ABSTRACT
All cells have programmed limits on their size and how they react to things surrounding them. Unicellular organisms are able to sense certain chemicals and signals from other organisms in their vicinity. When cells form communities, they also communicate in order to maintain the ordered community. Cells often divide and the daughter cells are different. When they divide but do not separate, this is the beginning of multicellularity, a characteristic that is polyphyletic (Figure 16.1), that is, it evolved independently in many lineages. Because multicelluarity involves complex cell communication, it is likely that the basic genes and proteins that led to multicellularity existed long before organisms became multicellular. Also, it is possible that the characteristic has been horizontally transferred one or more times during evolution, or alternatively that it has been lost on many branches. However, for acquisition via horizontal gene transfers, there would have been multiple transfers of large gene clusters, which is unlikely. Also, there is a great deal of evidence that the basic mechanisms that led to multicellular development existed in bacteria and archaea very early in the evolution of life on the Earth. Bacteria and archaea live their lives surrounded by other cells of the same, as well as different, species. Some form filaments where many cells are joined together in multicellular assemblages, and some produce more than one cell type. Molecular mechanisms have evolved in these organisms that allow them to chemically recognize when there are other cells like themselves in the same area, when there are different cells in the area, where cells are signaling that there is food present, where the food is, and where the dangers are (e.g., toxins, predators, heat, cold). These chemical sensory mechanisms are the basis for interacting with their environment and other cells, and they provide the basis for interactions within a multicellular organism. Throughout evolution, the cells that possessed the genes that express these functions were favored in competitions for nutrients and for avoidance of dangers. In fact, some bacteria form multicellular bodies and different types of cells based on their interactions with the environment and other cells. Some bacteria form long chains of cells (e.g., Streptococcus spp.) and some form spores, whereas others form more than one cell type attached to each other (e.g., Caulobacter crescentus, Figure 16.2). The multicellular forms have been important to the survival and propagation of these species, and cell-to-cell signaling is vital to these processes.
