Although Margulis’s theory of symbiotic evolution and synthesis theory of neo-Darwinism are representative scientific theories that explain evolution, they have been in conflict with each other because of their different basic assumptions about the mechanism by which the life lineage is formed. Whereas Margulis presupposes symbiotic cell fusion as a major factor in the formation of a life lineage, Neo-Darwinism attaches greater significance to genetic variation within a single species. Most of the studies dealing with the two theories take the position that there is a limit to the explanatory power of the symbiotic evolution theory, and it is regarded as a theory in opposition to neo-Darwinism. Unlike previous studies, this study tried to present the rationale on the premise that the theory of symbiotic evolution has the same level of explanatory power as neo-Darwinism in the process of explaining evolution. This is because, although the superiority of explanatory power differs by period, speciation develops in various ways in the long history of life, and it is suggested that evolution can be properly explained only when both explanatory methods are accepted. Furthermore, I tried to show the fact that the theory of symbiotic evolution could go one step further from Neo- Darwinism by explaining the possibility that it could be extended to Gaia, an organic explanation for the Earth's ecosystem. To this end, it was explained that autopoiesis, which regulates interactions between cells and individuals in the process of symbiotic evolution, has been extended to a level that encompasses the control system between various organisms/ environments constituting the entire global ecology and is presented as a universal principle. This discussion on the connection between symbiotic evolution theory and Gaia can contribute to evoking the ontological fact that we human beings are just one member of Gaia, looking at the earth as an organic unity.