The idea of endosymbiotic theory was published by Lynn Margulis back in the 1960s. Margulis argued that eukaryotic cells evolved from prokaryotic cells through symbiogenesis and by forming mutual symbiotic relationships. At the time, claims such as these, especially those of a female scientist, were extremely controversial because they contradicted Darwin’s theory of evolution which was accepted as a fact in society at the time. Scientists have been debating whether endosymbiotic theory or evolution by natural selection created the first eukaryotic cell. But now that Margulis’ theory of symbiogenesis has been proven time and time again, I think that endosymbiotic theory is the way eukaryotic cells are formed, and are not formed by Darwin’s evolution by natural selection.

Lynn Margulis’ endosymbiotic theory was based on the fact that eukaryotic cells evolved from prokaryotic cells through symbiogenesis. In an informational video posted by The Royal Institution, they essentially confirmed endosymbiosis; The narrator talked about how during endosymbiosis, one cell was engulfed by another cell and formed a mutual symbiotic relationship. This smaller cell that had been engulfed eventually formed the mitochondria in current eukaryotic cells. Another reason that endosymbiotic theory is plausible is that Margulis believed the prokaryotic cells started by having symbiotic relationships and therefore formed a eukaryotic cell. According to the Smithsonian National Museum of Natural History, Margulis believed, "Microbes began living inside other microbes, functioning as organelles for them. Mitochondria, the organelles that process food into energy, evolved from these mutually beneficial relationships." This shows that the mitochondria and chloroplasts were once independent prokaryotic cells that began a symbiotic relationship with a larger host cell that engulfed it.

Many scientists questioned the theory of symbiogenesis and believed eukaryotic cells evolved from natural selection. Charles Darwin had thought of a popular theory involving natural selection; his idea summarized that organisms evolved due to changes in their environment that caused them to have to adapt. Organisms with favorable traits survive and pass those traits down. However, an article from ThoughtCo stated, “However, other evolutionary scientists assert that it [Darwin’s theory of evolution by natural selection] is actually cooperation between individuals and not always competition that drives evolution.” This proves that Darwin’s theory is flawed and that there is more evidence to support Margulis and endosymbiotic theory. Another thing to consider is the structure of organelles in eukaryotic cells, such as the mitochondria and chloroplasts. According to Khan Academy, "The DNA molecules found in mitochondria and chloroplasts are small and circular, much like the DNA of a typical bacterium." A known fact about mitochondria and chloroplasts is that the structure of their DNA is similar to those of prokaryotes; they have circular DNA and their own ribosomes. The similarities shared between the DNA in these organelles are significant to support the endosymbiotic theory. 

With all of the works shown, it is safe to say that Margulis’ endosymbiotic theory was validated by the fact that eukaryotic cells were formed by mutual symbiotic relationships that ended with prokaryotic cells getting engulfed by larger cells. Even though the endosymbiotic theory was not accepted by the scientific community when it was originally released, the amount of information humans have about prokaryotic and eukaryotic cells would not be possible without symbiogenesis. All in all, I think that the formation of eukaryotic cells is best supported by endosymbiotic theory about the evolution of eukaryotic cells and not that they evolved from natural selection.