Evolution of the Hominoid Vertebral Column by Scott
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EVOLUTION OF THE HOMINOID VERTEBRAL COLUMN BY SCOTT A. WILLIAMS DISSERTATION Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Anthropology in the Graduate College of the University of Illinois at Urbana-Champaign, 2011 Urbana, Illinois Doctoral Committee: Associate Professor John D. Polk, Co-Chair Assistant Professor Charles C. Roseman, Co-Chair Assistant Professor Laura L. Shackelford Professor Steven R. Leigh Professor Lyle W. Konigsberg ABSTRACT This is a study of the numerical composition of the vertebral column, the central structure of the vertebrate body plan and one that plays an instrumental role in locomotion and posture. Recent models of hominoid vertebral evolution invoke very different roles for homology and homoplasy in the evolution of vertebral formulae in living and extinct hominoids. These processes are fundamental to the emergence of morphological structures and reflect similarity by common descent (homology) or similarity by independent evolution (homoplasy). Although the "short backs," reflecting reduced lumbar regions, of living hominoids have traditionally been interpreted as homologies and shared derived characters (synapomorphies) of the ape and human clade, recent studies of variation in extant hominoid vertebral formulae have challenged this hypothesis. Instead, a "long-back" model, in which primitive, long lumbar regions are retained throughout hominoid evolution and are reduced independently in six lineages of modern hominoids, is proposed. The recently described skeleton of Ardipithecus ramidus is interpreted to support the long-back model. Here, larger samples are collected and placed in a larger phylogenetic context than previous studies. Analyses of over 8,000 mammal specimens, representing all major groups and focusing on anthropoid primates, allow for the reconstruction of ancestral vertebral formulae throughout mammalian evolution and a determination of the uniqueness of hominoid vertebral formulae. This survey, in combination with analyses of intraspecific diversity and interspecific similarity, suggests that reduced lumbar regions are homologous in extant hominoids. Furthermore, hominoid vertebral formulae are unique among primates and relatively unique among mammals in general. Hominins likely evolved five lumbar ii vertebrae from a short-backed ancestor with an "African ape-like" vertebral profile. By the appearance of Australopithecus, hominins evolved a cranial placement of the diaphragmatic (one that bears a change in articular facet orientation) vertebra, which generates a functionally longer lower spine while maintaining five lumbar vertebrae. In light of these findings, it is proposed that bipedalism evolved in a party arboreal, partly terrestrial African ape-like locomotor context. iii For Milena and Oliver, who provided much-needed distraction and kept me on track, and for Evan, my doppelganger, nephew, and fellow naturalist. iv ACKNOWLEDGMENTS I am fortunate to have had invaluable guidance, help, and support from family, friends, colleagues, and mentors throughout my education, culminating in the form of this dissertation; as such, many thanks and acknowledgments are owed. I address these chronologically. My parents have unhesitantly encouraged my education, from childhood to my decision to declare my undergraduate majors in Anthropology and History, through to my enrollment in graduate school for a then- undetermined number of years. A large supply of animal books and toys and frequent camping trips to the woods piqued my interest in nature and its mysteries. My mom, Denise, was particularly helpful in middle school, when she faithfully and thoroughly checked my homework every night, and later when she made sure I enrolled in college. My dad, Bob, encouraged me to think freely and supported with gusto my pursuit of all things historical and scientific; his influence on and interest in my academic career have been unmatched, even by my academic advisers. Most of all, my parents have always encouraged me to pursue my interests unstifled by the thralls of societal expectations and the status quo. Other family members have been equally supportive and influential. My sister, Jen, who inherited all of the real talent – art, music, and the like – in my family, has been an inspiration and a great friend. Donnie Riggs and Sandy Wehmann Williams have assumed the roles of supportive step-parents. I thank the major influences of my undergraduate education at Kent State University, Marilyn Norconk, C. Owen Lovejoy, John Harkness, W. Frank Robinson, and the late, great Olaf Prufer. John's and Frank's incredibly intriguing and fiery lectures, respectively, piqued my interest in anthropology in the first