what is alive?

Arlin Stoltzfus arlin at is.dal.ca
Wed Sep 18 09:36:17 EST 1996


Ed Rybicki wrote:

> > Arlin Stoltzfus wrote:
> > > I don't understand this definition.  In my way of thinking,
> ...
> Mario re-iterated:
> > Indeed, I wish I had a better working definition of what 'information'
> > is, myself. But by analogy with cultural information (which I consider
> ...
> > Furthermore, all parasites and all symbionts require input from other
> > organisms to copy their information.
> 
> ...and not only parasites and symbionts, if you think about it.
> Which organisms are capable of making a living off an environment
> containing no other living thing, or organic matter?
> Chemolithoautotrophs and photoautotrophs.  The rest of us parasitise
> them, in one way or another.  So let's not be too proud about
> defining autonomic organisms as living to the exclusion of humble
> little viruses, shall we??

Let me make this a bit more clear.  To start, a definition of
life based solely on genetic 'information' doesn't work. If 
life is nothing but genetic 'information', then one must admit 
that a computer disk with a file containing the complete sequence of 
Methanococcus jannaschii is just as much alive as a cell with
the Methanococcus jannaschii genome inside it.  They have the 
same genetic 'information' right?  

Clearly, this definition is absurd, because the 
so-called 'information' in an organism means nothing 
without a physical instantiation that is acted upon by
physical processes.  As George Williams has written, an 
organism is 'a place where certain processes occur'.  In the 
case of an HIV virus or a computer disk representing the 
sequence of an HIV virus, the 'information' just sits there, 
doing nothing.  Indeed, as Mario suggested, it may be important
to ask whether the entity is capable of 'copying' its information.
In the case of viruses, it is clear that the genetic information
is NOT copied by the virus particle per se (which usually has
no enzymes) but by the virus-host combination.  The virus-
copying process involves substrates and catalysts provided
by the host within the confines of its own body.  Therefore,
by the 'copying information' definition, the isolated 
virus particle is not alive, though we might consider it a
propagule of a living host.  According to Ed, if we accept
Mario's 'copying information' definition, then we are all dead,
because we are dependent on other organisms.  But this view
implies that there is no way to distinguish modes of dependency,
or to distinguish parasites from
non-parasites, something that biologists seem to have done
for hundreds of years now without too much difficulty.  

Arlin
-- 
Arlin Stoltzfus
Department of Biochemistry
Dalhousie University
Halifax, Nova Scotia B3H 4H7 CANADA
(email) arlin at is.dal.ca 
(phone) 902-494-3569 
(fax) 902-494-1355



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