people like Isidore of Seville and the Venerable Bede in the
6th and 7th centuries, respectively. The one cited most often was
published in 1650 by James Ussher, then archbishop of Armagh in
Ireland. He used the number of ‘begats’ in the Book of Genesis to
calculate the precise year of the act of Creation, which, according to
his arithmetic, was in 4004 bc. Subsequently another theologian
John Lightfoot, of Cambridge University, England, refined Ussher’s
estimate and declared that the act of Creation took place precisely
at 9 a.m. on 23 October 4004 bce. Geology, and especially the work
of James Hutton, provided an alternative calendar, suggesting the
earth and its inhabitants were substantially older than this.

Human Evolution

opportunity to see previously hidden rock formations. Pioneer
geologists such as William Smith and James Hutton paved the way
for Charles Lyell in 1830 to set out a rational version of the history
of the earth in The Principles of Geology. Lyell’s book influenced
many scientists, including Charles Darwin, and it helped establish
fluvialism and uniformitarianism as alternatives to biblically based
diluvial explanations for the state of the landscape. Fluvialism
suggested that erosion by rivers and streams had reduced the height
of mountains and created valleys and thus played a major role in
shaping the contours of the earth. Uniformitarianism suggested
that the processes that shaped the earth’s surface in the past, such
as erosion and volcanism, were the same processes we see in action
today. Lyell also championed the principle that rocks and strata
generally increase in age the further down they are in any relatively
simple geological sequence. Barring major and obvious upheavals
and deliberate burial, the same principle must apply to any fossils or
stone tools contained within those rocks. The lower in a sequence of
rocks a fossil is, the older it is likely to be.
The implications of the new science of geology were profound.
There was no need to invoke the biblical floods or divine
intervention to explain the appearance of the earth. The pioneer
geologists of the time also suggested that it would have taken the
processes that are shaping the earth’s surface today a lot longer than
the 6,000 years implied by the Genesis narrative to make the
changes the pioneer geologists had observed.

Fossils
Classical Greek and Roman writers had recognized the existence of
fossils but they mostly interpreted them as remnants of the ancient
monsters that figure prominently in their myths and legends. By the
18th century geologists began to accept that life-like structures in
rocks were the remains of extinct animals and plants, and that there
was no need to invoke supernatural reasons for their existence. The
association of the fossil evidence of exotic extinct animals with
12

creatures closely related to living forms in the same strata effectively
refuted the diluvial theory, for as I noted earlier in the chapter the
latter does not allow for any mixing of modern and ancient, or
antediluvial, animals.

A catalogue of life
The same explorers and traders who had returned to Europe with
tales of the behaviour of primitive people also brought back
descriptions and sometimes suitably preserved specimens of many
exotic plants and animals. When these discoveries were added to
the more familiar plants and animals from Europe, they made for a
perplexing array of plant and animal life. The living world badly
needed a system for describing and organizing it. Several schemes
were put forward, notably one by John Ray who introduced the
concept of the species. However, the one that has stood the test
of time was devised by a Swede called Karl von Linné, a name we
know better in its Latinized form, Carolus Linnaeus.
Classification schemes try to group similar things together in
increasingly broad, or inclusive, categories. Think of the following
example of a classification of automobiles. It has seven levels, or
categories; it begins with the most inclusive category and ends
with a small group. The levels are ‘Vehicles’, ‘Powered Vehicles’,
13

Finding our place

In addition to the important conclusions reached by pioneer
geologists about the history of the earth, several other factors
influenced 17th- and 18th-century scientists to consider alternatives
to the Genesis account of human origins. Explorers were returning
from distant lands with eye-witness accounts of modern humans
living in crude shelters, using simple tools, and existing by hunting
and gathering. This was so far from the state of humanity in their
homeland that European travellers described the people they
observed as living in a state of ‘savagery’. According to the Genesis
narrative, no human beings created by God should be living in
such a state.

Human Evolution

‘Automobile’, ‘Luxury Car’, ‘Rolls-Royce’, ‘Silver Shadow’, and ‘1970
Silver Shadow II’. The Linnaean classification system also
recognizes seven basic levels. The most inclusive category, the
equivalent of ‘Vehicles’ in our example, is the kingdom, followed
by the phylum, class, order, family, genus, with the species being
the smallest, least inclusive, formal category. Linnaeus’ original
seven-level system has been expanded by adding the category
‘tribe’ between the genus and family, and by introducing the prefix
super- above a category, and the prefixes sub- and infra-, below it.
These additions increase the potential number of categories below
the level of order to a total of 12.
The groups recognized at each level in the Linnaean hierarchy are
called ‘taxonomic groups’. Each distinctive group is called a ‘taxon’
(pl. ‘taxa’). Thus, the species Homo sapiens is a taxon, and so is the
order Primates. When the system is applied to a group of related
organisms, the scheme is called a Linnaean taxonomy, usually
abbreviated to a taxonomy. The Linnaean taxonomic system is also
known as the binomial system because two categories, the genus
and species, make up the unique Latinized name (e.g. Homo
sapiens = modern humans; Pan troglodytes = chimpanzees) we
give to each species.
You can abbreviate the name of the genus, but not the species. So
you can write H. sapiens and P. troglodytes, but not Homo s. or
Pan t., as there can sometimes be more than one species name in
that genus that begins with the same first letter, such as Homo
sapiens and Homo soloensis.

Evidence of connections
Trees are common metaphors. In religion, for example in
Christianity, the Great Chain of Being is sometimes represented as a
tree. Modern humans are on top of the tree, with other living
animals placed within the tree at heights corresponding to their
level of complexity. However, in contemporary life sciences the Tree
14

of Life is not a metaphor: it is taken more literally. In a modern
scientific Tree of Life the relative size of the part of the tree given
over to any particular group of living things reflects the number of
taxa, and the pattern of branching within the tree reflects the way
scientists think plants and animals are related.

Proteins are the basis of the machinery that makes other molecules,
like sugars and fats, and ultimately the tissues that make up the
components of our bodies, such as muscles, nerves, bones and teeth.
In 1953 James Watson and Francis Crick, with the help of Rosalind
Franklin, discovered that the nature of proteins, the building blocks
of our bodies, is determined by the details of a molecule called DNA
(short for deoxyribose nucleic acid). Scientists have shown since
that DNA transmitted from parents to their offspring contains
coded instructions, called the genetic code. This, in large measure,
determines what the bodies of those offspring will look like. These
developments in molecular biology meant that instead of working
out how species are related by comparing traditional morphology,
or by looking at the morphology of protein molecules, scientists
could determine relationships by comparing the DNA that dictates
the structure and shape of proteins.
15

Finding our place

When the first science-based Trees of Life were constructed in the
19th century, the closeness of the relationship between any two
animals had to be assessed using morphological evidence that could
be studied with the naked eye or with a conventional light
microscope. The assumption was that the larger the number of
shared structures the closer their branches will be within the TOL.
Developments in biochemistry during the first half of the 20th
century meant that, in addition to this traditional morphological
evidence, scientists could use evidence about the physical
characteristics of molecules. The earliest attempts to use
biochemical information for determining relationships used
protein molecules found on the surface of red blood cells and in
plasma. Both these lines of evidence emphasized the closeness of
the relationship between modern humans and chimpanzees.

Human Evolution

When these methods, first traditional anatomy, then the
morphology of protein molecules, and finally the structure of DNA
(the details of how DNA is compared are given below) were applied
to more and more of the organisms in the Tree of Life it became
apparent that animal species that were similar in their anatomy also
had similar molecules and similar genetic instructions. Researchers
have also shown that, even though the wing of an insect, and the
arm of a primate look very different, the same basic instructions are
used during their development. This is additional compelling
evidence that all living things are connected within a single Tree of
Life. The only explanation for this connectedness that has
withstood scientific scrutiny is evolution; the only mechanism
for evolution that has withstood scientific scrutiny is natural
selection.

