<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> <P> Multicellularity allows an organism to exceed the size limits normally imposed by diffusion: single cells with increased size have a decreased surface - to - volume ratio and have difficulty absorbing sufficient nutrients and transporting them throughout the cell . Multicellular organisms thus have the competitive advantages of an increase in size without its limitations . They can have longer lifespans as they can continue living when individual cells die . Multicellularity also permits increasing complexity by allowing differentiation of cell types within one organism . </P>

In order for an organism to develop specialized tissue the organism must be which of the following