Using experimental biomechanics to reconstruct the evolution of hominin locomotor postures
Authors: Raichlen David A. , Gordon Adam D. 
Published: June 2021

Debates over the evolution of hominin bipedalism, a defining human characteristic, often revolve around whether early bipeds walked more like humans, with energetically economical extended hind limbs, or more like apes with flexed hind limbs. Here, we detail experimental data from humans and chimpanzees that links anatomy with lower limb posture and the energy costs of walking. Using these anatomical links, we examine the fossil record to determine when adaptations evolved that allowed for extended limb bipedalism.To complement anatomical studies of hominin fossils, we apply evolutionary biomechanics to the 3.6 million year old hominin footprints at Laetoli, Tanzania, which represent the earliest direct evidence of hominin bipedalism. We compare footprints from the original Lateoli Site G (n=8) and the newly described Site S (n=8) with experimentally generated footprints from modern humans (n=8 subjects) made using both extended and flexed limb, or Bent‐Knee, Bent‐Hip (BKBH) biomechanics. In our modern human sample, footprints generated during BKBH walking leave deeper toe relative to heel impressions compared with footprints generated during normal, extended limb walking (linear mixed effects model: p<0.001). The difference in proportional toe depth (calculated as log10[maximum fore‐foot depth/maximum heel depth]) is linked to the mechanical consequences of BKBH walking. During walking, the center of pressure (COP; the point of ground force application) moves from the heel at touchdown to the forefoot during toe‐off, and the forces applied to the ground determine, to some degree, the depth of the impression under the COP. As the COP travels past the mid‐foot the human heel rises due to the presence of a stiff longitudinal arch, and forces following heel rise deform the substrate under the toes. From experimentally generated force‐plate records, we demonstrate that the COP passes the mid‐foot earlier in the step in BKBH walking, leading to larger forces under the toes and relatively deeper impressions.Using linear mixed effects models, we show that prints from Site G and the newly described prints from Site S at Laetoli are most consistent with weight transfer patterns from extended limb biomechanics. Proportional toe depths do not differ significantly between the Laetoli sites (G vs. S: p=0.998), and do not differ between Laetoli and modern humans walking with an extended limb gait (p=0.195). However, the print morphology at Laetoli does differ significantly from those made by humans walking with a flexed limb (p<0.001) due to the uniquely deep toe depressions that occur in humans walking with this more ape‐like gait.Thus, using experimental data from living humans, we interpret morphology from both the Laetoli footprints and from early hominin skeletons to propose extended limb bipedal posture evolved by 3.6 Ma. These results represent the earliest direct evidence of kinematically human‐like bipedalism currently known. Thus, our results, suggest that selection acted to increase the energy economy of bipedalism early in human evolution and that efficient extended limb bipedalism evolved long before the appearance of the genus Homo.Support or Funding InformationFunded by the L.S.B. Leakey Foundation and the University of ArizonaThis abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.