Evolution – an explanation for the Tree of Life
Evolution means gradual change. In the case of animals this usually
(but not always) means a change from a less complex animal to a
more complex animal. We now know that most of these changes
occur during speciation, which is when an ‘old’ species changes
quite rapidly into a ‘new’, different, species. Although the Greeks
were comfortable with the idea that the behaviour of an animal
could change, they did not accept that the structure of animals,
including humans, had been modified since they were
spontaneously generated. Indeed Plato championed the idea that
living things were unchanging, or immutable, and his opinions
influenced philosophers and scientists until the middle of the
19th century.
A French scientist, Jean Baptiste Lamarck, in his Philosophie
Zoologique published in 1809, set out the first scientific explanation
for the Tree of Life. In the English-speaking world Lamarck’s ideas
were popularized in an influential book called Vestiges of the
Natural History of Creation (1844). We know that Vestiges
influenced the two men, Charles Darwin and Alfred Russel Wallace,
16

who, independently, hit upon the concept that the main mechanism
driving evolution was natural selection.

Charles Darwin made two seminal contributions to evolutionary
science. The first was the recognition that no two individual animals
are alike: they are not perfect copies. Darwin’s other related
contribution was the idea of natural selection. In a nutshell, natural
selection suggests that, because resources are finite, and because of
random variation, some individuals will be better than others at
accessing those resources. That variant will then gain enough of an
advantage that it will produce more surviving offspring than other
individuals belonging to the same species. Biologists refer to this
advantage as an increase in an animal’s ‘fitness’. Darwin’s notebooks
are full of evidence about the effectiveness of the type of artificial
17

Finding our place

Charles Darwin’s contributions to science did not include the idea
of evolution. What Darwin contributed was a coherent theory about
the way evolution could work. As we will see, Darwin’s theory of
natural selection accounts for both the diversity and the branching
pattern of the Tree of Life. Other books that influenced Darwin’s
thinking were Robert Malthus’s Essay on the Principle of
Population (1798) and Charles Lyell’s Principles of Geology.
Malthus stressed that resources are finite and this suggested to
Darwin that imbalances between the resources available and the
demand for them might be the driving force behind the selection
needed to make evolution happen. Lyell’s fluvial explanation for the
evolution of the surface of the earth was much like the gradual
morphological change that Darwin suggested was responsible for
the modification of existing species to produce new ones. Darwin
was also goaded into action by the work and philosophy of William
Paley. Paley was a champion of the notion that animals were so well
adapted for their habitat that this cannot have been due to chance.
He suggested that they must have been designed, and if so there
must be a designer, and that the designer must have been God.
Paley provoked Darwin to think about an alternative to the former’s
creationist interpretations.

selection used by animal and plant breeders. Darwin’s genius was to
think of a way that the same process could occur naturally.
Selection, and thus evolution, will only work if, in the case of natural
selection, the offspring of a mating faithfully inherits the feature, or
features, that confer(s) greater genetic fitness. What Darwin did not
realize (nor for that matter did any other prominent contemporary
biologist) was that while he was putting the finishing touches to the
Origin of Species, the genetic basis of variation and the essential
rules of inheritance were being painstakingly worked out in a
monastery garden in Brno, in what is now the Czech Republic.

Human Evolution

The flowering of genetics
The discipline of genetics was established on the basis of deductions
made by Gregor (this was his Augustinian monastic name, his
original forename was Johann) Mendel about the collection of
artificially bred pea plants he maintained in the garden of his
monastery. Mendel presented the results of his breeding
experiments to the Natural Science Society in Brno in 1865, but he
did not use the terms gene (meaning the smallest unit of heredity)
or genetics. The word gene was not coined until 1909, nine years
after Mendel’s pioneering experiments came to the notice of
evolutionary scientists. It was Mendel’s good fortune that his
various plant breeding experiments provided several examples of a
simple one-to-one link between a gene and a trait – these are called
single gene, or ‘monogenic’, effects.
Mendel’s simple dichotomies, yellow or green, smooth or wrinkled,
are called ‘discontinuous’ variables. In primate and hominin
paleontology we normally have to deal with ‘continuous’ variables
such as the size of a tooth, or the thickness of a limb bone. These
have smooth, curved, distributions, not the neat columns that result
from Mendel’s data. How do you get continuous curves from
discontinuous columns of data? The answer is that many genes are
involved in determining the size of a tooth, or the thickness of a
18

limb bone, so that what looks like a curve is in reality the
combination of many sets of columns.

Our closest relatives

Among the tales of exotic animals brought home by explorers and
traders were descriptions of what we now know as the great apes,
that is, chimpanzees and gorillas from Africa, and orangutans from
Asia. Aristotle referred to ‘apes’ as well as to ‘monkeys’ and
‘baboons’ in his Historia animalium (literally the ‘History of
Animals’), but his ‘apes’ were the same as the ‘apes’ dissected by the
early anatomists, which were short-tailed macaque monkeys from
North Africa.
One of the first people to undertake a systematic review of the
differences between modern humans and the chimpanzee and
gorilla was Thomas Henry Huxley. In an essay entitled ‘On the
relations of Man to the Lower Animals’ that formed the central
section of his 1863 book called Evidence as to Man’s Place in
Nature, he concluded the anatomical differences between modern
humans and the chimpanzee and gorilla were less marked than the
differences between the two African apes and the orangutan.
19

Finding our place

Not so long ago a book on human origins would have devoted a
substantial number of pages to descriptions of the fossil evidence
for primate evolution. This was in part because it was assumed that
at each stage of primate evolution one of the fossil primates would
have been recognizable as the direct ancestor of modern humans.
However, we now know that for various reasons many of these taxa
are highly unlikely to be ancestral to living higher primates. Instead,
this account will concentrate on what we know of the evolution and
relationships of the great apes. It will review how long Western
scientists have known about the great apes, and it will show how
ideas about their relationships to each other, and to modern
humans, have changed. It will also explore which of the living apes
is most closely related to modern humans.

Human Evolution

Darwin used this evidence in his The Descent of Man published in
1871 to suggest that, because the African apes were morphologically
closer to modern humans than to the only great ape known from
Asia, the ancestors of modern humans were more likely to be found
in Africa than elsewhere. This deduction played a critical role in
pointing most researchers towards Africa as a likely place to find
human ancestors. As we will see in the next chapter, those who
considered the orangutan our closest relative looked to South-East
Asia as the most likely place to find modern human ancestors.
Developments in biochemistry and immunology during the first
half of the 20th century allowed the search for evidence about the
nature of the relationships between modern humans and the apes
to be shifted from traditional morphology to the morphology of
molecules. The earliest attempts to use proteins to determine
primate relationships were made just after the turn of the century,
but the first results of a new generation of analyses were reported in
the early 1960s. The famous US biochemist Linus Pauling coined
the name ‘molecular anthropology’ for this area of research. Two
reports, both published in 1963, provided crucial evidence. Emile
Zuckerkandl, another pioneer molecular anthropologist, described
how he used enzymes to break up the protein haemoglobin from
blood red cells into its peptide components, and that when he
separated them using a small electric current, the patterns made by
the peptides from a modern human, a chimpanzee, and a gorilla
were indistinguishable. The second contribution was by Morris
Goodman, who has spent his life working on molecular
anthropology, who used techniques borrowed from immunology to
study samples of a serum (serum is what is left after blood has
clotted) protein called albumin taken from modern humans, apes,
and monkeys. He came to the conclusion that the albumins of
modern humans and chimpanzees were so alike in their structure
that you cannot tell them apart.
Proteins are made up of a string of amino acids. In many instances
one amino acid may be substituted for another without changing
20

the function of the protein. In the 1960s and 1970s Vince Sarich
and Allan Wilson, two Berkeley biochemists interested in primate
and human evolution, exploited these minor variations in protein
structure in order to determine the evolutionary history of the
molecules, and therefore, presumably, the evolutionary history of
the taxa being sampled. They, too, concluded that modern humans
and the African apes were very closely related.

Interrogating the genome

Sequencing methods have been applied to living hominoids and the
number of studies increases each year. The genomes of several
modern humans and a few chimpanzees have been sequenced.
Information from both nuclear and mtDNA suggest that modern
humans and chimpanzees are more closely related to each other
than either is to the gorilla. When these differences are calibrated
using the ‘best’ palaeontological evidence for the split between the
apes and the Old World Monkeys, and if we assume that the DNA
differences are neutral, the prediction is that the hypothetical
ancestor of modern humans and the chimpanzee lived between 8
and 5 MYA. When other, older, calibrations are used, the predicted
date for the split is somewhat older (e.g. >10 MYA).

