Arboreality, terrestriality and bipedalism

Abstract

The full publication of Ardipithecus ramidus has particular importance for the origins of hominin bipedality, and strengthens the growing case for an arboreal origin. Palaeontological techniques however inevitably concentrate on details of fragmentary postcranial bones and can benefit from a whole-animal perspective. This can be provided by field studies of locomotor behaviour, which provide a real-world perspective of adaptive context, against which conclusions drawn from palaeontology and comparative osteology may be assessed and honed. Increasingly sophisticated dynamic modelling techniques, validated against experimental data for living animals, offer a different perspective where evolutionary and virtual ablation experiments, impossible for living mammals, may be run in silico, and these can analyse not only the interactions and behaviour of rigid segments but increasingly the effects of compliance, which are of crucial importance in guiding the evolution of an arboreally derived lineage.

Figure 1.

(a) Diagram of the plantar aspect of the human foot, showing the position of the cuboid peg and illustrating the ‘windlass mechanism’, whereby the five slips of the PA are tensed by the curvature of the metatarsal heads as the metatarsophalangeal joint dorsiflexes. Similarly, the spring ligament is tensed by plantad motion of the head of the talus. Both mechanisms act to stiffen the median longitudinal arch during stance. (Figure modified from image from Primal's ‘Anatomy TV’). (b) Peak pressures, in false greyscale, where lighter tone indicates higher pressure, during bipedalism of a clinically normal subject recorded by Nike Inc., courtesy of J.-P. Wilssens of RSscan International. Dots indicate the path of the centre of pressure under the foot. (c) vGRF curve calculated from the same data for the individual featured in (a).

Figure 2.

(a,b) Chart showing the limb lengths and (c,d) moments of inertia for hominoids and horse. Forelimb moments of inertia are about the shoulder joint and hindlimb moments of inertia are about the hip joint. Great ape data are from Isler et al. (2006), human data are from Winter (1990) using a median male height and weight from the GEBOD database (Cheng et al. 1994), horse data from Buchner et al. (1997).

Figure 3.

Total tendon in both hind limbs as a fraction of body mass. Data are based on information from Pierrynowski (1995), Payne et al. (2006), Williams et al. (2007, 2008) and Wareing et al. (submitted).

Figure 4.

(a) Chart showing the maximum velocity and (b) the cost of locomotion of human running simulations where the elastic properties of the hindlimb tendons are manipulated. AT, Achilles tendon. (Sellers et al. 2010).