Beginning of Essay
About 2.5 billion years ago, oxygen began slowly to accumulate in the atmosphere, as a result of the photosynthetic activity of the cyanobacteria. Those prokaryotes that were able to use oxygen in ATP production gained a strong advantage, and so they began to pro ....

Middle of Essay
.... its organization into a number off distinct chromosomes, and complex organelles, among which are chloroplasts and mitochondria. Scientists believe that eukaryotic organisms such as the protists evolved from the prokaryotes. There are two main theories which describe how this transition may have occurred. The first is the endosymbiotic theory, or enosymbiosis, and the other is the autogenous theory, or autogenisis. These two theories are not mutually exclusive, meaning one or the other could account for different parts of eukaryotic cells. The endosymbiotic theory states that the formation of eukaryotic cells were symbiotic associations of proka ....