Reassessing Viral Origins: From Escaped Genes to Degenerated Microbes

Three main hypotheses have been proposed to explain the origin of viruses: the exogenisation (escape) hypothesis, suggesting that mobile genetic elements gained infectivity and autonomy; the degeneration hypothesis, proposing that viruses arose through gene loss from more complex, possibly cellular ancestors; and the virus-first hypothesis, which argues that viruses are ancient, pre-cellular entities. This review evaluates these models in light of molecular, structural, and ecological evidence. Key considerations include the lack of homologues for many viral proteins, the presence of giant DNA viruses with extensive gene repertoires, the conservation of capsid structures across diverse viruses, and the universal dependence of viruses on living hosts. Also discussed is the vast diversity of the global virosphere revealed by recent metaviromic studies, particularly in marine ecosystems, where viruses play key roles in structuring microbial communities and driving biogeochemical cycles. Such findings highlight that viruses are integral components of biological systems rather than merely parasitic outliers. Although no single hypothesis fully explains the origin of all viruses, their extraordinary genetic and functional complexity suggests a unified evolutionary theory may forever remain elusive. Rather, understanding the origins of viruses requires integrating genomic traits with ecological roles across their wide diversity.