<P> The Colonial Theory of Haeckel, 1874, proposes that the symbiosis of many organisms of the same species (unlike the symbiotic theory, which suggests the symbiosis of different species) led to a multicellular organism . At least some, it is presumed land - evolved, multicellularity occurs by cells separating and then rejoining (e.g. cellular slime molds) whereas for the majority of multicellular types (those that evolved within aquatic environments), multicellularity occurs as a consequence of cells failing to separate following division . The mechanism of this latter colony formation can be as simple as incomplete cytokinesis, though multicellularity is also typically considered to involve cellular differentiation . </P> <P> The advantage of the Colonial Theory hypothesis is that it has been seen to occur independently in 16 different protoctistan phyla . For instance, during food shortages the amoeba Dictyostelium groups together in a colony that moves as one to a new location . Some of these amoeba then slightly differentiate from each other . Other examples of colonial organisation in protista are Volvocaceae, such as Eudorina and Volvox, the latter of which consists of up to 500--50,000 cells (depending on the species), only a fraction of which reproduce . For example, in one species 25--35 cells reproduce, 8 asexually and around 15--25 sexually . However, it can often be hard to separate colonial protists from true multicellular organisms, as the two concepts are not distinct; colonial protists have been dubbed "pluricellular" rather than "multicellular". </P> <P> Some authors suggest that the origin of multicellularity, at least in Metazoa, occurred due to a transition from temporal to spatial cell differentiation, rather than through a gradual evolution of cell differentiation, as affirmed in Haeckel's Gastraea theory . </P> <P> About 800 million years ago, a minor genetic change in a single molecule called guanylate kinase protein - interaction domain (GK - PID) may have allowed organisms to go from a single cell organism to one of many cells . </P>

What type of cells do multicellular organisms have