A consideration of the evolutionary trends in the blue-green algae with palaeontological evidences

Abstract

Blue-green algae are known right from the Precambrian to Recent. Evolutionary concepts have largely been based on comparative morphology of extant forms as revealed by culture studies. Broadly speaking two trends came into vague: (i) the ‘retrogressive’ concept elucidated by Geitler (1925), and (ii) the ‘progressive’ one laid out well by Fritsch (1942). Because relatively few fossil blue-green algae were then known, palaeobotanical evidence played only a small part in these approaches. Korde (1971) was the first to present a consolidate picture of the phylogenetic trends as revealed by fossils. Evidences that one looks for among the fossils relate to the origin of the filamentous condition, appearance of the heterocysts and the origin of heterotrichous habit and true branching. Most of the trends seen in the present day filamentous organization appeared in the Precambrian. Trichomatous condition developed in a major way in the Precambrian itself. The first uncontroverted record of a heterocyst apparently comes from Cambrian strata. The hormogone and spore types appeared earlier than the heterocyst. Records of heterocystous forms seem to point to an evolution of heterocystous types later. These forms parallel the homocystous morphotypes known earlier. True branching types seem to have come much later than the Cambrian. The fossil record of these forms seems to clearly support a ‘progressive’ type of evolution in two phyletic lines, the homocystous/aheterocystous line preceding the heterocystous line. This evidence clearly supports Elenkin’s concept of parallel evolution but not his ‘retrogressive’ projections. Reduction may be relevant only in smaller clines. The blue-green algae as a whole seem to be a conservative group where major developments occurred very early before the end of Proterozoic and do not exhibit any great degree or rate of change during later periods as do many other groups of biota. More work on these fossils is the need especially in the Indian subcontinent.