Definition of Life

birney at molbiol.ox.ac.uk birney at molbiol.ox.ac.uk
Sun Feb 13 15:41:19 EST 1994



Julian Blanc wrote:
> The best definition I heard was given by John Maynard Smith. A living 
> thing must a physical dissipative structure having the following 
> characteristics:
> 
> 
> 	- Variation 
> 	- Reproduction
> 	- Heredity
> 
> This definition excludes fire, vortexes, planets, replicating clay crystals, 
> priests and cellular automata. It includes, however, free replicating 
> informational molecules such as nucleic acids, which is pretty neat. 
> 
> Julian Blanc
> 

I think this thread is quite interesting but fracturing into what has become
semantic arguments: that is should definitions of life be a practical
matter, so that we can call things with certainty alive or not or should 
they represent some inherent property, which a priori need not depend on
any particular chemistry or physical structure...

	The interesting point, in my opinion, about John Maynard's Smith
(who after all should know about these things) is that he restricts it to
_physical_ things: presumably he is trying to eliminate such things as
computer viruses and the like. 
	One could argue that this is just a 'practical' decision, because
we want our definition to exclude such things (else we might get into moral
quandrums every time we virus scanned a disk... ;) ). Alternatively there is
something different between computer viruses which exist due to the willful
act of some charming hacker (rather than evolution: is this an important
difference?) and us. 

	Again I would like to use the example of the chain letter: it 
satisfies the above criteria. (by chain letter I mean the things telling
you to write it on to the ten more  people else the recipient falls foul
of terrible luck etc). It has a physical structure, it reproduces (albeit
using humans as an unwitting host...but so do many, many parasites which
are very much alive), it has variation and hereditary. So John Maynard Smith
would call chain letters alive...

	So...is a chain letter alive, and if it is not, what criteria is
it not satisfying?

PS I realise it doesn't contain nucleic acids   :)

ewan

birney at molbiol.ox.ac.uk





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