Pre-Cambrian Life: Prokaryotes and BGA

The earliest bacterium-like organisms proliferated, giving rise to a great variety of forms, some of which were capable of photosynthesis. From these arose the oxygen-producing cyanobacteria approximately 3 billion years ago.

Bacteria are called prokaryotes, meaning literally “before the nucleus.” They contain a single, large molecule of DNA not located in a membrane bound nucleus, but found in a nuclear region, or nucleoid. The DNA is not complexed with histone proteins, and prokaryotes lack membranous organelles such as mitochondria, plastids, Golgi apparatus, and endoplasmic reticulum. During cell division, the nucleoid divides and replicates of the cell’s DNA are distributed to the daughter cells. Prokaryotes lack the chromosomal organization and chromosomal (mitotic) division seen in animals, fungi, and plants.

Bacteria and especially cyanobacteria ruled earth’s oceans unchallenged for 1 to 2 billion years. The cyanobacteria reached the zenith of their success approximately 1 billion years BP, when filamentous forms produced great floating mats on the oceans’ surfaces. This long period of cyanobacterial dominance, encompassing approximately two-thirds of the history of life, has been called with justification the “age of blue-green algae.” Bacteria and cyanobacteria are so completely different from forms of life that evolved later that they were placed in a separate taxonomic kingdom, Monera.

Carl Woese and his colleagues at the University of Illinois discovered that the prokaryotes actually comprise at least two distinct lines of descent: the Eubacteria (“true” bacteria) and the Archaebacteria also called Archaea. Although these two groups of bacteria look very much alike when viewed with the electron microscope, they are biochemically distinct. Archaebacteria differ fundamentally from bacteria in cellular metabolism, and their cell walls lack muramic acid, which is present in the cell walls of all Eubacteria. The most compelling evidence for differentiating these two groups comes from the use of one of the newest and most powerful tools at the disposal of the evolutionist, sequencing of nucleic acids. Woese found that Archaebacteria differ fundamentally from other bacteria in the sequence of bases in ribosomal RNA. Woese considers the Archaebacteria so distinct from the true bacteria that they should be considered a separate taxonomic kingdom, Archaea. The Monera then comprise only the true bacteria.

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