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Australopithecus Afarensis a Thesis Submitted in Partial Satisfaction Of CALIFORNIA STATE UNIVERSITY, NORTHRIDGE VALIDITY OF THE SPECIES: Australopithecus afarensis A thesis submitted in partial satisfaction of the requirements for the degree of Master of Arts in Anthropology by James Braxton May, 1983 The Thesis of James Braxton is approved: California State University, Northridge ii ACKNOWLEDGEMENTS I wish to thank the following people for their support and guidance. First, the members of my committee. Second, my family for their generous support, ideas, and allowing me the time to complete my endless obsession. Finally, to Nancy Murray who generously donated her time, her computer expertise, and for solving my constant program errors. iii TABLE OF CONTENTS Acknowledgements • iii List of Tables vi List of Figures vii Abstract • viii CHAPTER I. INTRODUCTION 1 II. LITERATURE REVIEW • 4 Biostratigraphical and Geochronological Evidence 4 Osteological Remains 7 Morphological Characteristics • 10 Jaw and Dental Characteristics 10 Cranial Characteristics • 14 Postcranial Morphology 15 Biological Species Concept 17 The International Code of Zoological Nomenclature and the Taxonomic Status of Hadar-Laetoli Fossils .• 17 Summary • 20 iv Table of Contents (continued) II I. MATERIALS AND METHODS 22 Materials--Fossil Hominid Sample • 22 Comparative Sample • 25 Measurement Data • 28 Methods 28 Univariate Analysis 31 Multivariate Analysis 31 Fossil Hominid Sample 32 IV. RESULTS • 34 Univariate Analysis Histograms: Fossil Hominid Sample Variation 34 Multivariate Analysis: Fossil Hominid Sample 41 Canonical Variate Analysis 48 Summary of Results 50 v. DISCUSSION 55 Morphological Characteristics • 55 International Code of Zoological Nomenclature • 56 Geographic Isolation 57 Statistical Implication • 58 VI. CONCLUSION 61 Literature Cited • 64 v LIST OF TABLES Table Page 1. Comparison Between A.afarensis and later Australopithecus and Early Homo • • • • • • • • 11 2. Fossil Hominid Specimens Included in the Analysis and their Geologic Origin • • • • . 23 3. Taxonomic Assignments of the Fossil Hominid Specimens • • • • • • • • • • • • • • • • 26 4. Fossil Hominid Sample Data - Maxillary Dentition . 29 5. Fossil Hominid Sample Data - Mandibular Dentition . 30 6. Fossil Hominid Sample Data • . 33 7. Principal Component Analysis -Maxillary Dentition • 42 8. Principal Component Analysis - Mandibular Dentition 43 9. Principal Component Scores - Maxillary Dentition • • 44 10. Principal Component Scores - Mandibular Dentition 46 11. F Test of Mahalanobis D2 -Maxillary and Mandibular Dentitions • • • • • • . 49 12. Jackknifed Classification - Maxillary and Mandibular Dentitions • • • • • • • • • . 53 vi LIST OF FIGURES Figure Page 1. Geological Column of the Hadar Formation Histograms 6 2. Maxillary Dentition - Canine . 35 3. Maxillary Dentition - p3 . 36 4. Maxillary Dentition - Ml 37 5. Mandibular Dentition - Canine 38 6. Mandibular Dentition - P3 39 7. Mandibular Dentition - M1 40 8. Bivariate Plot of Principal Component Scores - Maxillary Dentition • • • • • • • • • • • • . 45 9. Bivariate Plot of Principal Component Scores - Mandibular Dentition •••••••••••• . 47 10. Bivariate Plot of Canonical Variate Analysis - Maxillary Dentition • • • • • • • • • • • • . 51 11. Bivariate Plot of Canonical Variate Analysis - Mandibular Dentition • • • • • • • • • • • • . 52 vii ABSTRACT VALIDITY OF THE SPECIES: Australopithecus afarensis by James Braxton Master of Arts in Anthropology This thesis is an attempt to test the hypothesis that Austra­ lopithecus afarensis is not significantly different from any other Plio-Pleistocene hominid. This problem is of interest to anthro­ pologists because corroborating the hypothesis of the proposed taxon A.afarensis has important implications for interpretations of early hominid phylogeny. Analyses of fossil material from Hadar and Laetoli suggest the following: 1) A.afarensis is the earliest known bipedal hominid, 2) the extreme degree of morphological variation exhibited by A.afarensis can most likely be attributed to sexual dimorphism (rather than the Hadar-Laetoli fossils representing more than one species), and 3) controversy over the taxon A.afarensis has caused a reinterpretation of early hominid evolution. The validity of the proposed species, A.afarensis, was evaluated by a critical review of viii the literature and statistical analyses of dental measurements of a representative sample of Pliocene and Pleistocene hominids. Uni­ variate and multivariate statistical methods were utilized to compare the variability exhibited by A.afarensis with that of the Fossil Hominid sample. The results of these analyses suggest that A. afarensis is a valid taxon. ix CHAPTER I INTRODUCTION Fossil remains of the Pliocene and Pleistocene hominids that