Answer:
According to this hypothesis, viruses evolved early in Earth's history from fundamental replicative molecules that formed in the “primordial soup” as the planet began cooling. These molecules also led to the evolution of cellular organisms—the viral hosts—either in parallel or at a later stage of evolution.
That would have been 3 or so billion years ago, early in earths history.
The reproductive system is different from other body systems in that the reproductive system functions in the survival of a species while all other body systems function to ensure the survival of the individual.
Drastic change in pH or temperature because the enzyme reaction may become denatured.
It looks like the respiratory and digestive systems.
