Looking back over the vastness of evolutionary time, one realizes that the only constant is change. Every form of life changes, both within the life span of its individuals and within the life span of its species. The capacities of the human mind have changed in much the same way that cognitive capacities with children change with age. The existence of distinct phases of mind over the course of evolution is hard to dispute. The question is not one of whether phases or categorical stages of mental development exist but rather of where the transitions are. On this question there is relative agreement: the major cognitive shifts occurred around 4.5 mya, 2.0 mya, and 200,000 years ago (kya).25 These "breaks" can be seen both in major morphological change (in brain and body) and in cultural-behavioral change. In my model of cognitive evolution, I borrow from and add to other theoretical attempts to describe phases of human cognitive evolution, primarily those of Merlin Donald, Richard Klein, Steven Mithen, and Sue Parker and Michael McKinney. Even the relatively atheoreti-cal Richard Klein places the temporal and species breaks in much the same places as the more theoretical writers (for example, Donald, Mithen), but he does not place descriptive labels on these phases. My goal is to provide a straightforward yet not overly simplistic model for the phases of hominid cognitive evolution by modifying and clarifying existing models and theories rather than fundamentally changing them.
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