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Key Words brain size, social cognition, theory of mind, social group size, culture
Abstract The social brain (or Machiavellian Intelligence) hypothesis was proposed to explain primates' unusually large brains: It argues that the cognitive demands of living in complexly bonded social groups selected for increases in executive brain (principally neocortex). The evidence for this and alternative hypotheses is reviewed. Although there remain difficulties of interpretation, the bulk of the evidence comes down in favor of the social brain hypothesis. The extent to which the cognitive demands of bonding large intensely social groups involve aspects of social cognition, such as theory of mind, is explored. These findings are then related to the evolution of social group size, language, and culture within the hominid lineage.
INTRODUCTION
In the century and a half following the discovery of the first Neanderthals, the focus of palaeoanthroplogy has been on the who's who of hominid evolution. How species are defined has come to occupy the central place in that story, with anatomy perhaps inevitably being the central plank of that endeavor. As appropriate as this has been, it does overlook the fact that what makes us human is not our bodies but our minds. The story of hominid-and hence ultimately human-evolution is thus one that must be told in terms of the evolution of mind. As Lewis-Williams (2002) points out, there have been only two serious attempts to grapple with this problem (Donald 1991, Mithen 1996), and both of these have inevitably been somewhat speculative in nature. I here offer a preliminary attempt to grapple with this problem. My focus is the intersection of brain, mind, and language in hominid evolution.
Ever since Jerison's (1973) seminal study, it has been recognized that primates have unusually large brains for body size. Moreover, within the primates, some species have disproportionately large brains for body size, one of these of course being humans. In general, however, differences in brain size do not reflect proportional increases in all brain components. Rather, the size of the neocortex accounts for most of the deviation from overall trend lines (Finlay & Darlington 1995). Primates have larger brains than other species mainly because they have larger neocortices. In effect, then, when asking "Why do primates have unusually...