Are Two Legs Better than Four? New Approaches to Studying Human Locomotor Evolution

David Raichlen - Department of Anthropology at University of Arizona  

Abstract: Recent advances in the study of evolutionary biomechanics allow us to reexamine essential questions in human evolution.  Here, I will address two major questions using novel approaches: why did bipedalism evolve?, and was running an important element of human evolution?  First, I will use a combination of experimental data and biomechanical modeling to explore the importance of energetics in the evolution of bipedal locomotion.  New experimental data from adult chimpanzees allow us to understand, for the first time, the determinants of ape locomotor energetics.  Combining the experimental data with biomechanical models, I will explore how anatomical changes impact ape energetics and therefore how selection may have altered early hominin anatomy to make bipedalism an energetically attractive alternative to ape-like quadrupedalism. 

Second, I will examine human running from a novel perspective: the evolution of neurobiological motivation.  Recent research suggests that humans evolved endurance running capabilities similar to those of cursorial quadrupeds (i.e. the endurance running hypothesis).  Testing the endurance running hypothesis generally consists of identifying morphological convergences between humans and quadrupeds.  However the comparative approach is constrained by biomechanical differences between human and quadrupedal running.  Instead of examining anatomical convergences between humans and quadrupeds, I will explore the motivation to run, and specifically whether selection can generate endurance running by enhancing neurobiological reward mechanisms.  Since running is both energetically costly and increases risks of injury, selection may have used neurobiological rewards to motivate running behaviors in cursorial mammals, including humans.  Discovering convergence in the motivation to run represents a clear test of the endurance running hypothesis.  Thus, for both bipedal walking and running, novel approaches in evolutionary biomechanics hold the promise of answering long-standing questions, and testing new and controversial hypotheses.