place. It was in John's introductory courses that I was first v introduced to the work of Owen and Olaf. I consider myself fortunate to have had the opportunity to sit through a countless number of Olaf's lectures, taking in his marvelous stories and hurriedly jotting down extremely eloquent, albeit sometimes crude, quotes on politics, philosophy, and all things anthropology. Owen's presence and writings were highly influential on the formation of my interests in functional morphology, locomotion, bipedalism, and biological anthropology in general. Marilyn's courses, zoo trips, and assumption of the role of my "unofficial" undergraduate adviser was instrumental in my decision to apply to graduate school to work with Dan Gebo at Northern Illinois University. Dan was an excellent teacher and adviser and continues to be a close friend and mentor. The depth and well-roundedness of his knowledge of all aspects of biological anthropology are astounding and turned out to be a wonderful resource for an ambitious and unfocused student. Dan's guidance and courses, especially primate anatomy and evolution, helped me focus in on a master's thesis topic and are largely responsible for my interests in the evolution of orthogrady. It was also in Dan's class that I was first introduced to the lumbar transverse process and its intriguing differences among hominids, hylobatids, and non-hominoid primates, and between the mystery apes of the early Miocene, Proconsul and Morotopithecus. I consider myself lucky that my short time at NIU coincided with that of Art Durband, who taught human osteology and paleontology. Art is an extremely talented teacher and I learned a lot from him. Fellow (and senior) graduate students Kat Blake, Kierstin Catlett, and Amanda Zika were excellent role models and are still close friends. Amanda, in particular, was an endless source of entertainment. At the University of Illinois, I have had the good fortune of taking courses and working with a diverse group of faculty, including my co-advisers, John Polk and Charles Roseman, committee members Laura Shackelford, Lyle Konigsberg, and Steve Leigh, and Paul Garber, vi among others. John was initially my sole adviser and the PI of my NSF Doctoral Dissertation Improvement Grant, and I thank him for his guidance, support, and remarkable ability to move furniture and draft letters of support at the drop of a dime. It was in his course on interpreting behavior in the fossil record that I was first introduced to the existence and significance of the diaphragmatic vertebra, to which an entire chapter of this dissertation is dedicated. Some of my most humbling experiences in graduate school took place in Charles's courses. Charles seems to have an uncanny ability to detect when a student does not know an answer or is not paying attention, which he acts on without fail. His influence became an integral part of my graduate training and understanding of evolution; after all, Charles introduced me to morphological integration and quantitative genetics and grounded my thinking firmly in evolutionary theory. Lyle Konigsberg and Laura Shackelford taught me human gross anatomy. One would be hard-pressed to find a more kind and skilled pair of anatomists. Laura's faithful attention to three anthropologists amongst a lab full of needy and highly competitive medical students kept us both on track and sane. Although Laura joined my committee relatively late in the game, she has been incredibly proficient, constructive, and meticulous, and her advice and thorough input improved my dissertation substantially. Lyle's sharp wit and dry sense of humor generated much-needed laughs during 8 o'clock lectures and in the dissection lab. I had the rare privilege of being Lyle's T.A. for human osteology, a significant portion of which was spent on the floor of his office, rooting through boxes of cremains for appropriate test material, attempting to find the happy- medium between Lyle's "that's a little too easy" and "hmm, that's pretty tricky." Steve taught two very influential courses, on ontogeny and phylogeny and the history of biological anthropology, and, along with Milena Shattuck, introduced me to the evolutionary theory of aging and to longevity and senescence as fundamentally important aspects of life history evolution. vii Fellow graduate students have provided a reliable support base and have become close friends and colleagues. Mark Grabowski and I entered the program at Illinois at the same time and have been guinea pigs for many "experiments" – among other things, we are our shared co- advisers' first graduate students. But being their guinea pigs beats being Charles's mice or John's sheep. Our dissertation writing group, consisting of Mark, Milena Shattuck, Krista Milich, and Petra Jelinek, was extremely