It's my understanding that in the transition from single celled to multi-celled organism a lineage will transition between stages where existing in a colony of cells is optional to one where it is obligate. Integral to this process is the parts of the colony taking on specialized roles in the colony's survival. First is tissues, and eventually full blown organs.
With that as background (please correct any misconceptions), which job tends to be taken over by specialized cells first in the transition? The three main ones in every multicellular organism I can think of are: digestion, structure (including skin and any rigidity), and reproduction (please suggest roles if I missed any, or over-included). Is there a single answer to this, or does it vary between lineages?