I have heard that most (if not all) viruses evoloved out of
transposable elements in an organism's genome, but that viruses
have been around for almost as long as cells. Thus viruses
and cells did not originate *from* each other, but rather
with and by way of each other. Certain intermediate forms
between organismal DNA and viruses exist, encoding enzymes
(retrotransposases) that enable the transposon "selfish" DNA
to replicate *within* a cell but not infect other cells.
Gradually, the transposon beceoms more adept at serving its own
replication agenda. It becomes a competitor of the energy
spent on the maintenance of "good" DNA which actually *benefits*
the cell. Sometimes, however, a plasmid (why only in bacteria?)
may form out of the transpositional playgrond and encode something
like an antibiotic resistance gene. The plasmid may or may not
become part of a virus.
Events may occur that involve the element's gene swapping between
other viruses present--or between intracellular bacterial pathogens
suchs as the chlamydia.
Occasionally, an extremely rare event happens where a
transposon, retrotransposon, or a virus from the *extenal* world
permanently shuffles a gene fragment back into the chromosome with
benevolent results. If this happens in the germline cells of
an embryo, evolution has occured by the "horizontal" transfer
I sometimes wonder if this process is what (by and large) has
produced the huge amount of organismal diversity we see on planet
earth. This notion grates (a bit) against the idea that it is
merely the accquisition of simple point mutations over time which
enhance or detract from an organism's reproductive fitness.
If Nature invented the virus to be purely harmful, viruses
would not exist after 3-4 billion years of life's evolution!(?)
If you'll forgive this goofball anthropomorphy, viruses might be
how a large DNA molecule "talks" to its other distant cousins;
mostly, this talking amounts to nothing, but once in awhile the
results are staggeringly profound. For example, a virus
infects a tunicate in the Cambrian oceans and the result is
that a developmental switch fails to occur when the tunicate is
in the motile, larval stage (though reprduction is still possible
for these "larvae"). Lampreys, sharks, and bony fish rapidly appear
on the scene. Well, *comparatively* rapidly...
Possibly, metabolic stress due to starvation and UV radiation
ever-so-slightly tips the balance of a cell's replication machinery
to favor the existence of transposons or viruses. The teleology
is that the cell DNA "has nothing to lose" by allowing the more
chaotic and deleterious DNA elements to persist, since it will most
likely perish (anyway) from the environmental stress. However, the
shuffling may give one in 10^9 cells/organisms a greatly enhanced
chance of propogating. Something along these lines has been seen in
corn studies. "Transposon bursts" they are called. I wonder
if there are such things as "virus bursts."
I could ramble on and on. Someone who REALLY knows about this
ought to comment. If i have spoken irresponsibly, I apologize.
It's just that some interesting theories have come about by viewing
virology either ecologically or from a "DNA-centric" bent.