  This paper proposes that the distinctively human capacity for cumulative,
adaptive, open-ended cultural evolution came about through two
temporally-distinct cognitive transitions. First, the origin of Homo-specific
culture over two MYA was made possible by the onset of a finer-grained
associative memory that allowed episodes to be encoded in greater detail. This
in turn meant more overlap amongst the distributed representations of these
episodes, such that they could more readily evoke one another through
self-triggered recall (STR). STR enabled representational redescription, the
chaining of thoughts and actions, and the capacity for a stream of thought.
Second, fully cognitive modernity following the appearance of anatomical
modernity after 200,000 BP, was made possible by the onset of contextual focus
(CF): the ability to shift between an explicit convergent mode conducive to
logic and refinement of ideas, and an implicit divergent mode conducive to
free-association, viewing situations from radically new perspectives, concept
combination, analogical thinking, and insight. This paved the way for an
integrated, creative internal network of understandings, and behavioral
modernity. We discuss feasible neural mechanisms for this two-stage proposal,
and outline how STR and CF differ from other proposals. We provide
computational evidence for the proposal obtained with an agent-based model of
cultural evolution in which agents invent ideas for actions and imitate the
fittest of their neighbors' actions. Mean fitness and diversity of actions
across the artificial society increased with STR, and even more so with CF, but
CF was only effective if STR was already in place. CF was most effective
following a change in task, which supports its hypothesized role in escaping
mental fixation. The proposal is discussed in the context of transition theory
in the life sciences.
