In the transition to multicellularity what function becomes specialized first?
Lyons &amp; Kolter (2015) On the evolution of bacterial multicellularity. Curr Opin Microbiol 24: 21-28; emphasis mine:


  Multicellularity is one of the most prevalent evolutionary innovations
  and nowhere is this more apparent than in the bacterial world, which
  contains many examples of multicellular organisms in a surprising
  array of forms.
  
  cell-cell adhesion to form a new evolutionary unit, and intercellular
  communication leading to coordinated activity (...) are the minimal
  requirements to redefine a group of cells rather than any single cell
  as an "individual."
  
  [M]ulticellular magnetotactic prokaryotes (MMPs) [are the only known bacteria] without a unicellular phase in their lifecycle.
  
  [The key features of multicellularity are: cell differentiation,
  intercellular signaling, extracellular matrix, tight cell-cell
  junctions, coordinated movement, division of entire structure (Table
  1).]