There's Evidence for Multiple Origins of Viruses
- Viruses show enormous diversity in structure and genetic composition.
- This diversity suggests that viruses did not arise from a single common ancestor but instead evolved multiple times from different cellular origins.
- Their shared features likely result from convergent evolution, not shared descent.
The shared genetic code between viruses and cells supports a post-cellular origin for viruses.
Hypotheses for Virus Origins
1. Progressive (Escape) Hypothesis
- Viruses may have evolved from mobile genetic elements, like retrotransposons, which move within genomes.
- Retrotransposons and retroviruses (e.g., HIV) both use reverse transcription, where RNA is converted into DNA.
- Over time, retrotransposons may have gained capsid proteins, enabling them to exist independently as viruses.
Retrotransposons and HIV both rely on reverse transcription and genome integration, supporting the idea that retroviruses may have evolved from retrotransposons.
2. Regressive (Reduction) Hypothesis
- Viruses were once free-living cells that became parasitic and lost unnecessary genes over time.
- Some large viruses, like Mimivirus (0.75 μm, 1.2 million base pairs), still retain genes for enzymes and limited metabolic activity.
