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: Viral Evolution
Famous quotes containing the words theories and/or evolution:
“It takes twenty or so years before a mother can know with any certainty how effective her theories have been—and even then there are surprises. The daily newspapers raise the most frightening questions of all for a mother of sons: Could my once sweet babes ever become violent men? Are my sons really who I think they are?”
—Mary Kay Blakely (20th century)
“As a natural process, of the same character as the development of a tree from its seed, or of a fowl from its egg, evolution excludes creation and all other kinds of supernatural intervention.”
—Thomas Henry Huxley (1825–95)