DIVISION OF LABOR AND SPECIALIZATION. “The fitness advantage of such a division of labour is an important, if not crucial, factor in the emergence and evolution of multicellularity and cell specialization. In fact, the unicellular ancestors often already possess the prototypes of regulatory mechanisms that are needed to maintain cell specialization in multicellular forms.”
http://rspb.royalsocietypublishing.org/content/early/2011/12/05/rspb.2011.1999.short