21

Finding our place

The discovery of the chemical structure of the DNA molecule meant
that affinities between organisms could be pursued at the level of
the genome. This potentially eliminated the need to rely on
morphology, be it traditional anatomy or the morphology of
proteins, for information about relatedness. Now, instead of using
proxies researchers can study relatedness by comparing DNA. The
DNA within the cell is located either within the nucleus as nuclear
DNA, or within organelles called mitochondria in mtDNA. In DNA
sequencing the base sequences of each animal are determined and
then compared.

Implications for interpreting the human fossil record

Human Evolution

The results of recent morphological analyses of both skeletal and
dental anatomy, and the anatomy of the soft tissues such as muscles
and nerves, are also consistent with the very strong DNA evidence
that chimpanzees are closer to modern humans than they are to
gorillas. But some attempts to use the type of traditional
morphological evidence that is conventionally used to investigate
relationships among fossil hominin taxa did not find a particularly
close relationship between modern humans and chimpanzees.
Instead, chimpanzees clustered with gorillas.
This has important implications for researchers who investigate the
relationships among hominin taxa. They either need to use types
of information about skulls, jaws, and teeth that are capable of
confirming the close relationship between chimps and modern
humans, or they need to find other sources of morphological
evidence, such as information about the shape of the limb bones,
and see if those data are capable of recovering the relationships
among living higher primates supported by the DNA evidence.

Table 1. A traditional taxonomy (A) and a modern taxonomy (B) that
take account of the molecular and genetic evidence that chimpanzees
are more closely related to modern humans than they are to gorillas:
extinct taxa are in bold type © Bernard Wood

A. Superfamily Hominoidea (hominoids)
Family Hylobatidae (hylobatids)

Genus Hylobates
Family Pongidae (pongids)

Genus Pongo
Genus Gorilla
Genus Pan
22

Family Hominidae (hominids)

Subfamily Australopithecinae (australopithecines)
Genus Ardipithecus
Genus Australopithecus
Genus Kenyanthropus
Genus Orrorin
Genus Paranthropus
Genus Sahelanthropus
Subfamily Homininae (hominines)
Genus Homo

B. Superfamily Hominoidea (hominoids)
Family Hylobatidae (hylobatids)

Genus Hylobates

Subfamily Ponginae (pongines)
Genus Pongo
Subfamily Gorillinae (gorillines)
Genus Gorilla
Subfamily Homininae (hominines)
Tribe Panini (panins)
Genus Pan
Tribe Hominini (hominins)
Subtribe Australopithecina (australopiths)
Genus Ardipithecus
Genus Australopithecus
Genus Kenyanthropus
Genus Orrorin
Genus Paranthropus
Genus Sahelanthropus
Subtribe Hominina (hominans)
Genus Homo

23

Finding our place

Family Hominidae (hominids)

Chapter 3
Fossil hominins:
their discovery and context

As explained in Chapter 1, a hominin is the label we give to
anatomically modern humans and all the extinct species on, or
connected to, the modern human twig of the Tree of Life. In this
chapter I discuss what the hominin fossil record consists of, how
it is discovered and how it and its context are investigated.

The hominin fossil record
A fossil is a relic or trace of a former living organism. Only a tiny
fraction of living organisms survive as fossils, and until people were
buried deliberately, this also applied to hominins. We are almost
certain that the fossils that do survive are a biased sample of the
original population, and I discuss the implications of this in more
detail in the next chapter. Fossils are usually, but not always,
preserved in rocks. Scientists recognize two major categories of
fossils. The smaller category, trace fossils, includes footprints, like
the 3.6 MY-old footprints from Laetoli in Tanzania that I discuss
in Chapter 6, and coprolites (fossilized faeces). The larger category,
true fossils, consists of the actual remains of animals or plants. In
the hominin fossil record they so outnumber trace fossils that when
we use the word fossil it will normally apply to true fossils. Animal
fossils usually consist of the hard tissues such as bones and teeth.
This is because hard tissues are more resistant to being degraded
than are soft tissues such as skin, muscle or the gut. Soft tissues are
only preserved in the later stages of the hominin fossil record: for
24

example, the Bog People found in Denmark and elsewhere in
Europe.

Fossilization

For its hard tissues to be preserved as fossils, the bones and teeth of a
dead hominin would need to have been covered quickly by silt from
a stream, by sand on a beach, or by soil washed into a cave. This
protects the prospective fossil from further degradation and allows
fossilization to take place. Fossilization of a bone begins when
chemicals from the surrounding sediments replace the organic
material in the hard tissues. Later on, chemicals begin to replace the
inorganic material in bones and teeth. These replacement processes
proceed for many years, and in this way a bone turns into a fossil.
Fossils are essentially bone- or tooth-shaped rocks. In the meantime
the sediments that surround the fossil are themselves being
converted into rock. Teeth are already hard and durable in life, but
chemical replacement also occurs in teeth.
Diagenesis is the word scientists use to describe all the changes that
occur to bones and teeth during fossilization. Fossils from different
sites, and even fossils from different parts of the same site, show
different degrees of fossilization because of small-scale differences
in their chemical environment. When fossils are preserved in hard
rocks, and when they are freshly exposed, the fossils are very
durable. However, if it is exposed to erosion by wind and rain for
25

Fossil hominins: their discovery and context

The chances that an early hominin’s skeleton would have been
preserved in the fossil record are very small. Carnivores, such as the
predecessors of modern lions, leopards and cheetahs, would most
likely have had the first pick at the carcass of a dead hominin. After
them would have come the terrestrial scavengers, led by hyenas,
wild dogs and smaller cats, then birds of prey, then insects and
finally bacteria. Within two to three years – a surprisingly short
time – these organisms are capable of removing most traces of any
large mammal.

Human Evolution

any length of time, fossil bone can be as fragile as wet tissue paper.
In these cases researchers have to infiltrate the fragile bone with
liquid plastic, or its equivalent, in order to stop the fossil from
disintegrating. Obviously, deliberate burial greatly increases the
chance that skeletons will be preserved in good condition. It is one
of the main reasons why the human fossil record gets so much
better about 60–70 KYA.
Most hominin fossils are found in rocks formed from sediments laid
down by rivers, or on lakeshores, or in the floors of caves. Generally
older rocks (and thus the fossils they contain) are in the lower layers
and the younger ones are nearer the surface: this principle is called
the law of superposition. However, relative movement of rocks
brought about by tension and compression, such as the shearing
that occurs along faults in the earth’s crust, can confound this
general principle. Sedimentary rocks that form in caves are also
prone to being jumbled up in even more complex ways. Water that
percolates down from the surface can soften and then dissolve old
sediments. This produces Swiss-cheese-like cavities, which are then
filled by more recent sediments. So within caves new sediments may
be below old ones.
Earth scientists use the appearance, texture and distinctive
chemistry of rocks to describe and classify them. For example, they
might refer to one layer as a ‘pink tuff ’, or another as a ‘silty-sand’.
Just as there are rules for naming new species, there are rules and
conventions for naming the strata of a newly discovered
sedimentary sequence, and there is the equivalent of a Linnaean
taxonomy for rocks.
The layer of rock a fossil was buried in is referred to as its ‘parent
horizon’. Hominin fossils found within a particular rock layer are,
unless there is obvious evidence that they were deliberately buried,
considered to be the same age as that layer. A fossil found
embedded in a rock is described as being found in situ. Most
hominin fossils, however, have been displaced through erosion from
26

their parent horizon; these are called ‘surface finds’. In order to
reliably connect a surface find to its parent horizon, it helps if the
fossil still has some of the parent rock, or matrix, attached to, or
embedded in, it. This is why careful scientists never completely
clean the matrix from a fossil.

Finding fossil hominins

Palaeoanthropologists look where rocks of the right age (say back to
10 MYA) have been exposed by natural erosion. Erosion occurs in
places where the earth’s crust has been buckled and cracked as large
landmasses, called tectonic plates, are pushed together. The area
between major cracks, or faults, is forced downward, and the earth’s
crust on the outside of the major faults is thrust upwards. This is
how the floor and walls of rift valleys are formed. The faults that
define the sides of rift valleys are sometimes so deep that the liquid
core of the earth escapes through them. When it is under very high
pressure, the molten core escapes as in a volcanic eruption,
otherwise it ‘leaks’ slowly as a flow of molten lava. Usually volcanic
eruptions consist of ash (called tephra), which is rich in the
chemicals potassium and argon. Rocks formed from these ash
layers are called tuffs. Tuffs provide the raw material for the dating
of many East African hominin fossil sites. Tuffs also have a
distinctive chemical profile, or ‘fingerprint’, and this allows
geologists to trace a single tuff not only within a large fossil site, but
also across many hundreds of kilometres from one site to another.
27

Fossil hominins: their discovery and context

Where do palaeoanthropologists look for early hominin fossils? In
the 19th century Charles Darwin argued that, because the closest
living relatives of modern humans, the chimpanzee and the gorilla,
were both confined to Africa then it was probable that the common
ancestor of modern humans was also likely to have lived in Africa.
So, for the past 75 years, and especially the last 50 years, Africa has
been a focus of human origins field research. But researchers
cannot possibly search all of Africa. Are there particular places
where hominin fossils are likely to be found?

