HUMAN LIFE HISTORIES IN AN EVOLUTIONARY AND COMPARATIVE CONTEXT by Shannen Lorraine Robson A dissertation submitted by the faculty of The University of Utah in partial fulfillment of the requirements for the degree of Doctor of Philosophy Department of Anthropology The University of Utah August 2011 Copyright © Shannen Lorraine Robson 2011 All Rights Reserved The University of Utah Graduate School STATEMENT OF DISSERTATION APPROVAL The dissertation of Shannen Lorraine Robson has been approved by the following supervisory committee members: Kristen Hawkes , Chair 04/27/2011 Date Approved Douglas Jones , Member 04/27/2011 Date Approved James O’Connell , Member 04/27/2011 Date Approved Eric Rickart , Member 04/27/2011 Date Approved Ken Smith , Member 04/27/2011 Date Approved and by Elizabeth Cashdan , Chair of the Department of Anthropology and by Charles A. Wight, Dean of The Graduate School. ABSTRACT This dissertation utilizes life history theory to describe traits that are derived in humans through comparisons with other primate species. Modern human life histories are unique in that they are slower, exhibiting distinctly long postmenopausal life spans and later ages at sexual maturity as a result of a reduction in adult mortality since the evolutionary split the last Pan-Homo ancestor. Faster reproduction with shorter than expected interbirth intervals and earlier weaning ages are likely the result of cooperative breeding featuring postmenopausal grandmothers. Life history traits are distinguished from life history related variables (LHRVs) which are used to makes inferences about life history variables in extinct taxa. Body mass LHRV is a strong predictive life history proxy, but brain size and dental development are only weakly associated and inferences using them should be made with caution. Age at first birth is a central variable in demographic life history models as it identifies the beginning of fertility. For most mammals, age at first birth is closely aligned with the timing of physiological maturity. Humans live in varying ecologies that influence maturation rates and have marriage institutions that can constrain sexual access to fecund females. With few exceptions, the floor of the range of human age at first birth is remarkably consistent at about 17-18 years. Women who experience their first births before this age suffer maternal and infant costs. Heterogeneity, the inherent variation in individual quality, may have an important impact on the timing of life history events. Individuals of lower quality in severe conditions are prone to culling, leaving a subset of robust individuals who thrive in measurable ways. A test of this heterogeneity hypothesis is conducted using a subset of historic vital records from the Utah Population Database. Results show that mothers of twins have a more robust phenotype with lower postmenopausal mortality, shorter average interbirth intervals, later ages at last birth, and higher lifetime fertility than their singleton-only bearing counterparts. Thus, bearing twins may be a useful index of maternal heterogeneity. iv This dissertation is dedicated to Ann Kelsey and Dr. Lois Alexander Merkler. TABLE OF CONTENTS ABSTRACT ...................................................................................................................... iii LIST OF FIGURES ........................................................................................................ viii LIST OF TABLES ..............................................................................................................x ACKNOWLEDGEMENTS ............................................................................................. xii INTRODUCTION ..............................................................................................................1 Chapter 1 THE DERIVED FEATURES OF HUMAN LIFE HISTORY ..............................5 Summary ......................................................................................................................6 Derived human life history traits .................................................................................8 Life history contrasts ..................................................................................................12 Effects of derived human life history .........................................................................19 Conclusions ................................................................................................................32 Acknowledgements .....................................................................................................33 References ..................................................................................................................34 2 HOMININ LIFE HISTORY: RECONSTRUCTION AND EVOLUTION ........47 Abstract ......................................................................................................................48 Introduction ................................................................................................................48 Part I. Life history and life history-related variables of extant hominids ..................49 Part II. Inferring the life history of extinct taxa .........................................................60 Summary ....................................................................................................................71 Conclusion ..................................................................................................................71 Acknowledgements .....................................................................................................71 References ..................................................................................................................71 Appendix: Notes for body mass and brain size data as used in Tables 3-4 ................78 3 AGE AT FIRST BIRTH IN HUMANS: EVALUATION OF A DEMOGRAPHIC VARIABLE WITHIN THE CONTEXT OF LIFE HISTORY ........................................................................................................80 Abstract ......................................................................................................................81 Introduction ................................................................................................................82 Variation in human age at menarche .........................................................................84 Human adolescent subfecundity and the consequences of early teen pregnancy .......87 Variation in human age at first birth ..........................................................................90 Marital fertility ...........................................................................................................91 Great ape comparison ................................................................................................94 Evaluating Charnov’s life history model ...................................................................97 Discussion ..................................................................................................................99 Acknowledgements ...................................................................................................101 Literature cited .........................................................................................................102 4 TWINNING IN HUMANS: MATERNAL HETEROGENEITY IN REPRODUCTION AND SURVIVAL ..................................................................124 Abstract ....................................................................................................................125 Introduction ..............................................................................................................125 Demographic data and methods ...............................................................................126 Results ......................................................................................................................127 Discussion ................................................................................................................128 References ................................................................................................................130 vii LIST OF FIGURES 1-1 Phylogenetic relationships of the great ape species ............................................... 8 1-2 Neonatal weight relative to maternal weight ........................................................ 17 1-3 Percent of adult brain size achieved by age .......................................................... 23 2-1 Phylogenetic relationships of the great ape species .............................................. 50 2-2 Comparison of modern human and chimpanzee absolute (panel A) and relative (panel B) brain growth trajectories ....................................................................... 57 2-3 The more speciose (splitting) taxonomy .............................................................. 60 2-4 Estimated body mass plotted against first appearance date for the fossil hominin taxa recognized in the splitting taxonomy .............................................. 64 2-5 Estimated body mass plotted against first appearance date for the fossil hominin taxa recognized in the lumping taxonomy ............................................. 65 2-6 Estimated endocranial volume plotted against first appearance date for the fossil hominin taxa recognized in the splitting taxonomy ..................................... 67 2-7 Estimated