Evaluation of the Evidence The modern version of the endosymbiosis hypothesis was developed and promoted by Dr. Lynn Margulis in 1970. The term endosymbiosis comes from “endo” meaning “within,” and symbiosis which occurs when two different species benefit from living and working together (Genetic Science Learning Center). This theory proposes that the organelles of eukaryotic cells, specifically the mitochondria and chloroplasts, were once free-floating bacteria that were ingested by larger, prokaryotic bacteria by means of endocytosis.
The host bacteria benefitted from the engulfment of the organelles as they were able to carry out functions, such as cellular respiration, more efficiently than the host itself could. Therefore, the free-living bacteria were not digested, and consequently over time their DNA was combined with the host bacterial cell’s DNA to eventually develop a new form of cells called eukaryotes.
One of the most eminent pieces of evidence that supports the hypothesis is that although prokaryotic cells and eukaryotic cells are quite distinctive in physiological characteristics, some of the organelles of eukaryotes share a number of staggering similarities with prokaryotes. Like prokaryotes, the mitochondrion and chloroplast both replicate through means of binary fission, unlike the eukaryotic cells that contain them, which undergo mitosis.
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Moreover, prokaryotes and the abovementioned eukaryotic organelles have circular DNA, whereas eukaryotes have linear DNA. The size of the DNA is also a factor in the theory as the prokaryotic DNA and the DNA of the organelles is similar in size, and much smaller than that of eukaryotic cells. The dissimilarities between the physical characteristics of the DNA in the eukaryotic nucleus and the DNA in the organelles lead to the impression that the organelles were once bacterial (prokaryotic) symbionts (IUPUI Department of Biology).
However, since the DNA in the nucleus and the organelles replicate independently of each other, there is the possibility that random mutations may have occurred, thus causing the discrepancies in the DNA over a certain period of time. Therefore, this evidence is not conclusive proof that the organelles are possible descendants of ancestral bacteria. Additionally, chloroplasts and mitochondria are surrounded by a double membrane. This provides further evidence to the endosymbiosis theory because it suggests that these organelles were ingested by a primitive host (Genetic Science Learning Center).
To explain this circumstance, the host cell ingests the bacterium through phagocytosis – a form of endocytosis – which results in an additional lipid bilayer formed by the vesicle membrane, in addition to the bacterium’s inner plasma membrane (IUPUI Department of Biology). There is no other known cause as to why these organelles have double membranes, other than the widely-accepted endosymbiotic theory. In the field of science, a theory is a well-established explanation based on extensive experimentation and observation, and is generally accepted as fact by the scientific community (Genetic Science Learning Center).
Although some of the evidence provided in support of the endosymbiosis hypothesis is not irrefutable, there is more evidence is support of the theory, than there is against it. Therefore, in my opinion, the amount of research put into this topic over the years and the various evidences that have been unearthed in support of it, as well as the lack of proof against it, lead me to accepting this theory as conclusive. However, I also want to note that research is always being done, and in the near future it is possible that scientists may propose new, improved theories.
REFERENCES Genetic Science Learning Center (1969, December 31). The Evolution of the Cell. Retrieved on October 8, 2012 from http://learn. genetics. utah. edu/content/begin/cells/organelles/ IUPUI Department of Biology (2002, January 14). The Endosymbiotic Theory. Retrieved on October 8, 2012 from http://www. biology. iupui. edu/biocourses/N100/2k2endosymb. html Caprette, D. R. (2008). Evolutionary Origin of Mitochondria. Retrieved on October 8, 2012 from http://www. ruf. rice. edu/~bioslabs/studies/mitochondria/mitorigin. html
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