Sometimes hot volcanic ash falls not on the land but on water, and
the holes in the lumps of the volcanic pumice people buy for the
bathroom are caused by the air bubbles that form when hot ash falls
on water.

Human Evolution

Fossils are exposed on the sides and floors of the valleys that form as
streams and rivers erode their way through the blocks of sediment
that are thrown up at faults. Locations like these are called
‘exposures’, and the places on these exposures where fossils have
been found are called localities. In East Africa scientists look for
hominin fossils in rocks of the right age that have been exposed by
the combination of volcanic activity, called tectonism, and erosion
in and around the rift valley. Olduvai Gorge, in Tanzania, is
probably the best-known example of a rift valley site where both
tectonism and erosion have exposed rocks of the right age.
Early hominin fossils are found in a very different geological context
in southern Africa. Here, they are found in caves that form when
rain runs through cracks in the limestone. Small cracks expand into
big cracks, big cracks become cavities, and cavities coalesce to
become caves that then fill with soil washed in from the surface.
Leopards use the trees that grow in the entrances of the caves as a
place to hide carcasses, and hyenas use the entrances of such caves
as a den. Scientists think that most of the hominin fossils found in
the southern African caves were taken there by leopards or hyenas,
or by bone-collecting animals such as porcupines.
Although Africa is the major focus of fieldwork today, it was not that
way until well into the 20th century. Before that time the search for
human fossils was conducted in Europe and Asia. Europe was
where the first prehistorians lived and worked, so it is to be
expected that they would have taken advantage of any opportunity
that presented itself in their own region before looking for the fossil
remains of our ancestors in more exotic places. Just as in 1871
Charles Darwin predicted that Africa would be the birthplace of
humankind, Ernst Haeckel, a prominent German naturalist, in
28

1874 suggested that the presence of the orangutan, the only
non-African great ape, in what was then called the Dutch East
Indies (now Borneo and Sumatra in Indonesia) made that region a
likely birthplace for humanity. Two years before the publication of
Haeckel’s influential book, the naturalist Alfred Russel Wallace
(1872) had included detailed information about the morphology
and the habits of the orangutan in his book about the natural
history of the Malay Archipelago.
Haeckel’s logic and perhaps Wallace’s vivid descriptions of the
orangutan evidently appealed to a young trainee surgeon, Eugène
Dubois, for in the late 1880s he took a job in the region so he could
look for human ancestors. His most famous find, the top of a brain
case of a creature that had brow ridges unlike any seen on modern
humans, was recovered in 1891 in the bank of the Trinil River in
Java. Not all the human ancestors discovered in Asia were found in
sediments cut into by rivers. The famous Peking Man fossils came
from a cave at a site now called Zhoukoudian, near Beijing in China.
29

Fossil hominins: their discovery and context

3. C. K. (Bob) Brain demonstrating the complex stratigraphy at
Swartkrans, one of the southern African cave sites where early hominin
remains have been found

Human Evolution

Teamwork
The teams that nowadays look for hominin fossils in Chad, Ethiopia
or Eritrea must include a wide range of experts. In addition to
palaeoanthropologists, geologists, dating experts, and
palaeontologists who can identify and interpret the fossil remains of
the animals and plants found with the hominins, a
multidisciplinary team should include experts on the factors that
bias the fossil record, and may also include earth scientists who can
interpret the chemistry of the soils in order to reconstruct ancient
habitats. The team’s members have to travel to remote and
sometimes dangerous places where they, along with local hired
workers who help search for and excavate fossils, need supplies of
water, food, and fuel. Leaders of expeditions must have good
organizational skills in addition to their scientific qualifications. Big
expeditions to inaccessible Central and East African fossil sites are
expensive to mount, with the largest ones having annual budgets of
tens of thousands of dollars. The southern African cave sites are
mostly much more accessible. The majority lie within an hour’s
journey time by car from Johannesburg or from Pretoria. This
enables scientists to supervise research while working in
universities and museums in nearby cities.

Fossils rediscovered
Some dramatic hominin fossil discoveries are made in museums. It
is always worthwhile going through the collections of ‘non-human’
fossils recovered from a hominin fossil site. Even the best
palaeontologists can miss things as they sort through hundreds of
bone fragments. In the past when important hominin discoveries
were made they were sometimes sent away to experts for their
assessment, and unless great care is taken specimens can be
muddled or mislabelled. For example, records show that when a
remarkably complete skeleton of a Neanderthal baby was
recovered from the site of Le Moustier it was sent to Marcellin
Boule for an assessment of its age. However, all trace of the
30

skeleton seemed to have been lost until a researcher found the
bones of a neonate among the stone tools from the site of Les
Eyzies! Fortunately, some of the bones were still in their original
matrix and this matched rocks in the Vezere River, which runs past
Le Moustier.

Dating hominin fossils

Absolute dating methods are mostly applied to the rocks in which
the hominin fossil was found, or to non-hominin fossils recovered
from the same horizon. Researchers must take great care to
preserve the evidence that links a fossil to a particular rock layer.
Absolute dating methods rely on knowing the time it takes for
natural processes, such as atomic decay, to run their course, or they
relate the fossil horizon to precisely calibrated global events such as
reversals in the direction of the earth’s magnetic field. This is why
absolute dates can be given precisely in calendar years. The best
known of these absolute dating methods, radiocarbon dating, is
only appropriate for the later stages of human evolution. After
5,730 years (plus or minus 40 years) half of the carbon 14 there was
when the organism died has been converted to nitrogen 14 (this is
why this length of time is called its ‘half life’). Radiocarbon dating
has been used successfully for dating H. sapiens fossils from
Australia and Europe, but radiocarbon dates older than 40 KY are
unreliable because the amounts of radiocarbon left are too small to
be measured precisely.
Most of the hominin fossils from East African sites such as Olduvai
Gorge in Tanzania, Koobi Fora in Kenya, and Hadar in Ethiopia, are
31

Fossil hominins: their discovery and context

Geologists can usually work out the temporal sequence of fossils
within a small fossil site. But how do you compare the ages of fossils
found at localities hundreds of kilometres apart, and how do you
compare the ages of fossils from sites on different continents? To
answer these questions you need dating methods. These are divided
into two categories, absolute and relative.

Human Evolution

from horizons sandwiched between layers of volcanic ash, or tephra,
that are rich in isotopes of potassium and argon. Because
radioactive potassium and argon convert (or decay) into their
daughter products more slowly than carbon 14, potassium/argon
and argon/argon dating methods can be used on rocks that contain
fossils and stone tools from the early (older than 100 KY) part of the
hominin fossil record.
Palaeomagnetic dating uses the complex record of reversals of the
direction of the earth’s magnetic field. For long periods in its history
the direction of the earth’s magnetic field has been the exact
opposite of what it is now. The contemporary direction is called
‘normal’ and the opposite one ‘reversed’. Currents in the liquid core
of the earth cause these shifts in the direction of the magnetic field.
When the suspended particles settle prior to forming a hard
sedimentary rock, minute amounts of magnetic metal in the
particles mean that each of them behaves like a magnet. When they
settle they line up with the direction of the earth’s magnetic field at
the time, and give the rock as a whole a detectable magnetic
direction, or polarity. Researchers compare the sequence of changes
in magnetic direction preserved in the hominin fossil-bearing
sediments with the magnetic record preserved in cores taken from
the floor of the deep ocean (called palaeomagnetic columns) and try
to find the best match. Some sequences are seen more than once in
the reference column, so it helps if another absolute dating method
can be used to show researchers which part of the palaeomagnetic
record they should focus on. A long period of palaeomagnetic
stability is called a ‘chron’, and a relatively short-lived change in
magnetic field direction within a chron is called a ‘subchron’.
Olduvai Gorge was the first early hominin site to be dated using
magnetostratigraphy, and when subchrons were named and not
numbered as they are now one of them was called the ‘Olduvai
Event’.
Another group of absolute dating methods called amino acid
racemization dating uses biochemical reactions as a clock. For
32

example, eggshell contains an amino acid called leucine. When a
shell is formed initially all the leucine is in the L-form. However,
over time this L-form of leucine converts, or racemizes, at a more
or less steady rate to an alternate version, called the D-form.
Thus, the ratio of the two forms, plus the rate of conversion,
provides a date for when the shell was formed. Many later African
hominin fossil sites contain fragments of ostrich eggshell, and if
we make the reasonable assumption that the eggshell in a horizon
is the same geological age as any hominins it contains, then
ostrich egg shell (OES) dating can provide a potentially useful
method. Ostrich egg shell dating is one of several methods (others
are electron spin resonance, ESR, and uranium series dating,
USD) scientists use to date hominin fossil sites that are between
the ranges of radiocarbon and potassium argon dating. These
methods are particularly useful for dating sites between 300 and
40 KYA.
33

