The long and short of it: Leg length, aggression and the evolution of the human mind

22nd April 2013

Marianne Clarkson

BA(Hons), School of Anthropology, Archaeology and Sociology, James Cook University, October 2008

The well-developed and greatly expanded human mind is one thing that sets us apart from other organisms. Although there are varying theories as to when and how the mind evolved, many of them focus on the concept of modules or domains, which may have arisen in order to cope with or provide solutions to problems faced by our ancestors in the Plio/Pleistocene. Violence and aggression can be seen in our closest living relative, the chimpanzee, and can also be demonstrated in the fossil record as far back as the early Pleistocene. This allows for the premise that aggression was important in our ancestors’ lives, even before the panin-hominin split 6-7 million years ago. Using a Darwinian framework, this thesis focuses on how aggression may have helped to shape the cognitive changes that are assumed to have occurred with the demonstrable increase in brain size of Homo erectus.

Australopithecus and H. habilis both retained ape-like limp proportions, with longer arms and shorter legs. As well as allowing for a semi-arboreal lifestyle, these adaptations potentially increased balance and forearm strength, both important fighting attributes in species where there was intense male-male competition for females. These anatomical features, which would have allowed for bipedalism, but made it relatively inefficient due to the retention of shorter legs, remained stable for over 2 million years. This would seem to imply that aggressive ability exerted a strong positive sexual selection pressure for a prolonged period of time.

It is now well recognised that hominin evolution was not a simple linear affair and that for most of this time two or more different hominin species existed at the same time and in the same areas. About 2.5-2 mya the climate and environment appear to have changed from C3 woodlands to widespread C4 grasslands. This timeframe fits in with the appearance around 1.9 mya of a longer-legged habitual biped, H. erectus. Thus it seems that natural selection now favoured new adaptations that were suited to the new savannah environment, but that may have been detrimental to fighting ability. However, continuing body weight sexual dimorphism in H. erectus would still seem to imply that male-male competition for choosy females persisted in likely polygynous groups. Increased cognition could, therefore, have been the sexually selected random trait that provided another, more potent way of showing aggression and domination over rivals using stone tools. Cognitive changes in our ancestors occurred because they effectively provided solutions to problems, ecological changes and challenges from other hominins during the Pleistocene. This increasing cognition can be seen archaeologically in the increasingly modified and symmetrical Acheulean stone handaxes. Wynn has demonstrated that stone tools can be used to determine how cognitive ability changed over time; many of the other proposed uses for handaxes do not adequately explain all of their attributes. Aggression, however, can explain their over-modified appearance, their change over time, the fact that many are unused and why they may appear in huge numbers in some sites.

Marianne Clarkson
June 2010
Thesis Abstracts