Biomechanics of the Hominin Upper Limb: Entheseal Development and Stone Tool Manufacture

Abstract

The close association of the advent of tool behaviours with changes to the upper limb morphology of fossil hominins has lead researchers to hypothesise that stone tool manufacture and use represents an important selective pressure for the upper limb, and yet the makers of the first stone tools, the Oldowan technological complex, remain obscure. Entheseal complexity studies have suggested that morphological changes to muscle attachment sites might be a fruitful means to investigate activity patterns in past populations. The aims of this study were therefore two-fold. First, upper limb kinetics, kinematics, and normal activation patterns of 15 shoulder and elbow muscles were investigated using electromyography (EMG) in 16 novices during Oldowan stone knapping to identify which muscles were highly and regularly recruited during this behaviour. Second, upper limb entheses in 10 species of fossil hominin were analysed using fractal analysis to score entheseal complexity. These results were then compared with the EMG analysis to determine whether patterns of entheseal complexity mirror muscle recruitment patterns in living subjects. If so, it may be possible to identify who made the first stone tools.

The results of the biomechanical study indicate that the motion of the knapping arm in stone tool manufacture is a dynamic three-dimensional flexion-extension motion. The shoulder and elbow musculature is active primarily to produce acceleration of the arm segments to generate the strike force. The segments of the upper limb moved in a coordinated proximal-to-distal sequence. This motion originated with the shoulder proximally in the up-swing or “cocking” phase and was transmitted through to the distal limb segments (the wrist and hammerstone) in the down-swing phase. The principle strike force-generating muscles of the down-swing are Mm. latissimus dorsi, teres major, and triceps brachii. M. pectoralis major works during this phase to decelerate the rapidly extending arm to improve strike accuracy. The wrist flexor and extensor musculature, rather than producing specific motion of the wrist, appears to be highly recruited to stabilise the elbow and wrist against reactive forces from hammerstone impact.

The entheseal complexity analysis indicates that potential members of the tool-making guild include Australopithecus africanus, Australopithecus anamensis, Paranthropus robustus, Homo habilis, and Homo ergaster. While patterns of entheseal complexity in the fossil hominin upper limb do indeed mirror muscle recruitment patterns during stone knapping, the data is nonetheless equivocal as morphological evidence in at least two candidate species (one of which occurs 1.5 Ma prior to the first evidence of the Oldowan technological complex) suggests strong commitment to arboreality, calling into question the efficacy of entheseal complexity studies for identifying activity patterns in fossil hominins.