Multicellular? Most organisms on earth are unicellular[1] even now. My impression is that multicellular organisms evolved because the 'easy' evolutionary niches were already occupied by single-celled species, and it took new tricks to compete for new niches.
It's not known exactly how multicellular life came to be. It's happened only once in the history of the earth. Nuclear dna is from Archaea and Mitochondrial dna is from bateria. Lynn Margulis was the first to propose and substantiate symbiogenesis for this.