The Endosymbiont Hypothesis

An interesting question can produce some interesting answers. One of these is the question of what living things are made of. There are many possible answers, depending on who you ask. A young child asking his parents may simply be told: “Stuff”. A chemist may go into the detail of the basic common organic elements: Carbon, Hydrogen, Oxygen, Nitrogen and Phosphorous (Starr et al., 2009) and the different compounds that are formed. As biologists, we tend to look at the composition of living things at a relatively larger scale viz. from a scale of less than 0.1 nm to a scale of more than 1 nm (Starr et al., 2009a). I say relatively because, although this is still very small, it is much larger than the scale at which chemists work. This scale encompasses the realm of cells and cellular structures. The basic unit of life, as defined by the cell theory, pioneered by Schwann and Schleiden in 1838 (Mazzarello, 1999; Wayne, 2009), is the cell. Since that time, a lot of development has occurred in the quest to find the basis of life. It is now known that there are in fact two distinct kinds of cells, i.e. the prokaryotic and eukaryotic cells. The distinct differences between the two are easily discernible with modern technology such as electron microscopes. These differences include: the presence of a nucleus in the eukaryotes, and the absence of one in the prokaryotes; Eukaryotes also have distinct membrane-bound organelles, while prokaryotes do not (Stanier et al., 1970a; Mauseth, 2009). It is after discovery and examination of these organelles that first led to the Endosymbiont Hypothesis or Endosymbiont Theory, as it is sometimes called (both terms are used throughout). In this essay we shall look at the history of the theory, how it was formed, who the most influential collaborators were, and how it is applied to the evolution of certain organelles, in particular, the chloroplast and mitochondria.
The development of the Endosymbiont Theory occurred over a

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