The series of evolution depicted on Figure 1 is the about the Chlamydomonas species turning to a Volvox species. Aside from that interpretation, there are more theories that elaborate multicellularity in different descriptions. First is the symbiotic theory. In this theory, cells of different unicellular species derive mutual benefits from aggregation and evolve to become the first cell types of a multicellular organism (See Figure 2). Secondly, we have the coenocytial theory that denotes that a single cell with multiple nuclei evolves to partition each nucleus within a separate membrane thereby creating a multicellular organism. See Figure 3. Finally, we have the colonial theory. This is considered as the most plausible hypothesis. A single cell divides to produce two daughter cells, which fail, to separate and lead to the production of a colony of attached cells. Selective pressure then lead to specialization and segregation of germ and somatic cells. Alternatively, separate cells aggregate to form as a colony. (See Figure 4)
Outside the box, multicellularity itself has both advantages and disadvantages. Starting from the pros is intelligence and evolution. Being multicellular allows an organism to