Bipedality is the defining characteristic of the hominin clade. The pelvis is a fundamental component of the hindlimb locomotor system and underwent dramatic reorganization to facilitate the adoption of obligate bipedality. In spite of this, pelvic development, function, and evolution remain poorly understood. My research program aims to fill this gap by clarifying the proximate and ultimate causes of pelvic anatomy, and by using this information to more accurately reconstruct locomotion in fossil primates.
Pelvic adaptations to locomotion in primates
While correlations between locomotor behavior and bony pelvic anatomy have been observed, adaptive hypotheses that relate locomotor mechanics to pelvic morphology have not been rigorously tested. This project used three-dimensional landmark data to test adaptive hypotheses in a phylogenetically broad, comparative sample of over 800 human and non-human primate pelves from 67 taxa. I identified specific traits of the ilium and ischium that are adaptations to locomotion, which will increase the accuracy of reconstructions of locomotion in fossil primates.
Experimental biomechanics of pelvic loading
A theoretical model of pelvic mechanics is required to understand how pelvic form is related to function. I proposed and tested a theoretical mechanical model of pelvic stress resistance using an experimental biomechanics approach. This project involved work on primate cadaver specimens using in vitro strain gauges to determine how the pelvis resists stress during loading, which laid the groundwork for me to develop a general biomechanical model of pelvic stress resistance. This project was completed at the Spinal Biomechanics Laboratory at Barrow Neurological Institute in Phoenix, AZ (directed by Neil Crawford).
Morphological integration and modularity
The three bones of the pelvis (the ilium, ischium, and pubis) exhibit differences in the timing and genetic regulation of their development, and this may have an effect on pelvic morphological integration and evolvability. This project tested a hypothesis of developmental modularity of the ilium and ischiopubis and examined overall levels of morphological integration and the ability of the pelvis to respond to selection (i.e., evolvability). The results of this work demonstrate that the ilium and the ischiopubis are separate modular units that may result from their differing developmental genetic regulation, and that all primates share a common pattern of relatively low levels of pelvic integration and relatively high levels of evolvability (Lewton, 2012). These results suggest that the evolution of the primate pelvis, including the dramatic reorganization of the hominin pelvis that occurred during the Plio-Pleistocene, occurred via a developmental pathway that is common to all primates.
Metabolic cost of hip width on bipedality
It has long been assumed that a wide pelvis is costly during bipedal locomotion. The ‘obstetrical dilemma,’ proposed 70 years ago, posits that difficult in human childbirth is a result of a tradeoff between a pelvis just wide enough to accommodate a large-brained fetus, but not so wide as to impair bipedal locomotion. However, the metabolic cost of hip width has not been experimentally determined. This project analyzed the effects of pelvis width on the metabolic cost of human walking and running. Oxygen consumption and kinematic data during walking and running were measured in a sample of fit human males and females, and demonstrated that there is no relationship between hip width and metabolic cost of bipedality. Given the differences in pelvic dimensions and leg length between early hominins and genus Homo, and the inferred differences in locomotor patterns between them, our findings suggest that 1) possessing relatively wide hips was likely not a major cost for Australopithecus as a walking biped, 2) there is no support for the ‘obstetrical dilemma’ as currently defined (i.e., the tradeoff between economical locomotion and birthing large-brained infants), and 3) we need to test other hypotheses of selection pressures on hip width.
Collaborators: Anna G. Warrener (Harvard University), Herman Pontzer (Hunter College), Dan Lieberman (Harvard University)
I have also participated in several field research projects in East and South Africa, focusing on identifying new hominin fossil sites.
In 2005 I joined the Awash-Mille Survey Project, in the Afar Regional State of Ethiopia, led by Charlie Lockwood and Kaye Reed of the Institute of Human Origins at Arizona State University.
I recently surveyed for new hominin fossil sites in the Malmani Dolomites of South Africa, as part of the South African Palaeocave Survey project, led by Kevin Kuykendall of the University of Sheffield in the UK.
My research on pelvic morphology led me to the Beza Mahafaly Special Reserve in rural southwest Madagascar, which has an osteological collection of lemurs (the Beza Mahafaly Osteological Collection).