اصلی Human Evolution .A Very Short Introduction
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A very good complementary reading material for students in the same field.
29 June 2021 (15:33)
Bernard Wood HUMAN EVOLUTION A Very Short Introduction 1 3 Great Clarendon Street, Oxford o x 2 6 d p Oxford University Press is a department of the University of Oxford. It furthers the University’s objective of excellence in research, scholarship, and education by publishing worldwide in Oxford New York Auckland Cape Town Dar es Salaam Hong Kong Karachi Kuala Lumpur Madrid Melbourne Mexico City Nairobi New Delhi Shanghai Taipei Toronto With ofﬁces in Argentina Austria Brazil Chile Czech Republic France Greece Guatemala Hungary Italy Japan Poland Portugal Singapore South Korea Switzerland Thailand Turkey Ukraine Vietnam Oxford is a registered trade mark of Oxford University Press in the UK and in certain other countries Published in the United States by Oxford University Press Inc., New York © Bernard Wood 2005 The moral rights of the author have been asserted Database right Oxford University Press (maker) First published as a Very Short Introduction 2005 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, without the prior permission in writing of Oxford University Press, or as expressly permitted by law, or under terms agreed with the appropriate reprographics rights organizations. Enquiries concerning reproduction outside the scope of the above should be sent to the Rights Department, Oxford University Press, at the address above You must not circulate this book in any other binding or cover and you must impose this same condition on any acquirer British Library Cataloguing in Publication Data Data available Library of Congress Cataloging in Publication Data Data available ISBN 0–19–280360–3 978–0–19–280360–3 1 3 5 7 9 10 8 6 4 2 Typeset by ReﬁneCatch Ltd, Bungay, Suffolk 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 53 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. 55 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