Author's School

Graduate School of Arts & Sciences

Author's Department/Program

Anthropology

Language

English (en)

Date of Award

Spring 4-24-2013

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Chair and Committee

Erik Trinkaus

Abstract

Accurate, high velocity throwing is a skill unique to humans among living species. It likely provided an adaptive advantage for our hominin ancestors, either in the context of hunting, or protection from predators. Thus, understanding how variation in body form and anatomy influences throwing ability may provide insight into the evolution of human morphology. Research has been done on various forms of ball and javelin throwing, yet the biomechanics of spear throwing were completely unknown. Moreover, it has been suggested that early modern humans had lesser effective mechanical advantage: EMA, the ratio of moment arm to load arm) than Neandertals, as a result of selection for throwing, as this could provide increased joint angular velocity at the cost of strength. However the biomechanical principles underpinning this assertion had not been tested empirically. Thus, the purpose of this dissertation is to establish the patterns of kinetics and kinematics used during spear throwing, to examine the morphological correlates of throwing performance and strength in living humans, and to consider the implications of the patterns observed for Paleolithic fossil hominins.

Throwing performance, measured as the energy imparted to a thrown object: kinetic energy, Ke), was assessed in 41 experienced throwers throwing balls and spear-like objects. Joint linear and angular velocities, torques, and kinetic energies were calculated using high-speed infrared cameras. Subject anthropometrics, including body mass, height, and segment lengths, were measured externally, and arm muscle cross-sectional areas and EMA of the elbow and wrist were measured from magnetic resonance images. The maximum isometric strength of the elbow and wrist were measured using a load cell. Additionally, EMA of the elbow and wrist were estimated skeletally for available early Homo, Neandertal, Middle Paleolithic, Upper Paleolithic, and recent human specimens.

It was demonstrated that the shoulder experiences very low angular velocities and torques, and the elbow is relatively less mobile during spear as compared with ball throwing. Thus, the literature on the arm in ball throwing will be of less relevance to the evolution of throwing. Furthermore, there is no direct correlation between spear Ke and any morphological or anatomical variable of the arm: including EMA). Instead, spear Ke is primarily produced by the legs and torso, where musculature, leg length, and shoulder breadth are correlated with greater Ke. This implies that research on throwing should focus on the legs and torso, and variation in the morphology of the arm should not be used to infer differential selection for throwing in Paleolithic hominins.

No correlation was found between the EMA of the elbow or wrist and joint strength independent of muscle cross-sectional area. This is likely due to variation in muscle recruitment and activation, and because r is correlated with muscle size independent of body size. Furthemore, it was demonstrated that EMA cannot be measured reliably in the fossil record, as load arms are highly variable within and between individuals, even performing a highly trained activity like throwing. Consequently, they cannot be predicted from skeletal dimensions available in the fossil record. Furthermore, although there is a trend towards greater EMA of the elbow in Neandertals as compared with other fossil hominin samples, this variation is not functionally or adaptively relevant. No significant differences exist for moment arms between groups; rather, the difference in EMA is due to variation in load arms: ulna length). However, load arms measured skeletally are not relevant to living behaviors, and variation in load arms between these groups is due to known differences in brachial indices, which reflect climatic adaptations. Thus, EMA of the arm is not a useful measure of performance in Paleolithic fossil hominins.

DOI

https://doi.org/10.7936/K7NS0RVC

Comments

Permanent URL: http://dx.doi.org/10.7936/K7NS0RVC

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