PhD: Human evolution and the Ecomorphology of Tree Climbing

Bipedalism and the extraordinary manipulative abilities of the human hand, that are central to tool use and technology, are widely considered to be two of the major hallmarks of humanity. Their evolution was thought to be temporally separate; with short hands and precise manipulative abilities attainable only after the hands were freed from the demands of arboreal locomotion by the evolution of terrestrial bipedalism. However, new evidence suggests that significant arboreality was achieved by multiple species with short hands and bipedal adaptations from before the common ancestor of chimpanzees and humans to well into the genus Homo.

Arboreal and terrestrial habitats, and routinely moving between them, place sharply conflicting mechanical demands on the body and the fossil record is highly inconsistent about how hominins (living humans and their ancestors) remained effective in the trees (Johannson et al, 2017). Indeed many researchers have concluded that bipedal, tool making hominins lost completely the ability to utilise forest canopy resources (e.g. Latimer, 1991). However, the muscles of living apes are highly ’plastic’, which allows them to compensate for skeletal structure and exhibit demanding locomotor behaviours that are not reflected in the skeleton (Neufuss et al, 2014). This plasticity has recently been shown to be sufficient to permit modern humans to be effective arboreal climbers: indigenous (former hunter gatherer) climbers in Uganda have been found to have significantly longer fibres in their calf muscles than neighbouring non-climbing populations (Venkataraman et al, 2013). These allow the highly (dorsi)flexed ankle positions required for climbing vertical tree trunks, but there is no external skeletal evidence. Thus although early hominins are typically described as adapted to primary behaviours such as bipedalism, quadrupedalism or vertical climbing, muscle plasticity may have allowed a species adapted to any of these to also exhibit the others, without modification of their skeleton. The premise of this proposal is that we have substantially underestimated the muscle plasticity of fossil hominins, leading to reconstructions of locomotor repertoires that have been far too narrow and stereotyped.

This studentship will reconstruct the arboreal ecomorphology of fossil hominins by quantifying and comparing the ecological and biomechanical demands of arboreal locomotion in humans and gorillas to drive predictive computer models that can ’flesh-out’ the fossil record. The studentship has three interlinked Objectives:

O1. Quantify the arboreal locomotor ecology of indigenous human tree climbers and gorillas at project partner Martha Robbin’s fieldsite in Gabon

O2. Quantify the biomechanical performance of humans/ gorillas during arboreal locomotion in controlled conditions at UK zoos

O3. Use the outputs of Objectives 1 and 2 to create musculoskeletal models of human and gorilla climbing in a range of environments, and to morph the mechanics of human/ gorilla locomotion to extinct species and habitats, with the support of Co-Supervisor William Sellers.

This approach will reveal the extent to which early humans could exploit forest canopy despite their increasingly modern anatomy to develop a new paradigm for humans’ evolutionary history.