What is one of the greatest transitions in the history of life on Earth? 

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There are several transitions that have happened in the life form in the earth and they are very important since these events connect the evolution and transformation of early life forms to the present day earth. However, these are not claimed to be utmost true since there are several theories rejecting it. 

The formation the first RNA (~3500 billion years ago) - after the cooling of the earth's atmosphere, the energy of the environment is sufficient enough to form metabolic action thus creating sequences which was believed to be the first RNA. This was not actually supported by several scientists and is under series of experimentation. Another theory is postulating that the life form actually have started when a meteorite landed on earth bringing organic molecules that interacts with the earth atmosphere thus forming the first RNA and eventually unicellular organisms.

The endosymbiotic theory (~3.4-2 billion years ago) - this theory says that the cellular organelles such as mitochondria and chloroplasts are once living independently from each other. The early eukaryotic cells engulf the aerobic cells or cyanobacterium for food. However the cyanobacterium is able to convert the carbon dioxide in the atmosphere thus providing food for the eukaryotic cell. Then the two lived symbiotically with each other until the point that they can no longer live without each other. Millions of years after, they are already inseparable organisms which can reproduce and eventually complexes to other forms of organisms. Again, this is also a theory and no strong evidence to support it.

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