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
Famous quotes containing the words theories and/or evolution:
“The wise man regulates his conduct by the theories both of religion and science. But he regards these theories not as statements of ultimate fact but as art-forms.”
—J.B.S. (John Burdon Sanderson)
“By contrast with history, evolution is an unconscious process. Another, and perhaps a better way of putting it would be to say that evolution is a natural process, history a human one.... Insofar as we treat man as a part of nature—for instance in a biological survey of evolution—we are precisely not treating him as a historical being. As a historically developing being, he is set over against nature, both as a knower and as a doer.”
—Owen Barfield (b. 1898)