Endosymbiotic Theory
The endosymbiotic theory concerns the origins of mitochondria and chloroplasts, which are organelles
of eukaryotic cells. According to this theory, these originated as prokaryotic endosymbionts, which came
to live inside eukaryotic cells. The theory postulates that the mitochondria evolved from aerobic
bacteria (probably proteobacteria, related to the rickettsias), and that the chloroplast evolved from
endosymbiotic cyanobacteria (autotrophic prokaryotes). The evidence for this theory is compelling as a
whole, and it is now generally accepted.
History
The idea that the eukaryotic cell is a group of microorganisms was first suggested in the 1920s by the
American biologist Ivan Wallin. The endosymbiont theory of mitochondria and chloroplasts was
proposed by Lynn Margulis of the University of Massachusetts Amherst. In 1981, Margulis published
Symbiosis in Cell Evolution in which she proposed that the eukaryotic cells originated as communities of
interacting entities that joined together in a specific order. The prokaryote elements could have entered
a host cell, perhaps as an ingested prey or as a parasite. Over time, the elements and the host could
have developed a mutually beneficial interaction, later evolving in an obligatory symbiosis.
Dr. Margulis has also proposed that eukaryotic flagella and cilia may have arisen from endosymbiotic
spirochetes, but these organelles do not contain DNA and do not show any ultrastructural similarities to
any prokaryotes, and as a result this idea does not have wide support. Margulis claims that symbiotic
relationships are a major driving force behind evolution. According to Margulis and Sagan (1996), "Life
did not take over the globe by combat, but by networking" (i.e., by cooperation, interaction, and mutual
dependence between living organisms). She considers Darwin's notion of evolution driven by
competition to be incomplete.
Dr. Christian de Duve (winner of the 1974 Nobel Prize in Physiology or Medicine) proposes that
peroxisomes may be the first endosymbionts, which allowed cells to withstand the growing amounts of
free molecular oxygen in the Earth's atmosphere. Since peroxisomes have no DNA of their own, this
proposal is more speculative than the above ideas.
Evidence
Evidence that mitochondria and chloroplasts arose via an ancient endosymbiosis of a bacteria is as
follows:
Both mitochondria and chloroplasts contain DNA, which is fairly different from that of the cell nucleus,
and that is in a quantity similar to that of bacteria.
Mitochondria utilize a different genetic code than the eukaryotic host cell; this code is very similar to
bacteria and Archaea.
They are surrounded by two or more membranes, and the innermost of these shows differences in
composition compared to the other membranes in the cell. The composition is like that of a prokaryotic
cell membrane.
New mitochondria and chloroplasts are formed only through a process similar to binary fission. In some
algae, such as Euglena, the chloroplasts can be destroyed by certain chemicals or prolonged absence of
light without otherwise affecting the cell. In such a case, the chloroplasts will not regenerate.
Much of the internal structure and biochemistry of chloroplasts, for instance the presence of thylakoids
and particular chlorophylls, is very similar to that of cyanobacteria. Phylogenies built with bacteria,
chloroplasts, and eukaryotic genomes also suggest that chloroplasts are most closely related to
cyanobacteria.
DNA sequence analysis and phylogeny suggests that nuclear DNA contains genes that probably came
from the chloroplast.
Some genes encoded in the nucleus are transported to the organelle, and both mitochondria and
chloroplasts have unusually small genomes compared to other organisms. This is consistent with an
increased dependence on the eukaryotic host after forming an endosymbiosis.
Chloroplasts appear in very different groups of protists, which are in general more closely related to
forms lacking them than to each other. This suggests that if chloroplasts originated as part of the cell,
they did so multiple times, in which case their close similarity to each other is difficult to explain.