Evolution has had at least 20 separate attempts at forming multicellular organisms. Plants, animals and fungi are the most spectacular and complex achievements of this kind. There are essentially two ways to make a simple multicellular entity out of single cells: either a single cell divides and its offspring stick together, or several solitary cells aggregate to form a colony. Division and adhesion is characteristic of multicellular forms of aquatic origin, whereas aggregation is typical in terrestrially derived colonies.

What is multicellularity? An overall coordination of at least some key function is a necessary and sufficient condition for a colony of cells to qualify as multicellular. According to this view, most bacterial colonies are not multicellular, as they lack this overall coordination, despite occasionally undergoing self-organized, patterned growth. It is the presence of developmentally differentiated cell types in a colony that makes it truly multicellular.

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