Figure 1.

Schematic representation of the relative timing of the key events leading to the emergence of metazoan-type multicellularity. The figure emphasizes that the evolution of transcription factor modules defining different cell types of unicellular and colonial pre-Metazoa preceded the intron invasion that led to the rise of exon shuffling and the creation of cell–cell communication proteins that are essential for complex multicellularity of Metazoa.