Biochemistry: The Chemical Reactions of Living Cells, Volume 1 - D. Metzler 1980

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Evolution of Complex Organisms

The vast differences between prokaryotic and Eukaryotic Cells have sparked numerous hypotheses regarding the evolutionary relationships between these two major categories of living organisms. According to one popular theory, Cell/35.html">Mitochondria trace their origins back to aerobic Bacteria. It is hypothesized that following The Emergence of blue-green Algae and the accumulation of sufficient oxygen in the atmosphere, a Symbiosis developed between small aerobic bacteria and larger, obligately anaerobic bacteria. In this arrangement, the aerobic bacteria took up residence inside the anaerobic host cells, consuming all the free oxygen and thereby protecting the host Organism, for which oxygen was toxic. Over time, this relationship became permanent, leading to the evolution of eukaryotic cells containing mitochondria [28, 29]. Subsequently, a similar symbiotic association with blue-green algae may have given rise to METABOLISM/14.html">Chloroplasts in plant eukaryotes.

This perspective is supported by the fact that symbiotic relationships continue to exist among modern organisms. For instance, the Cytoplasm of the green paramecium (Paramecium bursaria) harbors the unicellular alga Chlorella, a typical green plantlet capable of independent living. This coexistence between Chlorella and Paramecium presumably originated by chance [28]. While biologists and biochemists readily embraced the Symbiotic Theory of mitochondrial origin, Raff and Mahler proposed an alternative hypothesis. They suggested that mitochondria arose instead from mesosomal membranes, and that their DNA derives from extrachromosomal genetic material (such as Plasmids or episomes; Chapter 15, Section G.7) commonly found in Prokaryotic Cells [30]. This question remains unresolved and continues to be widely debated [30–32].

Fossil remains of bacteria and blue-green algae have been discovered in deposits that, according to geochemical data, are over 3 billion years old. It is possible that the first eukaryotes emerged around the same time, eventually evolving through natural Selection into the more than one million species known today.