1 FACTORING THE PROTO-BIPED INFANT INTO THE ORIGIN OF BIPEDALISM Grayson Joy Gerrard March 2018 A thesis submitted for the degree of Doctor of Philosophy of The Australian National University © Copyright by Grayson Joy Gerrard 2018 2 Except where otherwise indicated by citation, all parts of this thesis represent my own original work _______________________________ 3 DEDICATION TO THE MEMORY OF COLIN GROVES 1942 - 2017 4 ACKNOWLEDGEMENTS I owe an enormous debt to the late Colin Groves, who encouraged this project from the outset and who chose to continue supervising it even when his health was failing. I am deeply grateful to him for everything he taught me about palaeoanthropology and primates, and for years of patience, kindness, support, constructive criticism, and mentoring. No matter how busy he was with his many other commitments, Colin was always there for his postgraduates, always their rock, and always unfailingly generous with his time. My own debt to him is particularly acute because I was one of the last few of these privileged individuals, and he was part-way through reading the final draft of this thesis before his last illness. I am also indebted to Dr. Phyll Dance, Colin’s wife, for many kindnesses during the years I was her husband’s student. Dr. Robert Attenborough acted as my co-supervisor. I thank him warmly for all his encouragement and support, his insistence on careful definitions, and his many constructive criticisms and suggestions for improvement. Most of all, I thank him for taking on the task of steering this thesis through its final, pre-submission stages. Neale Reynolds and Jennifer Grattidge extended hospitality and many forms of practical assistance during all my thesis-related visits to Canberra, and I thank them for being such good friends. I also thank Bec Scott of the National Zoo and Aquarium, Canberra, for granting permission for the siamang observation study which forms Appendix B; Jessica Walsh and staff of the off-campus service of ANU library for all their very efficient help with library resources; Liz 5 Walters (now retired from ANU) for extensive administrative support during the early stages of the thesis, and for being Liz. Lastly, I thank John Dawson for all his support on the home front and for all the sacrifices. Without John, there would have been no thesis. 6 ABSTRACT This thesis applies a single test method to a high number of proposed explanations (models) of hominin bipedalism. This has not been undertaken previously, due largely to the very variable ways in which the models have been built, and the disparity of their core ideas. Taken as a whole, however, the modelling literature contains a gap, and the thesis’ main claim is that it can be exploited to assess relative levels of tenability, using non-statistics-based hypothesis testing. Infants are defined here as individuals too young to survive without caregivers and older infants are defined as those who are not completely dependent on others for nutrition and transport. Most models have omitted infants, and others have included them only in minor or circumscribed ways. A species’ survival, however, depends on sufficient numbers of young reaching breeding age, and species which provide postnatal care need to provide adequate care. Any viable model of how hominin bipedalism evolved ought, therefore, to be able to accommodate appropriate levels of infant survival and adequate caregiving, and factoring infants and caregivers into the models can thus serve as a test method. The method cannot assess overall robusticity, because models which accommodate infants and caregivers might be untenable relative to other criteria, but it can narrow down the possible ways in which bipedalism might have evolved by identifying models which are untenable in one crucial way. Seventy-six models, written between 1871 and 2017, are identified. They are classified into 12 model types, three of which are treated as obsolete. Seventy-one models 7 are tested, 76.2% of which omit infants or include them only trivially. The models are tested either in blocs, according to type or variant, or individually. Forty-three tests are conducted. In order to factor a proto-biped infant into a model, it is necessary to have a clear idea of its likely basic characters. After its Introduction, Methods chapter and Literature Review, this thesis is therefore divided into two distinct parts. The first part extensively reviews available literature bases for character reconstruction, and one of these bases, the literature on nonhuman anthropoid infancy, is supplemented by a short-term captive study of clinging behaviour in an orthograde ape, presented as an appendix. The likely size of older proto-biped infants, relative to conspecific adult females, is