Form and function of the human and chimpanzee forefoot: implications for early hominin bipedalism

Abstract

During bipedal walking, modern humans dorsiflex their forefoot at the metatarsophalangeal joints (MTPJs) prior to push off, which tightens the plantar soft tissues to convert the foot into a stiff propulsive lever. Particular features of metatarsal head morphology such as "dorsal doming" are thought to facilitate this stiffening mechanism. In contrast, chimpanzees are believed to possess MTPJ morphology that precludes high dorsiflexion excursions during terrestrial locomotion. The morphological affinity of the metatarsal heads has been used to reconstruct locomotor behavior in fossil hominins, but few studies have provided detailed empirical data to validate the assumed link between morphology and function at the MTPJs. Using three-dimensional kinematic and morphometric analyses, we show that humans push off with greater peak dorsiflexion angles at all MTPJs than do chimpanzees during bipedal and quadrupedal walking, with the greatest disparity occurring at MTPJ 1. Among MTPJs 2-5, both species exhibit decreasing peak angles from medial to lateral. This kinematic pattern is mirrored in the morphometric analyses of metatarsal head shape. Analyses of Australopithecus afarensis metatarsals reveal morphology intermediate between humans and chimpanzees, suggesting that this species used different bipedal push-off kinematics than modern humans, perhaps resulting in a less efficient form of bipedalism.

Figure 1. Comparative morphology of Homo, Pan, and A. afarensis (AL-333) metatarsals (MT1–MT5).

Note that Homo is characterized by dorsal overlap of the distal articular surface onto the MT shaft and by wide flattening of the dorsal articular surface (arrows). AL-333-115 demonstrates Homo-like dorsal overlap but lacks mediolateral widening at the MT1, similar to the condition seen in Pan. Left column: distal view. Right column: medial view. Bar: 1 cm.

Figure 2. Examples of metatarsophalangeal joint motion during stance phase of bipedal steps from a chimpanzee (top) and a human (bottom).

The peak dorsiflexion angles at the end of stance were used in statistical analyses.

Figure 3. Average peak metatarsophalangeal joint dorsiflexion angles during push off in humans and chimpanzees.

Plus/minus one standard deviation is indicated by error bars. *Human angles are significantly different from both bipedal and quadrupedal chimpanzee angles (Mann-Whitney U test, P < 0.001). ⁺Chimpanzee bipedal and quadrupedal angles are significantly different from one another (Mann-Whitney U test, P = 0.001).

Figure 4. PCA scatterplot of PC1 vs. PC2 for MT1 (left) and MT2–MT5 (right).

PC1 tracks dorso-plantar orientation of the MT head whereas PC2 tracks mediolateral MT head breadth. High negative PC scores indicate dorsally oriented MT heads, whereas high positive PC scores indicate more plantarly oriented MT heads. PC2 captures overall MT head robusticity, with considerable overlap between species. Wireframes represent articular shape differences of observed extremes for MT1 (PC1: −0.19–0.16; PC2: −0.10–0.12) and MT2–5 (PC1:−0.12–0.15; PC2: −0.08–0.12).