Abstract
Close physical interspecific associations called symbioses are central for understanding both the evolution of animals, plants and all other branches of living creatures, and the shaping the Earth's physical features, i.e. the whole biosphere. Since the nature of the phenomenon has a fundamental importance as a unifying scheme, symbiotic research is occuring in all fields of biology.
Here, I briefly discuss the theoretical frame, definition and history of the concept of symbiotic interactions (Part I - What does the Symbiosis mean and what is its Importance for Biological Sciences); the nature of the smallest microorganisms (if they are organisms at all) - viruses, and their role in biosphere (Part II - Symbiotic Interactions between Viruses and Cellular Organisms); and the nature and ecological & evolutionary role of prokaryotes with particular regard to description of the whole domain, including historical perspective and some examples (Part III - Symbiotic Interactions between Prokaryotes and other Organisms).