Parasites at the origin of life.

This paper is concerned with parasitic virus-like particles and their hosts. It is proposed that parasitism must have occurred at an early stage of evolution, soon after the first self-reproducing systems had formed. When chemical building blocks for self-reproducing systems became scarce, current theories envision that some self-reproducing systems evolved the capability to synthesize materials for self-replication from chemical precursors in the environment. It is proposed that at about the same time parasitic systems (phages) arose that replicated at the expense of host systems by diverting host materials to the replication of their own genomes. With the aid of a mathematical model we demonstrate that host and phages can coexist in a stable equilibrium, depending upon the carrying capacity of the environment. If the latter falls below a threshold, then the parasites die out. A parasite that has the capability to integrate into the host genome is replicated along with it and thus escapes extinction during periods of population bottlenecks of the host population. The presence of phages creates evolutionary pressures favoring host defenses against them. Thus, modern bacteria are able to degrade most invading DNA (through restriction enzymes). Defense capabilities require a share of the genome, thus adding to the genetic complexity of organisms.