Re: Chain reactions and sparks in the origin of life

Perplexed in Peoria wrote:
"Tim Tyler" <seemysig@xxxxxxxxxxxxxx> wrote in message news:fqs1va$2gme$1@xxxxxxxxxxxxxxxxxxxxxx

I have no problem with topologies, and attractors being "genetic"
in principle - though of course there are practical problems about
whether such ideas are plausible OOL suggestions.

All modern life uses template copying, and it's far from obvious
that anything else will actually work -

Or rather, all known modern life uses template copying for the
biggest part of the work of heritability. Many instances of
non-template-based heritability exist though - from the classical
cortical inheritance in cilliates to the phenomena of epigenetic
'imprinting' using mechanisms of (non-template) methylation
of the DNA.

And since these things actually DO work, you probably mean to
say that it is far from obvious that these kinds of things can work
*well enough* on their own without also having a template-based
genetic system in operation to do the 'heavy lifting'.

That is pretty much what I mean. Non-nucleic acid inheritance
outside man may get into the news, but it seems to be mostly fluff.
A handful of bits, most of which are probably rapidly
trashed by selection - or regenerated from nucleic-acid
sources.

The things that work at all (prions, etc) still tend to
work rather like templates - with the components physically
standing next to each other and undergoing a semi-mechanical
imprinting process as the copying takes place.

e.g. see Genetic Takeover, page 74.

Oops - most of the discussion I /meant/ to reference is on p.72.
My humble apologies :-(

There, Cairns-Smith criticises the "autocatalytic set" idea.

The basic criticisms are: the more inherited bits you have the
more autocatalytic chemical sets you need:

"The trouble here is that each reproducing molecular species
only contributes a limited amount of genetic information (about
one bit [...])".

.....and the more chemicals you have in the system, the more you get
unwanted reactions between them:

"it is very hard to imagine in practice several autocatalytic
processes going on without them interfering with each other".

He describes these systems as having a "low ceiling" of
information-carrying capacity - while to get off the launch
pad, life would need open-ended heredity, with lots of bits.

Modern living systems do that by spatialising the genetic
information - so different locations have different meanings.
The proposal is that living systems have always done that.

This is a proven strategy, so the idea is a highly
conservative one.

I note that Maynard-Smith has some cold water to pour on
autocatalytic sets as well:

"It was the apparent lack of heredity that made Eigen (1971)
abandon his own idea." - "The Major Transitions..." p.71.

So, is a minimal (that is, barely sufficient for 'life')
metabolism-oriented genome simpler than a minimal template
oriented genome. Clearly, it is. The minimal
metabolism-oriented genome is one bit! That bit
'encodes' (embodies?) one complete autocatalytic cycle.
But the minimal template-oriented genome must be much
larger than one bit. Much, much larger if it needs to
indirectly specify the copying machinery and the
anabolic pathways for the alphabet elements.

Carrying more than one bit is not a disadvantage.

.....and as I am sure you know, in Cairns-Smith's idea,
"indirectly-specified copying machinery" and "synthesis
of components" are seen as an enormous and unnecessary
burden that no realistic primitive system could possibly
be expected to support.

He has "naked genes". They are synthesised without any
copying machinery at all - by natural self-assembly processes.
The required components are ubiquitous molecules, that
are synthesised prebiotically by geosynthesis.

The need to build, concentrate and purify complex chiral
mixes of thermodynamically unlikely compounds by prebiotic
means is a problem for other theories. Such tasks are
performed /far/ more easily by an existing ecosystem, than
they can possibly be on a bare, lifeless planet.

Anyway, if you define genes as being nucleic acids, or a
molecular-chain template replicator, or some such, then
the "metabolism-first" terminology would also make sense.

But that makes the Cairns-Smith idea a 'metabolism first'
scenario. I don't think that anyone wants that. [...]

My approach - as I may have mentioned - is just to ditch
the whole 'gene first' / 'metabolism first' terminology
as ill-conceived.

I favour definitions of 'gene' and 'metabolism' that
mean that an organism without genes - or without
a metabolism - cannot persist.

'Gene centric' / 'metabolism centric' would make more
sense from my point of view - if you /must/ categorise
theories along that kind of axis.

Cairns-Smith /did/ get many of his clues about the
OOL by considering aspects of genetics - while there
are others who apparently do not see the origin of
high-fidelity information copying as the key problem -
and seem to want to focus on the energy source of the
first organisms.
--
__________
|im |yler http://timtyler.org/ tim@xxxxxxxxxxx Remove lock to reply.

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