Fossil hominin footprints and the dynamics of footprint formation
Open AccessBipedalism is a fundamental modern human behavior that has profoundly influenced our anatomy, behavior, and ecology. Yet many questions about the evolution of bipedalism in our extinct ancestors remain unanswered. Paleoanthropologists studying fossil hominin skeletal anatomy have analyzed these data in many different ways only to arrive at conflicting conclusions regarding when and how hominin bipedalism evolved. New approaches are necessary to resolve these ongoing debates.The goal of this dissertation was to take a different approach to investigating long-standing questions about the evolutionary history of human bipedal locomotion, by analyzing fossil hominin footprints. Footprints are usually ephemeral but a few sets made by extinct hominin taxa have been uncovered, and they provide unmatched windows into the evolutionary history of hominin locomotion. These footprints preserve the only direct records of both the foot anatomy and gait of extinct hominin taxa, and recent discoveries have greatly increased the sample of footprint sites known in the human fossil record. They offer a new approach for investigating questions about fossil hominin bipedalism, which circumvents many of the problems that may be associated with functional interpretations of skeletal evidence. However, before these data can be used to inform questions about the evolution of human bipedalism, we need to understand how biomechanical variables are actually recorded in, and can be inferred from, footprint morphologies. In this dissertation, experiments were conducted with modern taxa in order to achieve a quantitative understanding of the mechanical process through which footprints are produced. With an understanding of how specific biomechanical variables affect this process, experimental results were then used to derive informed functional interpretations of fossil hominin footprint morphologies. Biomechanical experiments were conducted with habitually barefoot modern humans and with chimpanzees to develop the first quantitative framework directly relating patterns of footprint morphological variation to specific biomechanical patterns, while also considering the confounding effects of sediment mechanical properties. The morphologies of c.3.7 Ma fossil hominin footprints from Laetoli, Tanzania and c.1.5 Ma footprints from Ileret, Kenya were then quantified and compared to the footprints created by modern humans. Functional interpretations could be made in the context of experimental results, relating patterns of variation in footprint morphology to specific biomechanical causes. Results indicate that the Laetoli footprints, which represent the only direct evidence of Pliocene hominin bipedalism, were likely produced by a different form of bipedalism than is seen in modern humans. The Ileret footprints, however, which offer direct evidence of early Pleistocene hominin bipedalism, appear indistinct from the footprints of modern humans. These results support the hypothesis that important changes to hominin locomotion (and therefore also their anatomy, behavior, and ecology) occurred near the late Pliocene or early Pleistocene.
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