gogol

The ancestor of the metaphytans must have been eukaryotic, multicellular, photosynthetic, and aquatic. The first three characters are the most conservative or plesiosemic ones we can find in this kingdom. And the last character, though aposemic, since the metaphytans occur in marine and freshwaters as well as on land, is plesiosemic among those forms arguably close to ancestral forms. The multicellular algae are the plants that come most quickly to mind in response to these four requirements for metaphytan ancestry. What can we say about the red, the brown, and certain green algae in this regard?

The algae are not especially closely related. First, regarding the red algae, their lack of any motile cells and unique photosynthetic pigments place them closer to the prokaryotic blue-green algae than to the eukaryotes. It is thought that the red algae arose from the bluegreen algae; either through endosymbiosis or through transformation of a prokaryotic alga into a eukaryotic red algae. The problem here is much the same as for the origin of the protistans, in particular, the origin of the green algae. The plesiomorph of the red algae would have a relatively simple thallus and a simple fife cycle. A possible candidate might be Porphyra perforator. It must be emphasized, however, that there is a large gap--the gap separating prokaryotes and eukaryotes--between this red alga and the blue-green algae.

Turning next to the brown algae, we have a very difficult problem. These organisms suggest some similarity to the protistan golden algae (Chrysophycophyta). But there is no series of homologous semes that allows a convincing ancestor-descendant relationship to be established between the brown and the golden algae. There is a gap here; not as large as that between prokaryotes and eukaryotes, but nonetheless large. The first aspect of the problem is that there is no good candidate for a plesiomorph for the brown algae. All the brown algae are multicellular and have fairly complex to very complex life cycles. It is as if all the simpler ones have lost out by competition to the highly evolved ones. The result is that forms suggestive of the ancestral form are missing. The second problem is that we do not find any multicellular protists in the golden or other algaethat show convincing homologies with the brown algae. The photosynthetic pigments of the Phaeophycophyta, Chrysophycophyta, and Pyrrophycophyta are somewhat alike, but only in a general way. Photosynthetic products, cell wall chemistry, occurrence of flagella, and patterns of multicellular organization are not arguably homologous. We do not really know yet what to say about the ancestor and origin of the brown algae except that the origin was probably from the Protista and that the ancestor and its related forms are extinct.