  This paper summarizes efforts to computationally model two transitions in the
evolution of human creativity: its origins about two million years ago, and the
'big bang' of creativity about 50,000 years ago. Using a computational model of
cultural evolution in which neural network based agents evolve ideas for
actions through invention and imitation, we tested the hypothesis that human
creativity began with onset of the capacity for recursive recall. We compared
runs in which agents were limited to single-step actions to runs in which they
used recursive recall to chain simple actions into complex ones. Chaining
resulted in higher diversity, open-ended novelty, no ceiling on the mean
fitness of actions, and greater ability to make use of learning. Using a
computational model of portrait painting, we tested the hypothesis that the
explosion of creativity in the Middle/Upper Paleolithic was due to onset of
con-textual focus: the capacity to shift between associative and analytic
thought. This resulted in faster convergence on portraits that resembled the
sitter, employed painterly techniques, and were rated as preferable. We
conclude that recursive recall and contextual focus provide a computationally
plausible explanation of how humans evolved the means to transform this planet.