Fossil hominins: their discovery and context

4. Some of the methods used to date fossil hominins and the time
periods they cover

Human Evolution

Relative dating methods mostly rely on matching non-hominin
fossils found at a site with equivalent evidence from another site
that has been reliably dated using absolute methods. If the animal
fossils found at Site A are similar to those at Site B, Site A can be
assumed to the approximately the same age as Site B. Compared to
absolute dating methods, relative dating methods only provide
approximate ages for fossils. The use of animal remains for dating,
called ‘biochronology’, has been especially important for dating early
hominin fossils from the southern African cave sites. Nearly all of
these sites contain antelope and monkey fossils. Because the same
animals have been absolutely dated at key East African sites,
researchers can apply these dates to the layers that contain
equivalent fossils in the southern African caves. Biochronology has
also been used to date hominin fossil sites in Chad and at Dmanisi,
in Georgia.
Dendrochronology, the use of tree rings for relative dating, has
been used to improve the precision of carbon dating. Annual tree
rings are so reliable that they have been used to correct carbon
dates that have been affected by recent human-induced, or
anthropogenic, changes in levels of carbon isotopes in the
atmosphere.

Reconstructing past environments
Just as the contours of the earth’s surface are different than they
were several million years ago, past environments in a region are
not necessarily the same as those we see today. Researchers
reconstruct past environments using geological and
palaeontological evidence. Chemical analysis is used to tell whether
a soil was laid down in moist or dry conditions. Palaeontologists can
tell a lot about the palaeohabitat from the types of animal fossils
that are found along with the fossil hominins. They use both large
mammals and small micromammals (such as mice and gerbils) to
reconstruct past environments. Small micromammals are especially
useful because their geographical ranges are more restricted than
34

those of larger mammals, so they are likely to provide more precise
habitat reconstructions. Fossilized owl pellets are a good source of
information about micromammals because owls hunt small
mammals within a relatively small range. Researchers who use
larger mammals like primates to reconstruct past environments
have to be careful not to assume that the habitat preferences of the
ancestors were like those of their modern-day representatives.
For example, although modern colobus monkeys are mainly
leaf eaters who live in dense woodland, their ancestors lived in more
open habitats, so the presence of colobus monkeys at a 5 MY-old
site does not mean the same as finding contemporary colobus
monkeys.

Hominin evolution has taken place at a time when there have been
major changes in world climate. Researchers study climate change
by looking at deep-sea cores. Microscopic organisms called
foraminifera (usually shortened to ‘forams’) are suspended in the
water of the world’s oceans. These foraminifera take up two forms
of oxygen isotope: one of them, oxygen 16, is lighter, the other,
oxygen 18, is heavier. When global temperatures are higher more of
the lighter oxygen evaporates, so the ratio of the light to the heavy
form reduces: the opposite occurs when global temperatures are
cooler. Researchers use the proportions of the two oxygen isotopes
to track the temperature of the oceans, and they use ocean water
temperature as a proxy for global climate. But the climate in a
region is the result of a complex interaction between global climate
and local influences such as latitude, altitude, and the presence of
mountain ranges.
During the period from 8 to 5 MYA the earth experienced the
beginning of a long-term drying and cooling trend. Early hominin
evolution took place in Africa at the time of these climatic changes,
and the possible influence of climate change on the origin of the
hominin lineage will be explored further in Chapter 5.
35

Fossil hominins: their discovery and context

Global climate change

Human Evolution

5. Plot of oscillations in oxygen isotope levels during the past six million
years, showing that since 3 MYA the global climate has shown a general
cooling trend

Later in hominin evolution cyclical changes in global climate,
measured using deep sea cores, were superimposed on the
long-term cooling trend. Prior to 3 MYA global climate was subject
to 23 KY hotter/drier and cooler/wetter cycles. Around 3 MYA the
periodicity of these cycles switched to 41 KY and 1 MYA it switched
yet again to a 100 KY cycle. These 100 KY cycles are the ones
responsible for the periods of intense cold recorded in the northern
hemisphere during the past million years. These long cycles had
another important impact on human evolution because when so
much ice is locked up in the icecaps at the north and south poles, it
is inevitable that the sea level will fall. This would have exposed
much of what we call the continental shelf. Reductions in sea level
of this magnitude allowed modern human ancestors to migrate
from the Old World to both Australasia and the New World.

36

Chapter 4
Fossil hominins:
analysis and interpretation

Palaeoanthropologists use many methods to work out the
significance of newly discovered fossil evidence. The hominin fossils
must be assigned to a taxon, or taxa, the taxa must be classified,
their relationships to other fossil and living taxa worked out, and
their behaviour reconstructed.

Classification and taxonomy
Western science classifies all living things according to a scheme
devised in 1758 by the Swedish naturalist Carolus Linnaeus. The
basic unit of the scheme is the species, a group of morphologically
similar animals that consistently breed productively with each
other. Individual living animals all belong to a species, similar
species are grouped into genera, genera are grouped into tribes,
tribes into families, and so on up to categories like kingdoms.
Modern humans, Homo sapiens, belong in the species sapiens, the
genus Homo, and the tribe Hominini.
A subdiscipline of classification, called ‘nomenclature’, is devoted to
prescribing how names should be used in the Linnaean system.
There is a formal code for regulating nomenclature, and scientists
who think they have discovered a new species must follow this code.
Rules in the code govern the types of name that can be given to a
new species or genus. For example, the names of commercial
37

Human Evolution

products are prohibited: Burgerking ipodensis would not be an
acceptable binomial for a new hominin species. It is also important
to make sure that the name of an existing taxon is not inadvertently
used for a new taxon, otherwise they will be confused.
When researchers decide to introduce a new species, they have
to choose one fossil as its ‘type’ specimen. Usually a relatively
well-preserved fossil is selected from among those found at the time
of the initial discovery: it does not have to be a typical (i.e. an
average) member of the species. The significance of the type
specimen is that the taxon name is irrevocably attached to it. So,
for example, if the type specimen of Homo neanderthalensis was
found to be different from all the other fossils included in
H. neanderthalensis, then they would have to be assigned to a new
species, and it would need to be given a new name. The name
H. neanderthalensis cannot be used independently of the type
specimen; where it goes, the name goes, too. If researchers
eventually decide that a particular specimen should be moved to a
new species, then it takes its species name with it. Age counts in
nomenclature: if two type specimens end up in the same species, the
oldest name is the one that has to be used.
A species is an example of a taxon. All the Linnaean categories are
taxa, but when researchers write about ‘a taxon’ they are usually
referring to a species. How species are arranged in an increasingly
inclusive hierarchy (i.e. larger and larger clusters of species) is
called a taxonomy, literally a ‘scheme for taxa’. Taxonomic analysis is
the process of determining what taxon hominin fossils should be
put in. First, researchers have to decide whether a newly found
fossil belongs in an existing hominin taxon. Only if they are
convinced that it cannot be assigned to an existing species can they
begin to think of making a new species with a new name. The same
principles apply all the way up the Linnaean hierarchy, so
researchers should only establish a new genus if they are convinced
the new species cannot be accommodated in any of the existing
hominin genera, and so on up the Linnaean hierarchy.
38

