Walking on two feet (bipedal walking) is a hallmark of human behavior that matures gradually throughout development. The way in which the foot contacts the ground, and thus the way in which foot bones are loaded during walking, differ in toddlers and adults. As toddlers grow, the shape and structure of their internal (trabecular) bone also changes. The objective of this study is to document concurrent developmental changes in human foot bone loading and internal bone structure within a comparative context. Additionally, this study uses comparative data from living apes and humans to determine the nature and development of walking adaptations in early human ancestors (Australopithecus afarensis). This study consists of three main parts: 1) an experimental analysis of forces that load the foot during bipedal walking in human toddlers, 2) a micro CT scan analysis of trabecular structure in human and African ape foot bones that receive high forces during walking, and 3) a trabecular analysis of juvenile and adult A. afarensis foot bones. By identifying functionally significant anatomical correlates to bipedal walking within human foot bones, this study will help resolve debate regarding the extent to which bipedal walking in A. afarensis resembled that of modern humans. Moreover, this study will examine the bipedalism of A. afarensis in greater detail than previous studies because it will be the first to infer bipedal development from foot bones in this fossil species.
Beyond the scope of physical anthropology, this study has clinical implications for pediatric orthopedics including the study of normal and pathological bone changes in juveniles. This project will also expand research opportunities available to undergraduates by affording undergraduate students (particularly members of under-represented groups) the opportunity to obtain hands-on research experience by aiding in data collection.
Among primates, humans are unique in habitually walking on only two feet (bipedal walking). The evolution of bipedalism in the human lineage is among the most studied yet least resolved topics in physical anthropology. Paleoanthropologists rely on form-function relationships to interpret fossil foot bones. But testing such relationships is challenging. The purpose of this study was to test the relationship between the form of internal (trabecular) bone and the function of the foot from a developmental perspective. We documented concurrent developmental changes in 1) how foot bones are loaded during walking and 2) the internal structure of foot bones in order to infer the development of bipedalism in early fossil humans. Bipedalism begins around one year of age and matures gradually. While adults step down with their heel first and then propel themselves forward with their big toe (hallux) at the end of stance, new walkers lack a heel-strike at touchdown and hallucal propulsion at toe-off. In Part 1 of this study, we conducted an experimental analysis of forces that load the foot during bipedal walking in toddlers. Our toddlers ranged in age from 11 months to 4 years old. The youngest toddlers had been walking for only 2 weeks while the oldest toddlers had been walking for as many as 3 years. We found that in new walkers (with less than 1 year of walking experience), the location of center of pressure (COP, the location on the foot through which ground reaction forces travel) is behind the metatarsal heads (ball of the foot) when the foot pushes off the ground. By two years of age, the center of pressure has shifted forward to beneath the hallux. Since the COP is not located beneath the hallux just prior to push-off in inexperienced walkers, our data confirm that new walkers lack a propulsive hallucal toe-off. The presence of the COP beneath the hallux in experienced walkers suggests that, like adult humans, hallucal propulsion is present by the end of the first year of walking. As toddlers grow, the shape and structure of their trabecular bone also changes. Based on the results of our biomechanical study, we predicted that trabecular architecture in human foot bones would differ with walking experience due to more stereotypical loading as bipedal walking matures. In Part 2 of this study we tested for an association between foot bone loading that occurs during walking and trabecular bone architecture in experienced toddlers and adult humans by conducting a microCT scan analysis of the juvenile and adult human talus (ankle), calcaneus (heel), and hallucal metatarsal head. Additionally, we used a comparative approach by further investigating trabeculae in African ape (chimpanzee, bonobo, and gorilla) foot bones. Our comparisons of juvenile and adult pedal trabecular bone architecture (Part 2) corroborate findings from our biomechanics of toddler walking study (Part 1). In Part 1, we showed that inexperienced walkers lack a propulsive hallucal push-off, suggesting that the foot bones of new walkers do not receive strong forces associated with ankle extension (plantarflexion) during stance. In Part 2 we showed that trabecular architecture in adult and experienced toddler foot bones (> 2 years old) preserves evidence of joint tightening during powerful plantarflexion, while these features are lacking in 1-2 year old humans and African apes (who lack hallucal propulsion at lift-off). Results from comparisons within humans and comparisons between humans and African apes support the hypothesis that the juvenile human foot does not receive stereotypical loading during immature bipedalism. Our results reveal that developmental changes in trabecular bone reflect changes in walking development. With our large comparative database of living apes and human internal foot bone architecture, we can now determine the nature and development of walking adaptations in early human ancestors (Australopithecus afarensis). In Part 3 of this study we are conducting a trabecular analysis of the foot bones of a juvenile ("Lucy’s baby") and adult ("Lucy") A. afarensis. By identifying functionally significant anatomical correlates to bipedal walking within human foot bones, this study will help resolve debate regarding the extent to which bipedal walking in A. afarensis resembled that of modern humans. Moreover, this study will examine the bipedalism of A. afarensis in greater detail than previous studies because it will be the first to infer bipedal development from juvenile foot bones in this fossil species. Beyond the scope of physical anthropology, this study has clinical implications for pediatric orthopedics including the study of normal and pathological bone changes in juveniles. This project expanded research opportunities by affording six undergraduate students (including five females, four individuals from under-represented groups, and three first generation college students) the opportunity to obtain hands-on research experience by aiding in biomechanical data collection.
