Three Theories of Evolution
According to the progressive theory, segments of DNA escaped from existing cells, mutated and eventually evolved into modern viruses. Existing mobile genes such as plasmids and transposons support this theory.
The regressive theory postulates that viruses are "devolved" bacteria. Stemming from obligate intracellular parasites such as Chlamydia, viruses would have lost a large portion of their genetic code. This theory seems to best describe large double stranded DNA viruses such smallpox.
Based on the RNA world theory of life, the Virus First hypothesis proposes that viruses evolved from the very first segments of RNA, predating the early cells.
Read more about this topic: Evolutionary Virology
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