Since the recent
discussion regarding the definition of life, I have been thinking some about
this topic. I realized that one useful area to look at might be the
contentiousness in biology surrounding whether or not a virus is
"alive".
I had some offline conversations with Judith on this
topic, and she disagrees with some of my points. It seemed to both of us like an
interesting topic for list discussion.
After reading some
webpages, it seems to me that the common thinking goes roughly this way: a virus is not alive insofar
as an entity unto itself it cannot 1) reproduce,
2) is not a true cell, 3) it has no metabolism or internal
processes. And the argument on the other side is that once
a virus "takes over" a host cell, then it "controls" that cell and uses the
machinery of the cell to perform processes and reproduce, so in this (extended)
sense the virus is alive.
It strikes me,
though, that this debate is all wrong.
A virus on its own has no volition, no internal
metabolic processes at all. It is inert. It does not "seek" a host cell;
instead, it randomly floats around until it is in proximity to one. Once in
proximity, if the virus and cell are of the right types, the virus will become
attached (chemically bind?) to a receptor site on the cell. In response, the
virus then apparently initiates a fixed routine to inject its DNA/RNA into
the cell's cytoplasm.
To speak of this sequence in active volitional language, such
as "the virus seeks out a
host cell, uses its DNA/RNA
to take control of the host cell", seems to me entirely inappropriate, and assigns
volitional capabilities to a structure that has no such
capabilities.
I would argue that, on this level of description, a
virus is a Rosennean mechanism, not a complex system. The virus on its
own has no internal processes, much less any internal causal closed
loops of processes. Its one process of injecting DNA/RNA into a cell
is in reaction to the successful binding to the host. Crudely, it is
like a spring-loaded clamp and
hypodermic attached to a balloon. It floats around randomly until it comes in
contact with something (say, some organism) that trips the spring
clamp, which then causes the hypodermic plunger to move. This is an
entirely fixed and mechanical sequence.
Also, regarding viruses as
"evolving" entities. I think the idea that the "evolution" of viruses is
evidence that viruses are alive somehow is very weak. Prions (so-called
"misfolded" proteins) replicate and apparently also evolve (which may be what
allows them to sometimes pass between species), but no one (to my knowledge)
suggests that this indicates that prions are
alive.
So, the notion that once the virus is attached to the cell,
that the virus "takes the cell over" is erroneous. Just as it would be erroneous
to say that the balloon-clamp-hypodermic mechanism "takes over" an organism it
attaches to and injects with something that alters the processes in the
organism. Similarly, it would be erroneous to conclude that the
balloon-clamp-hypodermic is therefore now properly called "alive" because it has
attached itself to a living organism and has altered what the organism
does.
Instead, I think
the correct view is that the combining
of the virus and the host cell form a chimeric relationship:
the combination becomes a new creature.
This new creature has, as one of its
functions, to generate new copies of the virus. Depending on the type of virus,
this creature may then dissolve, turn cancerous, or continue to persist.
Rosen talked about hermit crabs in ch. 18 of
Essays and the way in which the crab adopts a snail shell and collects
sea anemones and becomes a new chimeric organism
with a new set of behaviors. It seems to me that this is
exactly the type of case that is
occurring in the virus-host situation. However, I think this has been historically
misinterpreted perhaps because of
the apparent similarity of the symptoms arising from
viral infections to bacterial
ones, and probably moreso because a virus
has RNA/DNA, which is taken as putative evidence of life. So,
the virus has been incorrectly seen as purposively usurping control
of the host, leading to the supposition that a virus is alive
somehow.
But the virus by
itself is much like the snail
shell by itself: it is inert and not
alive. When it has become combined with the crab, one cannot ask: is the
shell now alive or not by virtue of this
combining? The question is improper, since the shell is not now a thing by itself, but is now a
part of this larger organism, the chimera, and that
chimera is what is alive. Similarly, the virus does not become alive by
virtue of combining with the host cell; instead, both are subsumed
into this new larger chimera organism.
I hate to use the old saw, but a chimera is a case where one
must adhere to the principle "the whole is greater than the parts". Once
combined, the virus and cell cannot continue to be thought of as distinct
entities or parts if we want to explain and understand the behaviors of the
result of this combining. Otherwise, I think this only leads to the
confused thinking that perpetuates the virus live-or-dead
debate.
The tendency
toward reductionist thinking, and of thinking of the virus as a volitional creature, leads to one attempting to mentally
maintain the virus and host as distinct and separable entities, even after they
have combined. The generation or replication of copies of the initial virus also
seems to support this view. But I think that this is simply trying to make the
situation appear other than it actually is, which is that the (virus + host)
once joined are a new creature with new functionality (just as the (hermit crab + shell +
anemone) is a new chimeric creature). One
of those functions is to process raw materials into copies of the
original virus. And the question of "what is alive?" at that point is
answered by: "this new creature, the chimera, is what is
alive".
Regards,
Tim