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:
“The egoism which enters into our theories does not affect their sincerity; rather, the more our egoism is satisfied, the more robust is our belief.”
—George Eliot [Mary Ann (or Marian)
“Like Freud, Jung believes that the human mind contains archaic remnants, residues of the long history and evolution of mankind. In the unconscious, primordial “universally human images” lie dormant. Those primordial images are the most ancient, universal and “deep” thoughts of mankind. Since they embody feelings as much as thought, they are properly “thought feelings.” Where Freud postulates a mass psyche, Jung postulates a collective psyche.”
—Patrick Mullahy (b. 1912)