When the first cellular life forms stirred on the earth, viruses may have already been poised to infect them, writes Steve Farrar.
A radical hypothesis that seeks to bring order to the previously mysterious world of viruses implies that they may have become infectious in order to compete with the more advanced entities that followed them.
Dennis Bamford, professor of microbiology at Helsinki University, Finland, and colleagues Roger Burnett, at the Wistar Institute in the US, and David Stuart, at the Wellcome Trust Centre for Human Genetics in Oxford, have devised the theory while trying to construct viral lineages - a tree of life for viruses.
So far they have put together two, one of which includes some of the viruses that infect humans (such as a cold virus), algae and bacteria.
To do this, they have abandoned traditional approaches. In particular, they have ignored genetic-sequence analysis and focused on similarities between characteristic structures and functions of each virus.
This flatly contradicts Richard Dawkins's "selfish gene" theory, which argues that ultimately the survival of genes lies at the heart of evolution.
It is instead built around the idea that each virus has an innate "self" that a changing pattern of genes supports.
Professor Bamford said that the genetic sequences of different viruses had diverged so much that they were almost unrecognisable as possible relatives. But their basic structure and function may not have drifted so far.
The scientists looked at the architecture of the protein coats that surround the nucleic acid heart of each virus. Here they found similarities that hinted at viral lineages that infected widely separated life forms.
Professor Bamford further argued that the work implied that a number of different lineages arose independently of each other billions of years ago.
"I would propose that early viruses came before cellular life as self-assembling compartments carrying nucleic acids," he said.
"If what I'm saying is true, the entire virus taxonomy could be reorganised."
The research has been published in the journal Theoretical Population Biology .