As described in our article about the origins of life, prokaryotes (cells without a nucleus) [1] were surely the first living organisms on Earth. During the evolutionary process eukaryotes (cells with a nucleus) appeared as well. Many theories tried to explain the emergence of these two categories of organisms, who seem so different at first glance. The theory currently recognized by the scientific community is the “Endosymbiotic Theory” [2], which lets us explain the formation of eukaryotic cells and presented their possible chimeric origins.
Main differences between an eukaryotic and prokaryotic cell
Concept of endosymbiosis
Before diving deeper into this theory, it’s important to explain the concept of endosymbiosis. The word “endosymbiosis” in composed of a prefix endo-, which means “inside”, and a word “symbiosis” which describes a mutually beneficial relation between two organisms. We find many examples of this type of relation, like lichen: it’s a symbiotic relation between a fungus and an alga (seaweed) [3]. In the case of endosymbiosis, it’s an interaction where one organism lives inside another (the latter being known as the “host”). In this interaction both of them are dependent on another, which is beneficial, but also indispensable.
Endosymbiotic theory
The concept of endosymbiosis is fundamental for the understanding of the endosymbiotic theory. It not only partly explains how eukaryotic cells could have formed, but also, more particularly, the origin of two specific structures: mitochondria (present in all eukaryotes) and chloroplasts (specific to plants). These structures, belonging to the family of “organelles” (or cellular compartments) [4], would initially be prokaryotic cells, which were ingested by a predecessor of eukaryotic cells. In fact, ingestion by another cell is not harmless, as usually it’s used as a source of food. Usually the ingested organisms end up being digested. However, it can happen that the ingested cell is not destroyed, and starts living inside its predator, who now becomes a host. In the case of eukaryotes, they were ingested, later on lost their independence, and finally they are a simple organelle in the cells. According to endosymbiotic theory, this phenomenon took place twice in predecessors of eukaryotes. First endosymbiosis resulted in mitochondria, present in all cells with a nucleus. The second one, specific to plants, led to the development of chloroplasts.
Endosymbiosis leading to the creation of eukaryotic cells
What arguments support this theory?
Many arguments support endosymbiotic theory [5]. Still, it’s worth saying, that scientific theories are based on knowledge that sometimes cannot be directly proven, but are supported by numerous clues, which argue in favour of the theory. As a consequence, a theory cannot ever be considered as the exact truth, but it’s rather thought of as a concept that is the closest to the reality (as is the theory of evolution, for example).
The very first argument for the endosymbiotic theory is the fact, that mitochondria and chloroplasts are composed of DNA very similar to the one of some prokaryotes. What’s more, these structures are also able to transcribe and translate their own proteins, as an independent cell would do. These are the only two organelles with such functions.
The second argument comes from the size of these organelles; they have similar sizes to prokaryotes, to which they are already genetically related. In general, prokaryotic cells are much smaller than plant or animal cells, so it’s possible they were being ingested on regular basis by predecessor of today’s eukaryotes as a source of food.The third argument is based upon the division of organelles. There are different types of cell divisions. Mitochondria and chloroplasts divide by fission, common in prokaryotes [6], as opposed to the eukaryotic cells that divide by mitosis [7].
Cell division types
Finally, if we take a look at the membrane (barrier which encloses the cells and its different compartments) of these organelles, we can see another particularity. Their barrier is composed of two membranes, which could prove that their predecessor was a bacterium, integrated as illustrated below:
Phagocytosis is a process allowing cells to eat other cells
Advantages and common profit ?
In spite of having presented different arguments here, this idea can seem surprising at first. What profit could come from developing a relation with an organism that was meant to be just a meal? Firstly, it’s important to underline the rarity of such events, and their randomness. Next, we should remember that mitochondria and chloroplasts are structures that allow for energy production in the cell. Furthermore, it’s possible that the cells ingested by the eukaryotic predecessor managed to “convince” the host to keep them, by providing an additional source of energy. As for the ingested cell, living inside a host also has its up-sides, like good protection. Mutual advantages could have promoted a conservation of this symbiotic interaction, up to a point where two organisms became one.
If you wish to see a funny video with complementary information, check out this link: https://www.youtube.com/watch?v=FGnS-Xk0ZqU
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I started reading this because the title was a little clickbait-ey but wow am I glad I did. Really interesting article!
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