Genetics. Volvox genome shows it doesn't take much to be multicellular.
Pennisi
Elizabeth
E
D020418:Algal Proteins / Q000737:chemistry / Q000235:genetics; D005075:Biological Evolution; D016825:Chlamydomonas reinhardtii / Q000166:cytology / Q000235:genetics* / Q000254:growth & development; D048028:DNA, Algal / Q000235:genetics; D005796:Genes; D016678:Genome; D012091:Repetitive Sequences, Nucleic Acid; D017422:Sequence Analysis, DNA; D044446:Volvox / Q000166:cytology / Q000235:genetics* / Q000254:growth & development
D020418:Algal Proteins; D048028:DNA, Algal