The Evolution of the Advanced Hominid Brain [and Comments and Reply]

Abstract

A stochastic, hierarchical model for the evolution of the advanced hominid brain is developed. The fossil and geological data date the event to 1.6-1.8 million years ago. Palaeoclimatological evidence does not suggest that it coincided with significant ecological change. The cognitive capacities of "small" hominoid brains are reviewed in the context of pongid technology and carnivory and the evidence for hominid stone tool technology prior to the Plio/Pleistocene. Calibration information suggests that small-brained gracile australopithecines represented a successful long-term adaptation to their particular African ecozones. The evolution of the advanced hominid brain is, therefore, likely the result of ongoing stochastic genetic processes that keep the eukaryotic genome in a state of genetic flux without reference to the selection pressures generated by macroscopic biotic communities. Radiometric calibration of the fossil evidence viewed in conjunction with computer simulations of macroevolutionary events suggests that the time required for the appearance of the advanced hominid brain was short (in geological terms). The adaptive potential conferred upon the evolving Homo lineage was profound, and feedback effects that further promoted brain development were likely because of new dietary biases. This model is consistent with assumptions derived from both the neo-Darwinian modern synthesis and the punctuated-equilibria model. A speculative reconstruction of early hominid population structure describes the context in which either (a) a "fast gradualism" produced a macroevolutionary change or (b) a phenotype produced by a rapid burst of speciation was fine-tuned. To the extent that the model proposed is hierarchical, it is independent of the modern synthesis.