Decades of research and analysis has led microbiologists to conclude that individual
animal and plant cells are fundamentally more complex than bacterial cells. Years of
inquiry into how this complexity might have arisen has allowed for the development of a
single truly comprehensive explanation called endosymbiosis theory. This theory
predicts that very simplistic, bacterial (prokaryotic) cells must have physically joined
together to develop more complex, more highly functioning cells. One of the most
interesting, logical consequences of this theory reveals that neither plants nor animals
could have evolved if it were not for endosymbiosis.
Before the arrival of plants and animals, over three billion years ago, there was relatively
little variety in body plan. Most cells at this time were chemoautotrophs, they gathered
energy from the inorganic matter available to them deep within the ocean. These cells
were able to engulf specific molecular compounds and break them down chemically in
order to release the potential energy that they housed. This method of energy extraction
was anaerobic (did not necessitate oxygen) and was therefore very inefficient. It is
thought that these organisms might have depleted their chemical resources and that out
of this vacuum other more complex forms of cells evolved.
One of the first types of cells to evolve in response to the food shortage was another
kind of autotroph that used photosynthesis (yes, just like plants) to take energy from
sunlight and use it to create chemical bonds between available molecules in order to live
and grow. Some of these single celled organisms were very successful, and they
proliferated. These photoautotrophs, called cyanobacteria physically and genetically
joined with other single-cell life forms. The product of the symbiosis between these two
types of bacteria created more complex cells. These more behaviorally versatile
(eukaryotic) cells allowed for the development of some of the most evolutionarily
successful life forms, including algae and green plants. The chloroplasts (the main
cellular component involved in photosynthesis) that we observe within each cell of
modern day plants were originally a type of cyanobacteria. But the story does not end
Before green plants came to be, another type of early cell evolved. This cell was able to
burn energy in a much more efficient way, it burned chemicals aerobically (using oxygen
to perform oxidation). This type of organism was heterotrophic, meaning that it required
organic energy in the form of other living things (you and I are heterotrophic). Most
evolutionary biologists think that these types of organisms evolved to eat the
proliferating multicellular and unicellular cyanobacteria. Well, this type of energy
consumption proved to be wildly efficient and it allowed these heterotrophic cells a great
deal of energy that they could then turn into kinetic energy (the energy of movement).
Just like the cyanobacteria, these heterotrophic cells began to experiment with
multicellularity. Today, within the cells of each animal (including you and me) there
exists a mitochondrion which allows our cells to quickly and efficiently burn the organic
chemicals that we consume. These mitochondria are contained within our cells because
of endosymbiosis. They are symbiotic bacterial cells that have been employed for
hundreds of millions of years by animal cells to burn energy. These mitochondria are
not aliens though, they are as much a part of us as any of the other bacterial cells that
merged to create our eukaryotic, heterotrophic cells. One of the strongest pieces of
evidence that supports the theory of endosymbiosis is the fact that mitochondria (and
chloroplasts for that matter) have their own DNA, and this DNA is extremely similar to
You may have observed that plants do not seem to expend a great deal of energy in
comparison to animals, this is because they formed an alliance with a photosynthetic
symbiont instead of an aerobic one. Animals have more energy, but we have to move,
and work to procure it. Plants on the other hand can be sedentary, and they get their
food from the water and air, and their energy from the sun.
Animals are very much reliant on plants; most animals either eat plants or eat other
animals that eat plants. Plants even provide us with the oxygen that we need to burn the
food that we get from them. At the same time, plants are also very much reliant on us.
Just as we use the oxygen that they produce, plants use the carbon dioxide that we
exhale. Plants also use the dead bodies of animals as fertile soil to grow roots in and to
extract necessary minerals and nutrients from. For me, the most amazing relationship to
arise out of the endosymbiosis between bacterial cells is a distinctly different form of
symbiosis, one between animals and plants.
An organism capable of synthesizing its own food from inorganic substances, using
either light or chemical energy. Green plants, algae, and certain bacteria are autotrophs.
An organism, such as a bacterium or protozoan, that obtains its nourishment through
inorganic chemical compounds.
Symbiosis in which a symbiotic organism lives within the body of its partner. Also the
widely held theory that eukaryotic organelles were originally acquired by endosymbiosis.
An organism that cannot synthesize its own food and is dependent on complex organic
substances for nutrition.
An organism capable of synthesizing its own food from inorganic substances using light
as an source of energy. Green plants and photosynthetic bacteria are photoautotrophs.
A close association between two or more different organisms of different species that
may, but does not necessarily, benefit each member.
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