<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> <P> Genes borrowed from viruses have recently been identified as playing a crucial role in the differentiation of multicellular tissues and organs and even in sexual reproduction, in the fusion of egg cell and sperm . Such fused cells are also involved in metazoan membranes such as those that prevent chemicals crossing the placenta and the brain body separation . Two viral components have been identified . The first is syncytin, which came from a virus . The second identified in 2007 is called EFF1, which helps form the skin of Caenorhabditis elegans, part of a whole family of FF proteins . Felix Rey, of the Pasteur Institute in Paris has constructed the 3D structure of the EFF1 protein and shown it does the work of linking one cell to another, in viral infections . The fact that all known cell fusion molecules are viral in origin suggests that they have been vitally important to the inter-cellular communication systems that enabled multicellularity . Without the ability of cellular fusion, colonies could have formed, but anything even as complex as a sponge would not have been possible . </P>

Describe three levels of organization in complex multicellular organisms