The Running-Fighting Dichotomy and the Evolution of Aggression in Hominids

Abstract

A suite of functional tradeoffs involving specialization for fighting versus locomotor economy is hypothesized to have influenced the evolutionary pathways of hominids. Characters such as pronounced forelimb strength, robust distal limbs, short stature, wide hips, robust head and neck, and fully bipedal stance and gait gave australopithecines a body configuration that is consistent with specialization for fighting with the forelimbs. This, plus a high level of sexual dimorphism in body and forelimb size suggest that australopithecines were specialized for male-male aggression. Nevertheless, traits that made australopithecines effective fighters would also have made them inefficient runners by modern human standards. Given the evidence of specialization for male-male aggression in australopithecines, the evolution of high locomotor stamina in Homo represents a dramatic change in evolutionary trajectory. I suggest that the evolution of improved locomotor stamina in Homo was made possible by the invention of new weapon technology that reduced functional conflicts between specialization for economical transport and specialization for male-male fighting. Invention of the first effective weapons would have shifted selection for male-male aggression from favoring greater physical strength and agility to favoring creative innovation, allowing independent evolution of cursorial specialization. In this scenario, the emergence of Homo (1) is dependent on the invention of new weapons, and 2) represents both greatly improved distance transport and increased lethality in a lineage already highly specialized for fighting.