then estimated, based on data for extant anthropoids. All this information is then synthesized to reconstruct the infant and to guide the tests’ expectations of infant/adult differences and adequate caregiving behaviours. The second part of the thesis contains the 43 tests. The results show that 40.8% of models are untenable, including two whole model types, 43.7% are highly questionable, and only 11.3% accommodate infant survival well (4.2% did not support the test method). These results indicate that the ways in which bipedalism might have evolved are heavily constrained by the needs of infants. They also suggest that future models which consider the full implications of their proposed behaviours for infants and caregivers will have greater explanatory value than the majority of existing models. 8 TABLE OF CONTENTS Acknowledgements 4 Abstract 6 Table of contents 8 List of tables 14 List of figures 17 List of abbreviations 19 Chapter 1: Introduction 1.0 The topic area 20 1.1 Key terms 24 1.2 Aims 30 1.3 Overview of methods 31 1.4 Chapter outline 32 Chapter 2: Materials and methods 2.0 Models of bipedalism 34 2.1 The role of infants in the models 35 2.2 Test preparation: reconstruction of the proto-biped infant 35 2.3 Test procedure 36 2.4 Test results 39 Chapter 3: Literature review 3.0 Counting and classifying models of bipedalism 40 3.1 Review of model types 48 3.2 Overview of the literature 60 3.3 The role of infants 65 Chapter 4: Analogues for the proto-biped infant: extant nonhuman anthropoids 9 4.0 Nonhuman anthropoids as an analogue 70 4.1 Precocity 70 4.2 Life history (LH) 73 4.3 Carrying/Clinging 79 4.4 Nutritional dependence 85 4.5 Infant locomotion 89 4.6 Mortality 95 4.7 Allomothering 101 4.8 Conclusion 107 Chapter 5: Analogues for the proto-biped infant: Homo sapiens 5.0 Introduction 108 5.1 Hair reduction 109 5.2 Adipose tissue 114 5.3 Secondary altriciality 119 5.4 Childhood 124 5.5 Allomothering 126 5.6 Infant mortality 128 5.7 Clinging/Carrying 131 5.8 Locomotion 140 5.8.1 Factors which protract human locomotor ontogeny 146 5.9 Conclusion 148 Chapter 6: Analogues for the proto-biped infant: early fossil hominins 6.0 Introduction 149 6.1 Early hominin LH: consensus and debate 151 6.1.1 Old and new paradigms 151 6.1.2 Proto-biped LHs 153 6.2 Secondary altriciality 153 10 6.2.1 Secondary altriciality and brain sizes 153 6.2.2 Secondary altriciality and australopiths 157 6.2.3 Secondary altriciality and proto-bipeds 159 6.3 Childhood 160 6.3.1 Childhood in Homo 161 6.3.2 Childhood in australopiths 164 6.3.3 Childhood in proto-bipeds 165 6.4 Carrying/Clinging 6.4.1 Motor maturation and clinging 165 6.4.2 Arboreality and clinging and carrying 166 6.4.3 Proto-biped clinging and carrying 171 6.5 Infant locomotion 6.5.1 Australopith locomotor ontogeny 172 6.5.2 Proto-biped locomotor ontogeny 178 6.6 Allomothering 179 6.7 Conclusion 183 Chapter 7: Reconstructions of the chimpanzee-human last common ancestor (CHLCA) 7.0 Introduction 185 7.1 CHLCA reconstruction methods 186 7.2 Chimpanzee-like and non-chimpanzee-like CHLCAs 187 7.3 The fossil record and the CHLCA 192 7.3.1 Level of encephalization 193 7.3.2 Sociality 195 7.3.3 Gross LH pace 196 7.4 Controversy: inferring other aspects of the CHLCA from the fossil record 198 7.4.1 The controversial African ape last common ancestor (AALCA) 199 11 7.4.2 The controversial earliest hominins 200 7.4.3 Limitations of the fossil record for reconstructing CHLCA locomotion 201 7.4.4 The ambiguous fossil record 203 7.5 Precursor locomotion hypotheses 204 7.5.1 Current precursor locomotion hypotheses 207 7.5.2a Knuckle-walking (KW) 211 7.5.2b Arboreal pronogrady 211 7.5.2c Arboreal orthogrady 212 7.5.2d Current locomotor hypotheses: similarities and differences 212 7.6 Widely inferred characters of the CHLCA and contentious characters 214 7.7 CHLCA reconstructions and the proto-biped infant 221 Chapter 8: Proto-biped infant reconstruction 8.0 Introduction 222 8.1 Relative body size 8.1.1 Parameters of immature body size in proto-bipeds 223 8.1.2 Relative sizes of immature and adult anthropoids 227 8.2 The reconstructed proto-biped infant: aspects of infancy inferred from living and fossil analogues and CHLCA reconstructions 240 8.3 The reconstructed proto-biped infant: uninferrable aspects 249 8.4 The basic proto-biped infant 250 12 Chapter 9: Tests of model types as blocs: ‘Conserving Energy’, ‘Cooling’, ‘Standing-and-shuffling’ 254 Chapter 10: Tests of dual-variant model types by variant: ‘Exaptive Arboreality’ and ‘Sitting’ 269 Chapter 11: Tests of dual-variant model types by variant: ‘Versatility’ and ‘Water’ 287 Chapter 12: Tests of the Standing model type by five variants 311 Chapter 13: Tests of the ‘Display’ variant of the Standing model type, by version