represent the genus Australopithecus show considerable geographical, temporal, and morphological variation. Interpretation of these vari­ ations has led to many hypotheses, such as those proposed by: 1) Wolpoff (1973) and Brace (1979), who state that sexual dimorphism within one species can account for the known morphological variation exhibited by the genus Australopithecus, 2) Walker and Leakey (1978) who hypothesized that more than one species evolved, such that dif­ ferent australopithecene species may have been contemporaneous both in time and space, and 3) Robinson (1963), whose dietary hypothesis suggests that several genera may have co-existed simultaneously. Plio-Pleistocene sites from Eastern and Southern Africa have yielded fossil material for many hominids and among these remains, several australopithecine forms have been discovered. Australopithe­ cus africanus remains may be represented at Lake Turkana, Kenya, and Omo, Ethiopia, as well as the South African sites of Makapansgat and Sterkfontein (Howell, 1969; Leakey, 1978; Wolpoff (1971). However, Cronin et al. (1981) doubt the existence of A.africanus in the East African geographic area. Fossil material for A.boisei comes from Olduvai Gorge, Tanzania, and Omo, Ethiopia. Fossil remains for Aus­ tralopithecus robustus come from Lake Turkana, Kenya, as well as the 1 2 South African sites of Kromdraai and Swartkrans (Leakey, 1978; Wolpoff, 1971). These East and South African deposits represent time spans of 3.0 million to 1.5 million years for hominids finds (Howell, 1969; Leakey, 1966; Leakey, 1972; Tobias, 1976). Fossil remains of the proposed hominid Australopithecus afarensis were found at two Pliocene sites in East Africa (subsequent use of the name A.afarensis is for descriptive purposes only and not an advocated position). These sites are Hadar, Ethiopia, and Laetoli, Tanzania, whose hominid yielding deposits are dated between 3.65 and 4.2 million years (Johanson et al., 1978; Leakey et al., 1976). Interpreting the features of the fossil material for A.afarensis, Johanson and White (1979:325-327) suggest that dental, cranial, and postcranial characteristics show considerable differences from pre­ viously known Plio-Pleistocene hominids. Such features include: 1) large canines that project beyond the tooth row, 2) a diastema that interrupts the tooth row between the lateral incisors and 3 canines, 3) a C/P3 cutting complex, 4) sectorial P3 's, 5) P 's with two and sometimes three distinct roots, and 6) alveolar prognathism. Furthermore, fossil remains from Hadar and Laetoli are interpreted by Johanson (1980) to represent a new form of early hominid which is characterized by size variation, most likely due to sexual dimorphism. To test the null hypothesis that A.afarensis is not significantly different from all other Plio-Pleistocene hominids, the following points will be examined: 1) biostratigraphical and geochronological evidence, 2) current concepts on A.afarensis' taxonomic status, 3) anatomical comparisons between A.afarensis and other known australo­ pithecines, and 4) biometric analyses of dental measurements for 3 ~.afarensis and a representative sample of Pliocene and Pleistocene hominids. The validity of Weinert's classification of the Garusi I specimen as Meganthropus africanus will also be discussed (Johanson, 1980). Therefore, it is the purpose of this paper to test the null hypothesis that Australopithecus afarensis is not significantly different from any other Plio-Pleistocene hominid. The alternate hypothesis is that ~.afarensis is significantly different from all other known Plio-Pleistocene hominids. The paramount issue is whether the morphological differences between A.afarensis and other Plio-Pleistocene hominids are sufficient to justify a separate taxonomic classification for A.afarensis. Characteristics of the cranial, dental, and postcranial morphology can be used to ascertain the taxonomic status of fossil material. Teeth, either isolated or associated with other fossil material, along with mandibles and maxillae, represent the majority of fossil hominid remains from Pliocene and Pleistocene deposits. Dental morphology has proved to be invaluable when making taxonomic distinction between australopithecine species (Robindon and Steudel, 1973). To analyze dental variation, statistical (biometric) procedures were performed on measurements from a representative sample of Pliocene and Pleistocene hominids. Results from these statistical analyses were used to test the null hypothesis. CHAPTER II LITERATURE REVIEW The collection of fossil remains of the proposed hominid Australopithecus afarensis comes from two known Pliocene sites in Eastern Africa. These sites are Hadar, Ethiopia (11N, 40 30E) and Laetoli, Tanzania (3 128, 35 11E) which have been dated between
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