The Endosymbiotic Theory
In all living beings, there are two types of cells, prokaryotes and eukaryotes.
Prokaryotes are small, containing circular DNA which lies freely in the cytoplasm,
while eukaryotes are larger, and have linear DNA contained in a nucleus. Plants,
fungi, Protoctista and animals are made up of eukaryotic cells while bacteria are
prokaryotes.
Prokaryotes are believed to be the ancestors of eukaryotic cells. They were
present in approximately 2.7-billion-year-old rocks; hence they are older than
eukaryotes which appeared about 1.8 billion years ago. The two types of cells also
share many traits, for example, similar genetic codes, enzymes and plasma
membranes that could not have evolved independently in different organisms.
The endosymbiotic theory states that some of the organelles in eukaryotic
cells (mitochondria and chloroplasts) were once prokaryotic cells. The first
eukaryotic cell was probably an amoeba-like cell that got nutrients by phagocytosis
and contained a nucleus that formed when a piece of the cytoplasmic membrane
pinched off around the chromosomes. Some of these amoeba-like organisms
ingested prokaryotic cells that then survived within the organism and developed a
symbiotic relationship. The prokaryotes provided the eukaryotes with energy since
they were able to carry out complex metabolic reactions such as respiration and
photosynthesis. On the other hand, the eukaryote provided nutrients for the
prokaryotes. Prokaryotes are said to be endosymbionts.
This theory was introduced by a scientist named Lynn Margulis in 1970. Her
theoretical paper titled “On the Origin of Mitosing Cells” is considered a landmark
in modern endosymbiotic theory today, although it was originally rejected many
times until being accepted by the Journal of Theoretical Biology.
The idea behind this theory leads back all the way to 1883, where botanist
Andreas Schimper who was looking at the plastid organelles of plant cells using a
microscope, noticed that the plastids divided very similarly to the way some freeliving bacteria did.
Then, during the 1950s and 60s, scientists found that both mitochondria and
plastids inside plant cells had their own DNA. When scientists looked closer at the
genes in the mitochondrial and plastid DNA, they found that the genes were more
like those from prokaryotes. Hence, organelles are more closely related to
prokaryotes.
Lynn Margulis made a similar observation that the mitochondria in cells
resembled bacteria. Both bacteria and mitochondria had a double membrane, their
own circular DNA and similar methods of reproduction. Using this information as
well as that from other scientists, she developed the endosymbiotic theory. Even
though she was not the first scientist to make the observation, she was the first to
come up with concrete evidence that supports the hypothesis.
Some key features that support the theory about these organelles are that they
are both double membrane bound, are similar in size to prokaryotic cells, reproduce
by binary fission, have their own DNA that is circular and not linear nor enclosed in
a nucleus, and they have their own ribosomes which are the same size as those in
prokaryotes.