4 BIOL 1122 - Biology 2 for Health Studies Majors Discussion assignment 3

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It is believed that all eukaryotes have a similar ancestor in common and that they came about as a result of endosymbiosis (Rye et al., 2016). This theory posits that the mitochondria of eukaryotic cells started as the small cells in the symbiotic relationship whose job was to break down the waste of the large cell wall to produce energy. Other small cells possessed the ability to produce food with the use of sunlight and supplied the large cell with this by-product as well; these cells became the chloroplasts (LibreTexts, 2021). This was a theory developed by Lynn Margulis stating that one cell was engulfed by larger prokaryotic cells and continued to evolve until they were distinct (Biology, 2016). However, while this theory is the most common for explaining the evolution of eukaryotes, it is not universally accepted and there are other alternative theories. The hydrogen hypothesis unlike the endosymbiotic theory takes into consideration hydrogenosomes and anaerobic mitochondria. This theory predicts that eukaryotes that possess mitochondria or lost them come from a prokaryotic host. Hydrogen in its molecular form was produced as waste from the relationship between an anaerobic host and a eubacterium that were able to breathe. As a result, the eukaryotic tree should demonstrate how the aerobic and anaerobic forms are interconnected (Martin et al., 2015; Martin & Müller, 1998). Moreover, LibreTexts (2022) mentions the Endosymbiont Theory which postulates that when two symbiotic organisms become endosymbiotic it causes the fusion of genomes between various species resulting in horizontal gene transfer. It is believed that mitochondrial DNA and nuclear DNA originated from different ancestors. The mitochondrial DNA is said to have originated from bacterial circular genomes that were engulfed by the prokaryotic cells. This type of DNA degrades when the sperm fertilizes the egg or when the sperm is unable to fertilize the egg therefore, only the mother contains this type of DNA to donate to the daughter cell. This reasoning accounts for the fact that some eukaryotic cells resemble bacteria and others resemble archaea. Based on this theory, biologists have proposed that eukaryotic cells come from the fusion of genes from Archaea and bacteria. LibreTexts (2022) The fusion of genomes between bacteria and archaea results in the eukaryotic nucleus. Additionally, Baum & Baum (2014) propose an inside-out theory where they hypothesize that a prokaryote engulfed a bacterium that developed into mitochondria. In this autogenous model, it is suggested that the original prokaryotic cell already held the nucleus and all other constituents were developed as a form of protrusion called blebs. Bacteria were encapsulated by these blebs and their mitochondria would be used as food for them to grow and eventually fully engulf the bacteria. This theory is supported by the fact that prokaryotic cells are smaller than eukaryotic cells however, their nuclei are similar in size (Baum & Baum, 2014). There are so many other theories out there addressing the evolution of eukaryotic cells. There hasn’t been one that has been fully proven and universally accepted as yet. However, I believe the endosymbiosis theory to be more probable. The presence of DNA supports this since the DNA of the mitochondrion is similar to aerobic bacteria while the chloroplast’s DNA is similar to the photosynthetic bacteria. Additionally, the mitochondria and chloroplast are similar in size to bacteria. In sum, this theory provides a reasonable explanation for how prokaryotes were able to develop such complex eukaryotic organisms with distinct mitochondrial DNA. 

References: Baum, D.A., Baum, B. An inside-out origin for the eukaryotic cell. BMC Biol 12 , 76 (2014). https://doi.org/10.1186/s12915-014- 0076-2 Biology Professor. (2015, February 19). Endosymbiotic theory [Video]. YouTube. Endosymbiotic Theory LibreTexts. (2022, June 8). E ndosymbiotic Theory and the Evolution of Eukaryote s. LibreTexts. Retrieved from: 20.3C: Endosymbiotic Theory and the Evolution of Eukaryotes - Biology LibreTexts Martin, W. F., Garg, S., & Zimorski, V. (2015). Endosymbiotic theories for eukaryote origin. Philosophical transactions of the Royal Society of London. Series B, Biological sciences, 370 (1678), 20140330. https://doi.org/10.1098/rstb.2014.0330 Martin, W., & Müller, M. (1998). The hydrogen hypothesis for the first eukaryote. Nature, 392 (6671), 37–41. https://doi.org/10.1038/32096

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