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The parasite-host-environment system is dynamic, with several points of equilibrium. This makes it difficult to trace the thresholds between benefit and damage, and therefore, the definitions of commensalism, mutualism, and symbiosis become worthless. Therefore, the same concept of parasitism may encompass commensalism, mutualism, and symbiosis. Parasitism is essential for life. Life emerged as a consequence of parasitism at the molecular level, and intracellular parasitism created evolutive events that allowed species to diversify. An ecological and evolutive approach to the study of parasitism is presented here. Studies of the origin and evolution of parasitism have new perspectives with the development of molecular paleoparasitology, by which ancient parasite and host genomes can be recovered from disappeared populations. Molecular paleoparasitology points to host-parasite co-evolutive mechanisms of evolution traceable through genome retrospective studies.