(Andreas Schimper)

The Endosymbiotic Hypothesis wasn’t developed overnight by a single scientist.  The combined work of several researchers over a century of experimentation has led to the Hypothesis we know today.  The framework for the idea of endosymbiosis began with work done by Andreas Schimper in 1883.  Schimper, a botanist born in France in 1856, made the observation that the chloroplasts that are found in photosynthetic organisms had many similar characteristics to cyanobacteria.  Schimper never fully expanded upon this observation, but in a footnote in one of his journals he proposed the idea that the combination of two separate organisms may have given rise to photosynthetic organisms as we know them today.

This small observation by Schimper went unnoticed until 1905 when Russian botanist Konstantin Mereschkowski began to study Schimper’s work.  Mereschkowski did extensive work with lichens and this research, along with the work of Schimper, led Mereschkowski to develop the idea of symbiogenesis.  This hypothesis stated that complex organisms came to be through the relationships between less complex organisms.  This thought was predicated on the fact that the lichens Mereschkowski was studying exhibited a symbiotic relationship between fungi and algae.  While this idea was originally formed in 1905, Mereschkowski did not fully publish it until 1909 in his paper The Theory of Two Plasms as the Basis of Symbiogenesis, a New Study or the Origins of Organisms.

(Konstantin Mereschkowski)

Further headway was made in the 1920’s by Ivan Wallin, who extended the idea of endosymbiosis to include the origins of mitochondria.  Both Schimper and Mereschkowski focused on the transition from cyanobacteria, whereas Wallin looked at how aerobic bacteria became the mitochondria of eukaryotic cells.  These ideas were eventually supported in the 1960’s by the advent of electron microscopes as well as the discovery that both mitochondria and chloroplasts contained their own DNA, but when Wallin first proposed his hypotheses he was largely ignored.

The Endosymbiotic Hypothesis truly came into existence through the work of Lynn Margulis in the 1960’s and continuing into the 1980’s.  Margulis’ first seminal paper, The Origin of Mitosing Eukaryotic Cells, was published in 1966.  Much like Wallin before her, the ideas presented in her paper were widely ridiculed and dismissed.  At the time the idea of the “Modern Synthesis” stated that natural selection and mutation were the driving force behind the evolution of eukaryotic cells and new species.  Modern Synthesis believed that new species could only branch off from existing species; an explanation completely different from Margulis’ idea of evolution through cooperation between existing species.  After being rejected from 15 separate scientific publications, Margulis’ paper was eventually picked up by The Journal of Theoretical Biology and is now considered a critical piece of support for the modern theory of endosymbiosis.

Dr. Margulis continued her work throughout the following decades, with her 1981 paper Symbiosis in Cell Evolution being another landmark accomplishment.  This work focused more heavily on the origins of mitochondria, stating definitively that mitochondria were once free-living aerobic bacteria that were engulfed by larger cells.  This claim was supported by the fact that mitochondria and eukaryotic cells both began to appear roughly 1.5 billion years ago.  The prevailing belief is that as the atmosphere of the earth became more oxygen-rich, large anaerobic bacteria engulfed small aerobic bacteria in order to use the products of their cellular respiration.  The small aerobic bacteria also benefited from the protection of the large cell, so both organisms benefitted.  This symbiotic relationship resulted in a greater chance to grow and reproduce, and as the new cell evolved the engulfed aerobic bacteria eventually became the mitochondria that we see today.  Margulis’ argued that the complexity of life seen on the planet today could not have evolved solely through random mutation, and that this kind of Endosymbiotic relationship was critical for eukaryotic organisms to develop.

(Lynn Margulis)

One of the most heated points of debate Dr. Margulis faced was the origin of DNA in the cytoplasm of cells.  Margulis strongly believed that this DNA had originally been part of the endosymbiont bacteria that were engulfed by a larger cell.  Over time much of the bacterial DNA became one with the host cell DNA, which explains that while chloroplasts and mitochondria still contain DNA, they no longer contain all the genes necessary for autonomous life.  This hypothesis was further supported later in the 1980’s when it was discovered that the DNA found in the semi-autonomous organelles was of a different structure than the nuclear DNA of the eukaryotes.  In fact, the DNA in the chloroplasts and mitochondria bared striking resemblance to the circular structure found in prokaryotes.

The Endosymbiotic Hypothesis started with a tiny footnote in the work of Andreas Schimper, but as it worked its way through Mereschkowski, Wallin, and Margulis it grew and developed into a well-respected and widely accepted explanation for the origins of eukaryotic life.  Margulis herself received many accolades, notably being elected to the National Academy of Sciences in 1983 and receiving the National Medal of Science in 1999.  Through the combined endeavors of several scientists the Endosymbiotic Hypothesis has found its place in the scientific community.

(National Medal of Science)

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