Hypotheses regarding the selective pressures driving the threefold increase in the size of the hominid brain since Homo habilis include climatic conditions, ecological demands, and social competition. We provide a multivariate analysis that enables the simultaneous assessment of variables representing each of these potential selective forces. Data were collated for latitude, prevalence of harmful parasites, mean annual temperature, and variation in annual temperature for the location of 175 hominid crania dating from 1.9 million to 10 thousand years ago. We also (...) included a proxy for population density and two indexes of paleoclimatic variability for the time at which each cranium was discovered. Results revealed independent contributions of population density, variation in paleoclimate, and temperature variation to the prediction of change in hominid cranial capacity (CC). Although the effects of paleoclimatic variability and temperature variation provide support for climatic hypotheses, the proxy for population density predicted more unique variance in CC than all other variables. The pattern suggests multiple pressures drove hominid brain evolution and that the core selective force was social competition. (shrink)

Brain size has increased threefold during the course of human evolution, whilst body weight has approximately doubled. These increases in brain and body size suggest that reproductive rates must have slowed considerably during this period. During the same period, however, environmental heterogeneity has increased substantially. A central tenet of life-history theory states that in heterogeneous environments, organisms with fast life histories will be favoured. The human lineage, therefore, has proceeded in direct contradiction of this theory. This contribution attempts to resolve (...) this contradiction by recourse to Godfrey-Smith’s Environmental Complexity Thesis, which states that the function of cognition is to enable the organism to deal with environmental complexity. It is suggested that among slowly reproducing organisms the behavioural flexibility provided by advanced cognitive abilities is a fundamental component of adaptation to heterogeneous environments. In the human lineage this flexibility is manifest particularly in the increasing complexity of material culture. (shrink)

The target article provides an intermediate account of culture and freedom that is conceived to be curvilinear by treating economic development not as an adaptive outcome in response to climate but as a cause of culture parallel to climate. We argue that the extent of environmental variability, including climatic variability, affects cultural adaptation.