Researchers must be sure the measurements made on fossils
accurately reflect the size and shape of the bone or tooth before it
was fossilized. Bones and teeth crack if they are exposed to daily
cycles of heating and cooling. Rock matrix gets inside the cracks
and artificially enlarges the dimensions of a bone or tooth. Likewise,
if a fossil bone is exposed on the surface of the ground in dry and
windy conditions both before and after fossilization, sand grains
carried by the wind have a ‘sandblasting’ effect and remove part of
the outer layer of cortical bone. This erosion artificially reduces the
39

Fossil hominins: analysis and interpretation

Taxonomic analysis and the other methods of analysis described
below are based on a detailed assessment of the morphology of a
fossil. Its morphology, or phenotype, is what the fossil looks like,
both externally and internally. Morphology can be gross
morphology, which is what the eye can see unaided, or microscopic
morphology, which is what can be seen with a variety of types of
microscope. Researchers prepare detailed qualitative descriptions
of the size and shape of the fossil, but they also try to capture that
information in the form of measurements as a quantitative
description. In its simplest form quantitative descriptions consist of
distances between defined anatomical landmarks on the fossil:
these are called linear measurements. Laser beams and other
technologies borrowed from medical imaging now allow
researchers to capture details of the external morphology and the
internal structure of fossils much more precisely than in the past.
For example, Glenn Conroy, a palaeoanthropologist, and Charles
Vannier, a medical imaging specialist, both from Washington
University in St Louis, pioneered the use of computerized
tomography (or CT) imaging to study the internal structure of a
fossil hominin cranium from Taung in southern Africa.
Subsequently Frans Zonneveld, a medical imaging specialist from
Utrecht, and Fred Spoor, a palaeoanthropologist from University
College London, further developed these methods so that they can
now provide information about the inner ear. Researchers use these
data to help sort hominin fossils into species and to reconstruct
their posture and hearing.

Human Evolution

size of the fossil bone. The measurements and the non-metrical
morphology of a newly recovered fossil are compared with those of
similar specimens in existing fossil taxa. Closely related living
animals (in the case of hominins this means modern humans and
the African apes) are usually used as models to help decide how
much variation should be tolerated within a single species. But Cliff
Jolly, a primatologist from New York University who has spent 30
years studying what happens at the boundary between distinctive
groups of baboons, suggests that baboons and their close relatives
are in some ways a better analogue for hominin evolution. He
points out that not only are baboons more widespread than
chimpanzees and gorillas, but they are also similar to hominins with
respect to the pattern and timing of their recent evolutionary
history.

Reconstructing whole fossils from fragments
Hominin fossils several millions of years old are seldom found in
good condition. The brain case and the face are particularly fragile
and are easily trampled by hoofed animals and crushed by rocks
falling from the roof of a cave. Sometimes just one fragment of the
brain case is all that is left of a cranium. In a few cases more is
preserved, but if the pieces are tiny it is a challenge to reassemble
them. It is like a three-dimensional jigsaw puzzle with lots of sky, no
clouds and with no picture to help you. One option is to
painstakingly reassemble the pieces by hand, but this can take
hundreds of hours even by a skilled anatomist who knows every
detail of a skull.
Marcia Ponce de León and Christoph Zollikofer from the
Anthropological Institute of Zurich are both experts in a new
research area called ‘virtual anthropology’. They have used
computer power and advances in software design to devise an
alternative to reassembling hominin fossils by hand. The fossil is
scanned using a laser and a ‘virtual’ version is displayed on the
computer screen. Researchers can then move and rotate each piece
40

in any direction to see if any of the pieces fit. The software also
enables a missing piece on one side of the cranium to be replaced by
mirror imaging the equivalent piece from the other side. Zollikofer
and Ponce de León have recently used these methods to make a
virtual reconstruction of the cranium of Sahelanthropus tchadensis,
a potential early hominin. Similar software in conjunction with CT
scans enables structures buried deep in the bone, like the air
sinuses, the bony canals of the inner ear, or the roots of the teeth, to
be seen clearly.

Determining age and sex

The size and shape of the bones and teeth, the extent of muscle
markings, and the size and shape of the pelvis (although pelvic
fragments are rare in the hominin fossil record) are the usual ways
the sex of an individual fossil is determined. The underlying
assumption is that because in many non-human primates males are
larger than females, then early hominin males were also likely to
have been larger than early hominin females. This is one aspect of
sexual dimorphism, a term that refers to all the differences among
individuals that are related to their sex. However, when you are
dealing with a sparse fossil record overall size is not always a
reliable guide to sex.
There are also complications if one unthinkingly extrapolates
modern human sexual dimorphisms to early hominins. For
example, in modern humans many pelvic sex dimorphisms occur
41

Fossil hominins: analysis and interpretation

Even if one has a complete or nearly complete skeleton,
determining the sex and developmental age of hominin fossil
remains can be difficult. These difficulties are compounded when
all that remains are small fragments of a cranium. The age at death
of a fossil individual that has finished growing is difficult to
determine precisely. Dental development can help determine the
age of immature individuals, but once all the teeth are erupted and
the roots of the teeth are formed dental evidence is less useful.

because of compromises between the requirements of bipedalism
and the need in modern human females for space in the pelvis to
give birth to a large-brained infant. The same dimorphisms,
however, might not apply to small-brained early hominins who are
not bipedal in the same way that modern humans are: their pelves
may show a unique pattern of sexual dimorphism.

Human Evolution

Species and species identification
The most widely used scientific definition of a species is the
biological species concept (BSC) that is linked with the late Ernst
Mayr, a distinguished Harvard evolutionary biologist. This suggests
that a species is a ‘group of interbreeding natural populations,
reproductively isolated from other such groups’. This is all well and
good when you can observe living animals, and check who is mating
with whom, but it is self-evident that this method will not work
when we try to recognize species in the fossil record. However,
because members of the same species mate with each other and not
with members of another species, they resemble each other more
closely than they do individuals belonging to any other species.
Thus, in the absence of information about its mating habits, we can
use the appearance, structure, and (if any DNA is preserved) the
genetic make-up of an individual fossil to help allocate it to a
species.
But there are problems when researchers try to apply these
methods to the fossil record. The first difficulty is that we do not
have complete animals in the hominin fossil record. It is customary
to divide the components of animals into two categories, soft
tissues, such as muscles, nerves, arteries, and hard tissues, such as
bones and teeth. The fossil record for human ancestors is restricted
to the remains of the hard tissues, and many of these are just
fragments of bones and teeth. So the problem for
palaeoanthropologists is how to assign a fossil to a species when the
only evidence you have is several worn and broken teeth, or a piece
of jaw, or part of a thigh bone.
42

A good analogy is of a running race. A fossil is like a single still
photograph of a long-distance running race. But a long-lived
species may well be sampled several times during its history.
Palaeoanthropologists need to work out ways of telling whether
they are looking at several photographs of the same running race, or
single photographs of several different running races. In the case of
human evolution this means looking at collections of modern
human, and higher primate skeletons, and then using the size and
shape variation within those living taxa as a guide to how much
variation researchers should tolerate within a collection of fossils
assigned to a single species. If the variation is less than that seen in
the living taxa then there are good reasons to conclude that only one
species is represented in the collection of fossils. Because of the
extra time involved with fossil samples palaeoanthropologists try to
make an educated guess about the amount of variation they are
prepared to tolerate in their fossil sample before they declare that
43

Fossil hominins: analysis and interpretation

The second problem is time. Each species has a history, with a
beginning (speciation), a middle, and an end. Species either die
out without leaving any direct descendants (extinction), or they
become the common ancestor of one or more new ‘daughter’
species. The average fossil mammal species lasts for between one
and two million years. During such a long history the appearance
of that species is unlikely to stay the same. Random variation and
morphological responses to climatic variation will cause it to
change. But as long as its members only mate with members of
the same species then the species should continue to be
distinctive. However, even if a scientist spends their whole career
observing just one living species they will have studied that
species for just an instant during its existence. So the variation
you see in museum collections of skeletons belonging to a modern
species that have been collected over the course of a hundred
years, or so, is not an appropriate model for deciding how much
variation one should tolerate in a sample made up of fossils
collected at sites that span several hundred thousand years of
time.

the variation is ‘too great’ to be contained in a single species. But it
is only an educated guess.

