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The Endosymbiotic Theory
The exact origin of Eukaryotes is still under investigation, but one of the most
popular theories involves a symbiotic relationship between prokaryotes and a
pre-eukaryotic cell. This is known as the Endosymbiotic Theory, which originated
in 1883 with Andreas Schimper. Schimper hypothesized that cells had an
endosymbiotic nature. In 1905, Konstantin Merezhkovsky proposed the
argument that the plastids were
endosymbionts, further suggesting
that symbiosis is the driving force
behind evolution.1
In 1926, Merezhkovsky collaborated
with Ivan Walin and used the
Endosymbiotic Theory to explain the
origin of the mitochondria in all
eukaryotes through their book
publication titled, Symbiogenesis and
the Origin of Species. A more recent
proponent of this theory is Lynn
Margulis, who became famous through her research career that focused on this
concept.2
Figure 1. The plant cell (eukaryote).
The most important mechanism behind the Endosymbiotic Theory involves the
process of phagocytosis. Phagocytosis involves the ability of one cell to engulf
another cell, which likely initially evolved as a feeding mechanism. Before this
process evolved, material (e.g., food) was transported through the cell
membrane molecule by molecule. The evolution of phagocytosis gave cells the
ability to engulf entire cells, setting the foundation for the endosymbiotic theory.
According to Margulis, the pre-eukaryotic cell engulfed an aerobic bacterium
(through phagocytosis), but rather than digest and destroy the bacterium, a
symbiotic relationship was born. In this relationship, the aerobic bacterium
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provided energy through ATP (adenosine triphosphate) and the eukaryotic cell
provided an environment to live while protecting the new symbiont from harmful
environmental factors such as oxygen. Because almost all living eukaryotes
have a mitochondria, it is safe to assume that this event happened before plants
and animals split in the evolutionary lineage.3
After this first evolutionary leap came a
second that would separate the plant and
animal lineages forever. Through a second
symbiotic event that involved the eukaryotic
cell engulfing a cyanobacteria, the plants
would gain the ability to photosynthesize and
make their own food. This would categorize
them into the autotrophs and secure their
position at the bottom of the food chain,
regardless of how many evolutionary events
would take place from that point on.4
Figure 2. The chloroplast.
Evidence to support Margulis’s Endosymbiotic Theory has grown over the years.
This evidence includes (but is not limited to) the following:5,6,7
1. A double membrane surrounding
the organelles with an inner layer that
retains the bacteria’s characteristics
and an outer layer that retains
characteristics of the cell that
engulfed it.
2. Mitochondria and chloroplasts are
similar in size to prokaryotes.
3. Mitochondria and chloroplasts
have their own DNA and lack histone
proteins, the DNA is circular, and it is attached to the inner membrane just like in
prokaryotes.
Figure 3. The mitochondria.
4. Mitochondria and chloroplasts divide by fission, not mitosis.
5. The mitochondria, chloroplasts, and prokaryotes make proteins by similar
biochemical pathways that differ from those in eukaryotes.
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6. The mitochondria and chloroplasts are susceptible to certain antibiotics just
like prokaryotes. Eukaryotes are unaffected by these same drugs.2
1
G. S. Fliney, G. S. Hine, and M. Almanza. The Endosymbiotic Hypothesis. n.d.
http://endosymbiotichypothesis.wordpress.com/.
2
Ibid.
3
University of California. Understanding Evolution. n.d.
http://evolution.berkeley.edu/evolibrary/article/_0_0/endosymbiosis_04.
4
Ibid.
5
Indiana University. The Endosymbiotic Theory. n.d.
http://www.biology.iupui.edu/biocourses/N100/2k2endosymb.html.
6
G. S. Fliney, G. S. Hine, and M. Almanza. The Endosymbiotic Hypothesis. n.d.
http://endosymbiotichypothesis.wordpress.com/.
7
University of California. Understanding Evolution. n.d.
http://evolution.berkeley.edu/evolibrary/article/_0_0/endosymbiosis_04.
Pictures: Chloroplast:
http://commons.wikimedia.org/wiki/File:Chloroplast_in_leaf_of_Anemone_sp_TE
M_30000x.png
Plant cell:
http://commons.wikimedia.org/wiki/File:Plant_cell_structure.png
Mitochondria:
http://commons.wikimedia.org/wiki/File:Mitochondria_-_TEM.jpg
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