We looked in the last post about the structure and life cycle of viruses and we saw that viruses are very different from all other cellular life. So where did they come from?
So, if we go back a very long time ago, we have the Ancestral Virus, the original virus or type of virus to evolve. We also have LUCA, the Last Universal Common Ancestor of cellular life (the domains Archaea, Bacteria, and Eukarya, of which humans are a part). There are a few different hypotheses:
This theory suggests that viruses originated from the parasites of LUCA.
Evidence: This theory is popular because there are such vast morphological differences between viruses and cellular life that it makes sense that they would have evolved separately.
Some scientists have looked for evidence in analyzing the viral genome or the sequences of their proteins. However, this can be very difficult. Viruses evolve very quickly, and therefore any similarities or patterns in the genome could disappear pretty quickly.
This theory says that Ancestral Virus evolved from the escaped genetic material of LUCA.
Evidence: This theory came about after the realization that there are similarities in genetic material (they all use either DNA or RNA or both) between the three domains and viruses, which suggests that they did at some point have a common ancestor. Viruses may have evolved to gain the same genetic information, but then they split off before the evolution of the plasma membrane or ribosomes (part of the cellular component that creates proteins). Another piece of evidence for this theory is the fact that viruses give a cell their genetic information for the cell to then replicate and translate into proteins. This means that the genetic material of the virus and the replication machinery of the cell have to be somewhat compatible, suggesting they did have a common ancestor at one point.
This theory says that Ancestral Virus originated from cellular organisms that were parasites of other cellular life, and then they lost most cellular components, including enzymes and other machinery needed to do replication.
Evidence: Since the host cell provides much of the needed components to survive, the virus can get away with having a much smaller genome, which could cause it to, over time, lose genes.
Secondly, mimiviruses have been studied as evidence for the Reductive Theory. Mimiviruses have some morphology that resembles cellular morphology. They have a cell-like structure during development, a larger genome than most other viruses, some machinery that can translate genetic material, and they have both DNA and RNA. This suggests that maybe viruses used to have all these parts just like cells do, but then they lost it.
There still is not enough evidence for each individual theory to make a conclusive statement about the evolution of viruses (and there are definitely more theories out there), but each new piece of evidence helps us come a bit closer.
Holmes, Edward C. The evolution and emergence of RNA viruses. http://books.google.com/books?id=PN8oFf2Id4kC&printsec=frontcover&dq=The+Evolution+and+Emergence+of+RNA+Viruses&hl=en&src=bmrr&ei=XQPMTe23EMji0QGglMXZBg&sa=X&oi=book_result&ct=book-thumbnail&resnum=1&ved=0CDwQ6wEwAA#v=onepage&q&f=false
Bandea, Claudiu. The Origin and Evolution of Viruses as Molecular Organisms. Available from Nature Precedings (http://hdl.handle.net/10101/npre.2009.3886.1), 2009.