Passage IV

All multicellular animals can be grouped within the subkingdom Metazoa. Most metazoans are bilaterally symmetrical (they have right and left halves that are nearly identical). Biologists agree that metazoans evolved from protozoans (single-celled organisms) over 600 million years ago, but they do not agree on how this occurred. Three theories are presented.

Cellularization Theory

Some biologists believe that metazoans evolved from multinucleated protozoans (protozoans with many nuclei). Separate cells formed when membranes developed between adjacent nuclei.Later, cilia (hairlike structures), used for movement, grew from each cell. These ciliated protozoan ancestors then evolved into aquatic, bilaterally symmetrical metazoans similar to modern flatworms. Later, other metazoans evolved from these flatworm-like creatures (see Figure 1).

Colonial Theory

Other biologists suggest that many protozoans grouped together into a hollow ball, or colony. This colony evolved into the first metazoan. Each cell in the colony had a flagellum (movable tail) for movement. Initially, some of these cells were better suited for reproduction, and these became the reproductive cells of the metazoan. The first metazoans were aquatic, flagellated, radially symmetrical organisms (body parts arranged symmetrically central around a point, as in modern jellyfish). All later metazoans evolved from these jellyfish-like organisms (see Figure 2).

Separate Line Theory

Proponents of this theory argue that 2 separate protozoan lines led to metazoans. Radially symmetrical metazoans evolved from flagellated, colonial protozoans. Bilaterally symmetrical metazoans evolved from ciliated, multinucleated protozoans. The first metazoans of each group were jellyfish-like creatures and flatworm-like creatures, respectively. The 2 protozoan groups were only distantly related to each other, and the evolution of aquatic metazoans from the 2 protozoan groups occurred independently and at different times.