Multicellularity could have been achieved numerous times based on the premise that selection acts on phenotypes and how well certain combinations of traits work. In other words, even if cells adhere together using different mechanisms, or via different developmental pathways, if the results are cooperative aggregations of cells that function well and thus are able to survive better and, critically, produce more offspring than their unicellular counterparts, then these various evolutionary pathways could all be possible.
"The curtail point," emphasizes Niklas, "is that the evolution of multicellular organisms occurred multiple times and involved different developmental 'motifs,' such as the chemistry of the 'glues' that allow cells to stick together."
Certainly, one of the themes that Niklas drives home in his review is that natural selection acts on functional traits, so multicellularity could have evolved many times via different mechanisms and modes of development, and using different aspects of cellular biology.
However, there are certain sets of requirements that must be met in order for multicellularity to evolve. These include that cells must adhere to, communicate with, and cooperate with each other, and that cells must specialize in their functions (i.e., that not all cells do exactly the same thing, otherwise they would just be a group of cells or a colony). In order to make these things happen, cells must not reject each other. In other words, they must be genetically compatible to some extentanalogous to how our human bodies reject foreign items that are not recognized by our cells. This first step is termed "alignment-of-fitness."
|Contact: Richard Hund|
American Journal of Botany