A2.3 Virus origins - student reading
Evolution of Viruses
Although biologists have an understanding about evolution of present-day viruses, much less is known about the origin of the first viruses. When exploring evolutionary history scientists usually look at fossil records but viruses do not fossilize, so researchers must investigate how today’s viruses evolve by using biochemical and genetic information to create speculative virus histories.The diversity of virus structures supports the hypothesis that viruses don’t have a single common ancestor.
Several hypotheses about virus origins:
- One possible hypothesis, called the devolution hypothesis, proposes that viruses evolved from free-living cells. However, how this process might have occurred is a mystery.
- A second hypothesis, called the escapist hypothesis suggests that viruses originated from RNA and DNA molecules that escaped from a host cell. This hypothesis accounts for viruses having either an RNA or a DNA genome.
- A third hypothesis describes virus origins evolving alongside the cells they rely on as hosts using a system of self-replication similar to that of other self-replicating molecules. Studies of some plant pathogens support this hypothesis.
Common ancestors
This phylogenetic tree of the three domains of life (Bacteria, Archaea, and Eukarya) attempts to identify when various species diverged from a common ancestor. Finding a common ancestor for viruses has proven to be far more difficult, especially since they do not fossilize.
As technology advances, scientists may develop and refine further hypotheses to explain the origin of viruses.
The emerging field of molecular systematics attempts to do just that through comparisons of sequenced genetic material. These researchers hope to one day better understand the origin of viruses, by finding fragments of viral genomes in the genomes of species alive today. There more widely distributed viral gene sequences are, the more ancient they must be.