Where did viruses come from?
The probable multiple origins of viruses are lost in a sea of conjecture and speculation, which results mostly
from their nature: no-one has ever detected a fossil virus as a particle; they are too small and probably too
fragile to have withstood the kinds of processes that led to fossilization, or even to preservation of short
stretches of nucleic acid sequences in leaf tissues or insects in amber. As a result, we are limited to
studying viruses that are isolated in the present, or from material that is at most a few decades old. The new
science (or art) of virus molecular systematics is, however, shedding a great deal of light on the distant
relationships of, and in some cases on the presumed origins of, many important groups of viruses. For
example, picornaviruses of mammals are very similar structurally and genetically to a large number of small
RNA viruses of insects and to at least two plant viruses, and - as the insect viruses are more diverse than
the mammalian viruses - probably had their origin in some insect that adapted to feed on mammals at some
point in evolutionary time.
If one were to go far back into evolutionary time, a case could be made for descent from a single ancestor
of at least the replicase-associated functions of all viruses with positive-sense and negative-sense singlestrand RNA genomes; likewise, large DNA viruses like pox- and herpes viruses could be presumed to have
"degenerated" (as if a “renegade” stretch of DNA broke off) from cellular organisms, given that their
enzymes share more sequence similarity with sequences from cells than with other viruses or anything else.
Retroviruses, pararetroviruses, retrotransposons and retroposons all probably share a common origin
of the reverse transcription function, which in turn may be a living relic of the enzyme that enabled the
switch from a presumably RNA-based genetics to DNA-based heredity. (Note: Retroviruses use RNA as
their genetic material, but must make a DNA copy - using the enzyme reverse transcriptase - before actually
carrying out their life cycle. HIV is a retrovirus, and since we humans do not need to carry out the process
of reverse transcription in our cells, this is a major area of focus for HIV treatments.)
Whatever the implications of sequence relationship studies, it is very quickly apparent that there can have
been no single origin of viruses as organisms. For instance, there is no obvious way one can relate viruses
of the size and complexity of the Poxviridae [double-stranded linear DNA,130-375 kb, 150-300 genes] with
viruses like the Tobamoviridae [ss linear RNA, 6-7 kb, 4 genes], or either of these with the Geminiviridae [ss
circular DNA, 2.7 - 5.4 kb, 3-7 genes]. Thus, there can be no simple "family tree" for viruses; rather, their
evolutionary descent must resemble a number of scattered "bushes.” Viruses as a class of organisms must
therefore be considered to be polyphyletic in origin.
What viruses do have in common is a role as the ultimate "stripped-down" parasites: organisms which can
only undergo a life cycle inside the cells of a host organism, using at the very least the metabolic enzymes
and pathways and ribosomes of that host to produce virion (a single virus particle) components which get
assembled into infectious particles.
What is most frightening is that apparently new diseases are still erupting into our human environment at
regular intervals: new and often extremely virulent viruses, such as those causing Ebola, Marburg and
Congo-Crimean haemorrhagic fevers, hantavirus pulmonary syndrome and Korean haemorrhagic disease,
and of course, HIV1 and 2. These "emerging" and even "re-emerging" viruses are a great cause for concern
internationally, and the subject of a great deal of concentrated research.
From the Cartage Institute
http://www.cartage.org.lb/en/themes/Sciences/LifeScience/GeneralBiology/Microbiology/Viruses/Origins/Ori
gins.htm