Cause of the singular (or sole surviving) origin of animal-like complex multicellularity

Explain why phagocytic, animal-like complex multicellularity evolved only once or has only one surviving example, by evaluating intrinsic evolutionary difficulty, ecological priority effects, and extinction-driven information loss.

Background

Although complex multicellularity has multiple origins across eukaryotes, animal-like (phagocytic) complex multicellularity appears unique among extant clades. The authors propose alternatives to intrinsic improbability, including incumbency and information loss.

Determining the cause of this singularity informs whether uniqueness reflects active ecological inhibition, past diversity erased by extinction, or genuine evolutionary difficulty.