1.5 Metazoan Structure, Differentiation, and Model Organisms

The evolution of multicellular organisms most likely began when cells remained associated in small colonies after division instead of separating into individual cells. A few prokaryotes and several unicellular eukaryotes, such as Volvox (see Figure 1-3d), as well as many fungi and slime molds, exhibit such rudimentary social behavior. The full flowering of multicellularity, however, occurred in eukaryotic organisms whose cells became differentiated and organized into groups, or tissues, in which the different cells performed specialized functions.