The loss of the ‘pelvic step’ in human evolution

Human bipedalism entails relatively short strides compared with
facultatively bipedal primates. Unique non-sagittal-plane motions
associated with bipedalism may account for part of this discrepancy.
Pelvic rotation anteriorly translates the hip, contributing to bipedal
stride length (i.e. the ‘pelvic step’). Facultative bipedalism in
non-human primates entails much larger pelvic rotation than in humans,
suggesting that a larger pelvic step may contribute to their relatively
longer strides. We collected data on the pelvic step in bipedal
chimpanzees and over a wide speed range of human walking. At matched
dimensionless speeds, humans have 26.7% shorter dimensionless strides, and
a pelvic step 5.4 times smaller than bipedal chimpanzees. Differences in
pelvic rotation explain 31.8% of the difference in dimensionless stride
length between the two species. We suggest that relative stride lengths
and the pelvic step have been significantly reduced throughout
the course of hominin evolution.

与兼性两足灵长类相比，人类两足行走（bipedalism）的步幅相对更短。与两足行走相关的独特非矢状面运动，或是这一差异的部分成因。骨盆旋转可使髋关节向前平移，进而提升两足行走的步幅，即“骨盆步幅（pelvic step）”。非人灵长类在兼性两足行走时的骨盆旋转幅度远大于人类，这表明更大的骨盆步幅或许是其步幅相对更长的原因之一。我们采集了两足行走黑猩猩的骨盆步幅数据，以及人类在宽泛步态速度范围内的相关行走数据。在匹配的无量纲步态速度下，人类的无量纲步幅较两足行走黑猩猩短26.7%，骨盆步幅仅为后者的1/5.4。骨盆旋转的种间差异可解释两个物种间无量纲步幅差异的31.8%。我们认为，在整个人亚族（hominin）演化历程中，相对步幅与骨盆步幅均发生了显著缩减。