Human Evolution

Deciding how many species are represented in a collection of early
hominin fossils is made more difficult because biological variation
among hominins, including fossil hominins, is continuous.
Therefore where the boundaries between fossil taxa are drawn is a
matter of legitimate scientific judgement and debate. The discovery
of new fossils or the introduction of new analytical methods often
means that boundaries have to change, or palaeoanthropologists
have to reconsider the utility of their categories and labels. A new
species should be established only if there are really good grounds
for believing the new fossil evidence does not belong to an existing
species. There needs to be even stronger evidence to establish a
new genus.

Speciation
Some researchers think that new species are the result of gradual
change involving the whole population. This interpretation of
speciation is called ‘phyletic gradualism’, and the form of speciation
associated with it is known as ‘anagenesis’. Others see speciation as
the result of bursts of rapid evolutionary change concentrated in
a geographically restricted subset of the population. This
interpretation of speciation is called the ‘punctuated equilibrium’
model. In the latter model in the long interval between the periods
of rapid evolutionary change there should be no sustained trends in
the direction of morphological evolution, just ‘random walk’
fluctuations in morphology. Species formation in that mode is
called ‘cladogenesis’ and the term ‘stasis’ is used to describe the
periods of morphological stability between speciation episodes.
Almost all researchers now accept that most of the morphological
change involved in evolution occurs at the time of speciation.
In some circumstances speciation may be due to quite large-scale
changes in the genotype brought about by rearrangements in the
44

chromosomes. Researchers have suggested that this may have been
the mechanism underlying speciation in higher primates.
Particularly intensive periods of species generation and
diversification are called ‘adaptive radiations’. They tend to be
associated with an opportunity to exploit a new environment, or
when extinctions in other groups mean that adaptive opportunities
become available in an existing environment. At times like these
some lineages tend to generate more species than others, and they
are referred to as being ‘speciose’.
All species, including modern humans, will ultimately become
extinct. What is at issue is whether extinctions are determined by
the intrinsic properties of a species, or by extrinsic factors such as
changes in the environment, or by a combination of the two. These
competing hypotheses can be tested in the laboratory by varying
the conditions under which rapidly evolving organisms such as
fruit flies are kept. It can also be investigated by comparing the
fossil record with independent evidence about changes in past
climates.

45

Fossil hominins: analysis and interpretation

6. The two main hypotheses ‘phyletic gradualism’ and ‘punctuated
equilibrium’ about the timing of the morphological change that occurs
during evolution

Human Evolution

Splitters and lumpers
The taxonomy used in this Very Short Introduction recognizes a
relatively large number of hominin species, but not all researchers
recognize that many species. Researchers who subscribe to
taxonomies that recognize many species are called ‘splitters’. Those
who recognize fewer species are called ‘lumpers’. Both groups of
researchers are looking at the same evidence, they just interpret it
differently. Most disagreements among palaeoanthropologists
about how many species to recognize in the human fossil record are
due to differences in how they interpret variation. Researchers who
stress the importance of continuities within the fossil record
generally opt for fewer species, whereas those who stress
discontinuities within the fossil record will generally recognize
more species. However, when all is said and done, all taxonomies
are hypotheses. If scientists explain their taxonomy, then other
scientists can reinterpret the evidence in any way they choose, as
long as everyone makes it clear which fossil specimens they are
allocating to the species taxa they choose to recognize.

Cladistic analysis
Once the taxonomy of a new discovery has been worked out,
researchers move on to the next stage. This involves using cladistic
methods to work out how a fossil hominin taxon is related to
modern humans and to other fossil hominin taxa.
The technical term ‘clade’ refers to all (no more and no less) of the
organisms descended from a recent common ancestor. The smallest
clade consists of just two taxa; the largest includes all living
organisms. Cladistic analysis sorts taxa according to the amount of
morphology they share, but the morphology has to be of a particular
kind. To be helpful for working out relationships between closely
related species, the morphology used must be shared by two or
more taxa, but it must also vary within the group under
investigation, so that it can be used to break that group up into
46

Table 2. Two taxonomic hypotheses, one ‘splitting’ and one ‘lumping’, for the hominin fossil record.
Informal group

Splitting taxonomy

Age (MY) Type specimen

Main fossil sites

Possible and probable

S. tchadensis

7.0–6.0

hominins

O. tugenensis

6.0

BAR 1000’00

Lukeino, Kenya

Ar. ramidus s. s.

5.7–4.3

ARA-VP-6/1

Gona and Middle Awash, Ethiopia

TM 266–01–060–1 Toros-Menalla, Chad

Ar. kadabba

5.8–5.2

ALA-VP-2/10

Middle Awash, Ethiopia

Archaic and transitional

Au. anamensis

4.2–3.9

KNM-KP 29281

Allia Bay and Kanapoi, Kenya

hominins

Au. afarensis s. s.

4.0–3.0

LH 4

Belohdelie, Dikika, Fejej, Hadar, Maka,
and White Sands, Ethiopia; Allia Bay,
Tabarin, and West Turkana, Kenya

K. platyops

3.5–3.3

KNM-WT 40000

West Turkana, Kenya

Au. bahrelghazali

3.5–3.0

KT 12/H1

Bahr el ghazal, Chad

Au. africanus

3.0–2.4

Taung 1

Gladysvale, Makapansgat [Mb 3 and 4],
Sterkfontein [Mb 4], and Taung, South
Africa

Au. garhi

2.5

BOU-VP-12/130

Bouri, Ethiopia
Continued

Table 2 continued
Informal group

Splitting taxonomy

Age (MY) Type specimen

Main fossil sites

Archaic and transitional

P. aethiopicus

2.5–2.3

Omo 18.18

Omo Shungura Formation, Ethiopia;

P. boisei s. s.

2.3–1.3

OH 5

hominins (contd.)

West Turkana, Kenya
Konso and Omo Shungura Formation,
Ethiopia; Chesowanja, Koobi Fora, and
West Turkana, Kenya; Melema, Malawi;
Olduvai and Peninj (Natron), Tanzania
P. robustus

2.0–1.5

TM 1517

Cooper’s, Drimolen, Gondolin, Kromdraai [Mb 3], and Swartkrans [Mbs 1, 2,
and 3], South Africa

Pre-modern Homo

H. habilis s. s.

2.4–1.6

OH 7

Omo Shungura Formation, Ethiopia;
Koobi Fora, Kenya; ?Sterkfontein and
?Swartkrans, South Africa; Olduvai,
Tanzania

H. rudolfensis

2.4–1.6

KNM-ER 1470

Koobi Fora, Kenya; Uraha, Malawi

H. ergaster

1.9–1.5

KNM-ER 992

?Dmanisi, Georgia; Koobi Fora and
West Turkana, Kenya

H. erectus s. s.

1.8–0.2

Trinil 2

Many sites in the Old World e.g., Melka
Kunturé, Ethiopia; Zhoukoudian, China;
Sambungmacan, Sangiran, and Trinil,
Indonesia; Olduvai, Tanzania

H. floresiensis

0.095–

LB1

Liang Bua, Flores, Indonesia

0.018
H. antecessor

0.7–0.5

ATD6–5

Gran Dolina, Atapuerca

H. heidelbergensis

0.6–0.1

Mauer 1

Many sites in Africa and Europe, e.g.,
Mauer, Germany; Boxgrove, England;
Kabwe, Zambia

H. neanderthalensis 0.2–0.03 Neanderthal 1

Many sites in Europe, the Near East, and
Asia

Modern Homo

H. sapiens s. s.

0.2–pres

None designated

Many sites in the Old World and some in
the New World
Continued

Table 2 continued
Informal group

Lumping taxonomy Age (MY) Taxa included from the splitting taxonomy

Possible and probable

Ar. ramidus s. l.

7.0–4.5

Ar. ramidus s. s., Ar. kadabba, S. tchadensis, O. tugenensis

Au. afarensis s. l.

4.2–3.0

Au. afarensis s. s., Au. anamensis, Au. bahrelghazali, K.

