Modern Science have assimilated the theory of natural selection. This concept means that the organisms that are better prepared or adapted to their environment are the ones that are going to survive along the years and evolve. This process of natural selection occurs in every species. The factors that determine who is the best adapted depends on mutations (explain mutation concept) that can create new and better adaptations to the environment. Natural selection makes evolution possible and is usually associated with mutations in the DNA that start differentiating individuals that belong to the same species. However, some scientists and researchers considered another way of evolution and natural selection by the connection of two different lineages through “symbiosis”. The endosymbiotic theory, started by the scientists Lynn Margulis, explains the origins, the formation and the evolution of the eukaryotic cells.

In the 1960’s, studying the eukaryotic cell, Lynn Margulis realized that some organelles of animal and plant cells (eukaryotic cells) such as the mitochondrion had the same shape as a bacterium. Other scientists during the 19^th and 20^th century realized this similarity, but they were not able to explain or understand the reason of the likeness. Moreover, mitochondrion also had their own DNA different, being the only organelle that possessed this characteristic. Lynn Margulis devised a theory that explain the origin of this eukaryotic organelles.

The theory says that in fact the mitochondrion was originally a free prokaryotic cell. This bacterium possessed a nucleoid with DNA and RNA, plasma membrane and cell wall. By phagocytosis, a eukaryotic cell (that are much bigger than prokaryotic cells) englobed this bacterium and introduced it into his cytoplasm. The eukaryotic cell had the purpose of using the bacterium he had phagocyted to get energy and food. Nevertheless, the eukaryotic cell found that this prokaryotic organism could provide benefits such as the production of ribosomes or the cellular respiration. The prokaryotic cell that had been phagocyted by the eukaryotic also found that this one could provide materials, such as carbohydrates, lipids and proteins that were very helpful for the metabolism of the prokaryotic cell. This relation created by which both cells benefit from one and other is called “endosymbiosis”. The endosymbiosis is the symbiosis, a relationship between two organisms that take advantage from each other, but with the fact that one of the organisms lives inside the other. 

The Lynn Margulis theory is currently very accepted in the scientific community and explain the role that the organelles like mitochondrion or chloroplasts play inside the eukaryotic cells.