Functional morphology of the ankle and the likelihood of climbing in early hominins

Abstract

Whether early hominins were adept tree climbers is unclear. Although some researchers have argued that bipedality maladapts the hominin skeleton for climbing, others have argued that early hominin fossils display an amalgamation of features consistent with both locomotor strategies. Although chimpanzees have featured prominently in these arguments, there are no published data on the kinematics of climbing in wild chimpanzees. Without these biomechanical data describing how chimpanzees actually climb trees, identifying correlates of climbing in modern ape skeletons is difficult, thereby limiting accurate interpretations of the hominin fossil record. Here, the first kinematic data on vertical climbing in wild chimpanzees are presented. These data are used to identify skeletal correlates of climbing in the ankle joint of the African apes to more accurately interpret hominin distal tibiae and tali. This study finds that chimpanzees engage in an extraordinary range of foot dorsiflexion and inversion during vertical climbing bouts. Two skeletal correlates of modern ape-like vertical climbing are identified in the ankle joint and related to positions of dorsiflexion and foot inversion. A study of the 14 distal tibiae and 15 tali identified and published as hominins from 4.12 to 1.53 million years ago finds that the ankles of early hominins were poorly adapted for modern ape-like vertical climbing bouts. This study concludes that if hominins included tree climbing as part of their locomotor repertoire, then they were performing this activity in a manner decidedly unlike modern chimpanzees.

Fig. 1.

Lateral view of vertical climbing in an adult male chimpanzee. Notice the extreme dorsiflexion of the right ankle. The left leg has already pushed off the tree, and the left arm is reaching upward, meaning that much of the body weight is being supported on the highly flexed right ankle.

Fig. 2.

Width of the anterior aspect of the distal tibia in humans, African apes, and extinct hominins. The distal tibiae of chimpanzees and gorillas are mediolaterally wide along the anterior aspect and as a result have a trapezoid-like appearance in inferior view (see chimpanzee on bottom right). Humans, in contrast, have a more square-shaped articular surface to the distal tibia in inferior view (bottom left). All of the hominin tibiae studied (n = 12) are human-like and lack the wide anterior rim found in the distal tibiae of climbing apes. The box plots show the median (black bar), interquartile range (gray box), and overall ranges (whiskers). Outliers (circles) are defined as >1.5 times the interquartile range. Letters on the distal tibiae indicate anterior (A), posterior (P), medial (M), and lateral (L) aspects of the bone.

Fig. 3.

Angle between the plane of the ankle joint and the long axis of the tibia in humans, African apes, and fossil hominins. Chimpanzees and gorillas have an obliquely oriented tibia relative to the plane of the ankle joint, positioning the foot in inversion. Humans have a perpendicularly oriented tibia relative to the horizontal plane of the ankle joint. All fossil hominins, except for the pathological KNM-ER 2596, are decidedly human-like for this feature. The box plots show the median (black bar), interquartile range (gray box), and overall ranges (whiskers). Outliers (circles) are defined as >1.5 times the interquartile range. Under the graph, digital cross-sections of (left to right) human, KNM-KP 29285 (A. anamensis), StW 389 (A. africanus), KNM-ER 1481 (Homo sp.), and chimpanzee are shown. A line has been drawn perpendicular to the long axis of these tibiae to show the oblique tilt of the chimpanzee articular surface and the approximately perpendicular orientation of the articular surface in hominin and modern human tibiae.

Fig. 4.

Lines drawn through the long axis of the tibia, the axis of rotation of the ankle, and the superior plane of the ankle form a triangle. The angle formed between the long axis of the tibia and the axis of rotation of the ankle (B) is conserved between humans and the African apes (8). Thus, the angle formed between the axis of rotation of the ankle and the plane of the ankle (A), even if taken on an isolated talus, can be used to calculate the angle that the long axis of the tibia formed with the articular surface of the tibia (C). All of the hominin tali (n = 12) are human-like in possessing a low angle between the axis of rotation and the horizontal plane of the ankle, implying that these individuals would have also possessed a human-like perpendicularly oriented tibia. African apes, in contrast, have a larger talar angle, and thus an obliquely oriented tibia. The box plots show the median (black bar), interquartile range (gray box), and overall ranges (whiskers). Outliers (circles) are defined as >1.5 times the interquartile range. [Redrawn from Latimer et al. (8).]