  The emergence of complex patterns of organization close to the Cambrian
boundary is known to have happened over a (geologically) short period of time.
It involved the rapid diversification of body plans and stands as one of the
major transitions in evolution. How it took place is a controversial issue.
Here we explore this problem by considering a simple model of pattern formation
in multicellular organisms. By modeling gene network-based morphogenesis and
its evolution through adaptive walks, we explore the question of how
combinatorial explosions might have been actually involved in the Cambrian
event. Here we show that a small amount of genetic complexity including both
gene regulation and cell-cell signaling allows one to generate an extraordinary
repertoire of stable spatial patterns of gene expression compatible with
observed anteroposterior patterns in early development of metazoans. The
consequences for the understanding of the tempo and mode of the Cambrian event
are outlined.
