VII.

Evolution

Three theories have been put forth to explain the origin of viruses. One theory suggests that viruses are derived from more complex intracellular parasites that have eliminated all but the essential features required for replication and transmission. A more widely accepted theory is that viruses are derived from normal cellular components that gained the ability to replicate autonomously. A third possibility is that viruses originated from self-replicating RNA molecules. This hypothesis is supported by the observation that RNA can code for proteins as well as carry out enzymatic functions. Thus, viroids may resemble “prehistoric” viruses.