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April 30, 2007 — How do viruses originate?
—Vicki Massey, elementary specialist, Mesa (Arizona)
Public Schools
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Response
by Christian Schlieker
Whitehead postdoctoral researcher |
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Despite the fact that we understand a number of viruses
in great molecular detail, we know surprisingly little
about how they actually originated. Since viruses don’t
fossilize well—due to their small size and instability—we
don’t quite know what the ancestors of modern
viruses looked like billions of years ago. In the absence
of any information about the early days of viruses,
we have to resort to hypotheses that can be classified
as variations of one of the following three theories:
1) The virus-first hypothesis implies that
viruses represent a primitive, pre-cellular life form
that originated in the prebiotic world (i.e., before unicellular
organisms were present). Most scientists have dismissed
this idea, since all known viruses have parasitic life-styles
that critically rely on the host’s intracellular
machinery for their propagation, i.e., there is no
evidence that any virus can replicate independent of
its host. It thus follows that at least unicellular
organisms must have been present before viruses entered
the stage.
2) According to the escape theory, viruses
are fragments of cellular genomes that escaped from
their cellular environment. In this scenario, viruses
can be seen as infectious, “selfish” DNA/RNA
elements that eventually became autonomous. In
support of this idea, mobile genetic elements have
been identified in both uni- and multicellular organisms,
and these might be considered as viral precursors.
3) The reduction hypothesis says that viruses
are derived from unicellular organisms, which were
once endowed with the ability to self-replicate independently.
Following uptake by another cellular organism, these
virus progenitors lived as intracellular symbionts,
reminiscent of some organelles like mitochondria or
chloroplasts. Over time, the virus lost more and more
of its genome and ultimately established a true parasitic
life-style.
These theories are not mutually exclusive, and also don’t
necessarily apply to all viruses that we know. Viruses
have coevolved with their hosts over billions of years;
each virus has developed a unique toolbox to hijack a
specific host’s machinery to achieve parasitic
perfection. As a consequence, viruses are quite diverse
in many regards, and it is thus difficult to envision
a common ancestral origin of viruses. It is quite possible
that viruses that infect a bacterial cell developed independently
and at a different time than those that infect humans,
tobacco plants or other organisms.
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