Three-dimensional kinematics and the origin of the hominin walking stride

Abstract

Humans are unique among apes and other primates in the musculoskeletal design of their lower back and pelvis. While the last common ancestor of the Pan-Homo lineages has long been thought to be 'African ape-like', including in its lower back and ilia design, recent descriptions of early hominin and Miocene ape fossils have led to the proposal that its lower back and ilia were more similar to those of some Old World monkeys, such as macaques. Here, we compared three-dimensional kinematics of the pelvis and hind/lower limbs of bipedal macaques, chimpanzees and humans walking at similar dimensionless speeds to test the effects of lower back and ilia design on gait. Our results indicate that locomotor kinematics of bipedal macaques and chimpanzees are remarkably similar, with both species exhibiting greater pelvis motion and more flexed, abducted hind limbs than humans during walking. Some differences between macaques and chimpanzees in pelvis tilt and hip abduction were noted, but they were small in magnitude; larger differences were observed in ankle flexion. Our results suggest that if Pan and Homo diverged from a common ancestor whose lower back and ilia were either 'African ape-like' or more 'Old World monkey-like', at its origin, the hominin walking stride likely involved distinct (i.e. non-human-like) pelvis motion on flexed, abducted hind limbs.

Keywords: chimpanzees; hominin evolution; humans; kinematics; macaques; walking.

Figure 1.

The lower back and pelvis of (a) a macaque, (b) a chimpanzee and (c) a human. Macaques possess six or seven lumbar vertebrae, a wide sacrum and tall, more sagittally oriented ilia. Chimpanzees possess three or four lumbar vertebrae, a narrow sacrum and tall, coronally oriented ilia. Humans possess five lumbar vertebrae, a wide sacrum and short, sagittally oriented ilia.

Figure 2.

The local coordinate systems of the (a) macaque, (b) chimpanzee and (c) human pelvis and lower/hind limb segments shown in sagittal (left panel) and frontal (right panel) views. Models are positioned in neutral postures, with attendant marker sets.

Figure 3.

The pelvis (a,b) tilt, (c,d) list and (e,f) rotation angles, relative to the global coordinate system, over a walking stride (mean ± s.d.) for macaques (solid red line) and chimpanzees (solid blue line) (column 1), and for macaques, chimpanzees and humans (dashed black line) (column 2). Each stride begins and ends at ipsilateral ‘heel strike’ (i.e. foot touchdown). All vertical lines show the average stride event times for the chimpanzees, which were similar among the species. The broken vertical lines represent contralateral limb toe-off (M: 15%; C: 14%; H: 13%) and contralateral limb ‘heel strike’ (M: 50%; C: 48%; H: 51%), which define the double-support and single-support periods of a stride; the solid vertical line represents ipsilateral toe-off (M: 65%; C: 62%; H: 63%), which defines the start of the swing phase.

Figure 4.

The hip (a,b) flexion, (c,d) adduction, (e,f) rotation, (g,h) knee flexion and (i,j) ankle flexion angles over a walking stride (mean ± s.d.) for macaques and chimpanzees (column 1), and for macaques, chimpanzees and humans (column 2). Each stride begins and ends at ipsilateral ‘heel strike’ (i.e. foot touchdown). Vertical lines show the average stride event times for the chimpanzees, which were similar among the species. The broken vertical lines represent contralateral limb toe-off (M: 15%; C: 14%; H: 13%) and the contralateral limb ‘heel strike’ (M: 50%; C: 48%; H: 51%), which define the double-support and single-support periods of a stride; the solid vertical line represents ipsilateral toe-off (M: 65%; C: 62%; H: 63%), which defines the start of the swing phase.