ABSTRACT
A discussion of “key transitions” in the evolution of animals often invokes mental images of large-scale morphological or behavioural changes: the fi n-to-limb transition, avian beak shape changes, the transition from simply holding objects to using them purposefully as tools. These types of changes clearly occurred in evolution and had great adaptive value. Other types of changes, however, have also occurred in the morphologies and behaviours of single cells and cell lineages. A complete understanding of many “key transitions” involving new structures and new cell types must therefore incorporate the molecular genetic basis for the novel or modifi ed cell behaviours that can lead to novel structures.
