The Euglenophyceae are believed to be an ancient lineage of algae that includes some zooflagellate protozoa, which is supported by ultrastructural and molecular data, though the group is taxonomically contentious.
Some scientists consider the red algae, which bear little resemblance to any other group of organisms, to be very primitive eukaryotes that evolved from the prokaryotic blue-green algae (cyanobacteria). Evidence in support of this view includes the nearly identical photosynthetic pigments and the very similar starches among the red algae and the blue green algae.
Other scientists suggest that the red algae evolved from the Cryptophyceae, with the loss of flagella, or from fungi by obtaining a chloroplast. In support of this view are similarities in mitosis and in cell wall plugs, special structures inserted into holes in the cell walls that interconnect cells. Ribosomal gene sequence data from studies in molecular biology suggest that the red algae arose along with animal, fungal, and green plant lineages. The class Ulvophyceae is also ancient, whereas the classes Charophyceae and Chlorophyceae are more recent.
The class Dinophyceae is of uncertain origin and is taxonomically contentious. During the 1960s and ’70s the unusual structure and chemical composition of the nuclear DNA of the Dinophyceae were interpreted as somewhat primitive features.
Some scientists even considered the Dinophyceae to be mesokaryotes (intermediate between the prokaryotes and the eukaryotes) The Dinophyceae may be distantly related to the chromophyte algae, but ribosomal gene sequence data suggest that their closest living relatives are the ciliated protozoa.
It is likely that the Dinophyceae arose from nonphotosynthetic ancestors and that later some species of Dinophyceae adopted chloroplasts by symbiogenesis and thereby became capable of photosynthesis, although many of these organisms still retain the ability to ingest solid food, similar to protozoa.
Cryptophytes have flagellar hairs and other flagellar features that resemble those of the chromophyte algae; however, the mitochondrial structure and other ultrastructural features are distinct and argue against such a relationship.
The fossil record for the algae is not nearly as complete as it is for land plants and animals. Red algal fossils are the oldest known algal fossils. Microscopic spherical algae (Eosphaera and Huroniospora) that resemble the living genus Porphyridium are known from the Gunflint Iron Formation of North America (formed about 1.9 billion years ago).
The best characterized fossils are the coralline red algae represented in fossil beds since the Precambrian time (4.6 billion to 541 million years ago). Some of the green algal classes are also very old. Organic cysts resembling modern Micromonadophyceae cysts date from about 1.2 billion years ago. Tasmanites formed the Permian “white coal,” or tasmanite, deposits of Tasmania and accumulated to a depth of several feet in deposits that extend for miles.
Some green algae deposit calcium carbonate on their cell walls, and these algae produced extensive limestone formations. The Charophyceae, as represented by the large stoneworts (order Charales), date from about 400 million years ago.
Most species became extinct at the end of the Cretaceous Period (145 million to 66 million years ago), along with the dinosaurs, and indeed there are more extinct species of coccolithophores than there are living species.
The Chrysophyceae, Bacillariophyceae, and Dictyochophyceae date from about 100 million years ago, and despite the mass extinctions 66 million years ago, many species still flourish.
The Xanthophyceae may be even more recent, with fossils dating from about 20 million years ago, while fossil records of the remaining groups of algae, notably the Euglenophyceae and the Cryptophyceae, which lack mineralized walls, are negligible.