Au. africanus

3.0–2.4

Au. africanus

P. boisei s. l.

2.5–1.3

P. boisei s. s., P. aethiopicus, Au. garhi

P. robustus

2.0–1.5

P. robustus

H. habilis s. l.

2.4–1.6

H. habilis s. s., H. rudolfensis

H. erectus s. l.

1.9–0.018 H. erectus s. s., H. ergaster, H. floresiensis

H. sapiens s. l.

0.7–pres

hominins
Archaic and transitional
hominins

Pre-modern Homo
Modern Homo

platyops

H. sapiens s. s., H. antecessor, H. heidelbergensis, H.
neanderthalensis

subgroups, or clades. For example, the features that make all higher
primates mammals, such as the presence of nipples and warm
blood, are no use for sorting out detailed relationships among the
great apes. But to go to the other extreme, morphology that is found
only in one taxon cannot be used to work out the relationships
among taxa.

Cladistic analysis works on the assumption that if members of two
taxa share the same morphology, they must have inherited it from
the same recent common ancestor. This assumption is often
justified, but not always. We know that primates, including higher
primates, have experienced convergent evolution, a process by
which different lineages evolve similar morphology independently.
The term homoplasy refers to similar morphology seen in two
species but which is not inherited from a recent common ancestor.
For example, it is likely that thick tooth enamel evolved more than
once in human evolution, thus making it a homoplasy within the
hominin clade.

Fossil DNA
The newest form of analysis used to work out how hominin taxa are
related relies on the extraction and analysis of DNA. In your family,
closely related individuals, for example brothers and sisters, share
more DNA than do distant cousins. It is the same for taxa.
Individuals within a taxon should, on average, share more DNA
than two individuals drawn from different taxa. However, despite
the importance of DNA in our lives, fossilization quickly causes
nucleic acids to degrade. For example, after 50,000 years, only
51

Fossil hominins: analysis and interpretation

Two taxa that share specialized morphology are referred to as sister
taxa. That pair of sister taxa has its own sister taxon (for example
Gorilla is the sister taxon of the Pan/Homo clade) and so on. The
branching diagram that results is called a cladogram. The same
relationships can be represented in writing by using sets of
parentheses for sister groups (e.g. ( ( (Homo, Pan) Gorilla) Pongo) ).

Human Evolution

small amounts of DNA survive, and even this is broken into short
fragments. A team led by Svante Pääbo, a molecular biologist from
the Max Planck Institute for Evolutionary Anthropology at Leipzig,
was the first to recover DNA from a fossil hominin, and I will
consider fossil DNA evidence further when I discuss Neanderthals
in Chapter 7.
Researchers undertaking fossil DNA analysis must take particular
care to prevent and detect contamination. When people handle
fossils, they inevitably leave hair and skin cells on the fossil and
these are a potent source of contamination. Scientists must make
sure they are detecting DNA amplified from the fossil hominin and
not DNA from other sources. In a recent study of cave bear fossil
researchers detected more than twenty different modern human
DNA sequences on a single cave bear fossil. Tens, if not hundreds, of
people, will have handled most hominin fossils, especially those
found many years ago. Working out which of many DNA sequences
recovered from a modern human fossil really belongs to that
individual will be a challenge.

7. Comparison of the concepts of clades and grades as applied to living
higher primates
52

Grades

Functional and behavioural morphology
In addition to analysing fossils in order to classify them and arrange
them in a cladogram and then a phylogeny, palaeoanthropologists
also use the fossil record to work out the adaptations of hominin
species. They do this by trying to reconstruct how individuals
belonging to the same taxon lived their lives, and then they pool this
information with evidence about habitat and generate hypotheses
about how that species is adapted to its environment. Researchers
try to learn as much about an extinct animal as they would expect to
know about a living one. What did it eat? How did it move around?
Did it live in social groups, or was it solitary? Palaeoanthropologists
attempt to answer these questions by looking at functional or
behavioural morphology.
Functional morphology means looking at a bone or tooth and
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Fossil hominins: analysis and interpretation

Homoplasy complicates our attempts to sort early hominins into
clades. An alternative is to sort hominin taxa into grades. A grade is
a category based on what an animal does rather than what its
phylogenetic relationships are. So for example, Sport Utility
Vehicles is the equivalent of a grade, whereas all the cars produced
by the Ford Motor Company, including its range of SUVs, is the
equivalent of a clade. Grades may also be clades, but they are not
necessarily so. For example, leaf-eating, or folivorous, monkeys are
a grade and not a clade because folivorous monkeys from the Old
and New World are, respectively, just one component of much
larger Old and New World monkey clades. A clade must comprise
all the descendants of a common ancestor, not just some of them.
Palaeoanthropologists are more likely to agree about grades than
clades, but determining the branching pattern of the TOL is
something that must be pursued, even if the results are
controversial. I will refer to some of these controversies in later
chapters.

Human Evolution

considering what functions it performs best and most frequently.
For example, you would only need curved finger bones if you spent a
lot of time holding onto branches, so curved finger bones are a sign
that climbing was a part of that animal’s locomotion. The shapes of
finger joints and the length of the fingers and thumb also provide
clues about how well early hominins could have gripped objects.
Holding the shaft of a hammer needs a power grip, whereas the
ability to hold and use a small, sharp stone tool uses a precision grip
and a different combination of arm, forearm, and small hand
muscles. Similarly, the thighbones of animals that bear all their
weight on their hind limbs are differently shaped from those whose
weight is distributed across all four limbs.
Functional morphology can also help to reconstruct the diet of
early hominins. The shape of a tooth reflects what was eaten. Teeth
with large crowns, with low, rounded, cusps covered by thick
enamel are likely to have evolved to cope with a diet that included
tough food, or food that was enclosed in some sort of hard outer
coating, like the shell of a nut, that needed to be broken before the
contents could be eaten. Scientists use microscopes to look at
minute scratches not visible to the naked eye that are on all teeth.
Foods like tubers that grow in the ground contain a lot of grit, and
this leaves tell-tale gouges on the surface of the enamel. Sometimes
teeth get scratched when animals trample them, or when hard sand
grains are blown against them. But this type of damage should
affect the sides and not just the top, or occlusal, surface of a tooth.
When they look for clues about the diet of the early hominins by
looking for evidence of any microscopic scratches left by food
(called microwear), researchers must make sure that they do not
confuse these scratches made after death (post mortem) with the
scratches made during the life of the individual (ante mortem
microwear).
Direct evidence about the kinds of foods hominins ate comes from
stable isotope analysis. This form of analysis measures oxygen,
nitrogen and carbon isotopes in fossil bones or teeth and then
54

matches the pattern found in the fossil with the patterns seen in
living animals whose diets are known. For example, animals that
browse on leaves can be distinguished from those that graze on
grass and from those that are primarily carnivores. Using such a
method, Julia Lee-Thorp, an isotope chemist working at the
University of Bradford’s Department of Archaeological Sciences,
and her colleagues have shown that 1.5 MY-old Paranthropus
hominins from Swartkrans have stable isotope patterns that could
only come from eating meat, thus causing researchers to reconsider
earlier views that these hominins were primarily, if not exclusively,
vegetarians.

Over many decades palaeoanthropologists have accumulated
hominin fossils from thousands of individuals going back to
between 6 and 7 MYA. While this number may sound impressive,
the majority are concentrated in the later part of the hominin fossil
record. Besides this temporal bias, the hominin fossil record has
other biases and weaknesses. The science of working out these
biases and trying to correct for them is the topic of taphonomy.
Whereas some of the hardest parts of the skeleton such as the teeth
and the mandible are well represented in the hominin fossil record,
the postcranial skeleton, that is the vertebral column and the limbs,
and particularly the vertebral column and the hands and feet are
poorly represented. The relative durability of different parts of the
skeleton (for example, mandibles are generally heavier and are
made of denser bone than vertebrae) is partly responsible for the
differential preservation of body parts. Lighter bones like vertebrae
are likely to be swept along in the floods that follow torrential rain,
and then carried out into a lake, where they will be mixed in with
the fossilized bones of fish and crocodiles. In contrast, heavier bones
like skulls and jaws will fall to the bottom of the floodwaters, get
trapped in the stones on the bed of the stream or river, and are thus
preserved in sediments that preserve the heavier bones of other
terrestrial animals.
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Fossil hominins: analysis and interpretation

Gaps and biases in the hominin fossil record

Human Evolution

Another factor influencing differential preservation is which parts
of the carcass predators find most tempting. Leopards like to chew
the hands and feet of monkeys and, if extinct large carnivores had
similar preferences, then these parts of hominins would be in short
supply as fossils. Thus, we know more about the evolution of the
teeth of fossil hominins than we do about the evolution of their
hands and feet. Body size also has a significant influence