Viruses are highly diverse and appear to have evolved independently from many different cellular organisms within each of the major groups of life. Some viruses appear to have evolved from escaped components of cellular organisms, whereas other viruses may have evolved from parasitic cellular ancestors.
learning outcomes
You should be able to:
Explain why viruses are difficult to place within the tree of life.
Contrast the “escaped component” and “reduced parasite” hypotheses for the origin of viruses, and name major groups of viruses that best support each hypothesis.
Explain why phage therapy is once again an active area of research.
How does the size of viral genomes limit phylogenetic analyses of viruses?
The greatly reduced genomes of most viruses provide few sequences that can be compared with other organisms. Also, the genomes of many viruses evolve very quickly, making comparisons even more difficult. The reduced nature of viruses provides few morphological clues to their relationships, and their tiny size means they do not produce fossils. Viruses have evolved many times throughout the history of life, so they are related to organisms across the tree of life. For all of these reasons, it is often difficult to place viruses precisely on the tree of life.
What are the two main hypotheses of viral origins?
Many viruses probably represent escaped components of cellular organisms that now evolve independently of their hosts. Other viruses are likely to represent highly reduced, parasitic organisms that evolved from cellular ancestors but lost their cellular structures as they became independent of their cellular hosts.
How can viruses be used to treat some human diseases?
Viruses called bacteriophages, or “phages” for short, infect and kill bacterial cells. Phage therapy, first developed during World War I, involves applying these phages to kill pathogenic bacteria. Phage therapy was largely replaced by the use antibiotics in the 1930s and 1940s. With the increase in evolution of bacterial resistance to antibiotics, however, phage therapy is once again an active area of research.
It appears that the enormous diversity of viruses is, at least in part, a result of their multiple origins from many different cellular organisms. It may be best to view viruses as spin-