Endosymbiont Theory

The serial endosymbiotic theory of eukaryotic cell evolution, aggressively advocated by Lynn Margulis, revolutionized our concept of life on earth. She hypothesized that cell organelles like mitochondria, chloroplasts and basal bodies (9+2) of flagella have descended from free living bacterial species. The idea of endosymbiotic origin of mitochondria and plastid was prevalent even in the late 19th and early 20th century but soon faded until she revived it as a basic theme in eukaryote evolution. Her ideas were frequently greeted with skepticism and even hostility. However, with supportive experimental evidences the theory is broadly accepted, giving her due recognition.

Lynn Margulis was an American evolutionary theorist, biologist, science writer and above all an educator. In her 1967 article, ‘On the origin of mitosing cells’ she presented a symbiotic view of eukaryotic cell evolution. It was perhaps the first unified theory of eukaryogenesis (dubbed as serial endosymbiont theory). She subsequently elaborated the concept in her book, ‘Origin of eukaryotic cells’ and was the most creative challenger of mainstream Darwinism.

Margulis proposed that symbiosis acts as an evolutionary force and cooperative association had a greater role to play in evolution than competition oriented survival. While her endosymbiont theory on evolution of organelles is quite acceptable, her hypothesis that symbiosis is more important than survival of the fittest is still controversial. In her endosymbiont theory, small respiring bacteria parasitized larger anaerobic prokaryotes.These small ingested bacteria eventually evolved into organelles. It is likely that these proto-mitochondria invaded their hosts like the modern predatory bacteria, Bdellovibrio invades pray bacteria. 

The respiring (aerobic) parasite by consuming oxygen, would allow its host (anaerobic) to survive in conditions where it would otherwise die. The parasite would have also shared some energy (ATP) that it efficiently produced aerobically. In exchange, the large anaerobic host would have provided carbon sources for aerobic respiration. Over a course of evolution, everything that was not useful for survival of this relationship would have been lost /led to evolution of compensatory strategies. As oxygen began to increase in the atmosphere the anaerobic host became more and more dependent on protomitochondria to detoxify the gas. Therefore, what began as parasitism slowly evolved into an obligatory mutually beneficial partnership. These small respiratory bacteria eventually evolved into mitochondria of present day eukaryotic cells. Chloroplast is only found in plants and some protists. In evolutionary terms plastids are younger than mitochondria and evolved in some aerobic eukaryotes. The endosymbiont that subsequently became chloroplast are believed to be cyanobacteria. It is quite possible that some of these bacteria would have escaped from being digested inside the predator cell. In addition, the chloroplast progenitor would’ve provided an added advantage to the host. 

The resemblance between plastid and cyanobacteria structure and biochemistry is relatively more pronounced than the mitochondrion / αproteobacteria symbiont. Although the theory seems farfetched, we can see these events happening even today. For example, the ciliate protozoan Paramecium bursaria is often playing a host to the unicellular green algae, Chlorella. The alga living inside the Paramecium is like ‘pseudo chloroplasts’ providing food to the protozoan. Therefore the host prefers not to digest the green alga but establish a symbiotic partnership.