The French Version of OOL

From www.astrobio.net July 9, Mon.
Plus some of my comments.

It is this period of the Earthâ??s pre-history, the transition from small,
simple molecules to large, complex cells, that has been subjected to detailed
scrutiny in â??From Suns to Life: A Chronological Approach to the History of Life on
Earthâ?? edited by M. Gargaud et. al. and reprinted from Earth, Moon, and
Planets, Vol. 98/1-4, 2006. This is an ambitious book put together by a number of
professionals in the French astrobiology community and this review looks at
just the chapter on the pre-biotic world. Starting with the newly habitable Earth
and ending with the first true cell, known as the Last Common Ancestor - a
period that stretches from roughly 4.2 to 2 billion years ago â?? â??From Suns to
Lifeâ?? brings together the current thinking of this challenging subject.
What hits you immediately about this subject is the large amount of
uncertainty and the many different possible scenarios.

TH
No there is just one. Chemical response to the environment. EVERY step that
came after favored chemical reactions that stabilized or continued
chemical processes.

....... There is general agreement about the main players. Amino acids,
nucleosides and nucleotides as the small pre-cursors leading to peptides, proteins
and the long polymers of RNA and DNA. Energy sources are also required and there
are important supporting roles for fats, carbohydrates and inorganic ions
such as magnesium, iron and sodium. But just what happened and in what order is a
matter of much debate and likely to remain so for some time.

TH
Wrong approach IMO. You are setting up your own recipe and then trying
to find evidence for it in the facts. The basic premise is that life was on
a mission - that began with the above ingredients. It was not. The chemicals
that began life were not on a mission to get to you. They would only alter
if energy was forced on them. And they would only alter in ways that
were natural chemical responses to forced energy. The results were
the most stable reactions (those that lasted and were not destroyed.)

Broadly speaking- and if we do accept for now the roles of metabolisms and
energy transfer processes- , there are three main roles of biochemicals in
living organisms:
storage of genetic information, structure, and catalysis.

TH
Life is not on a mission to store genetic info, structure, and / or
catalysis. Stop
forcing mission on inert chemicals. And see that the only response any
prebiotic chemicals can do is to react to forced energy. We should look for
how forced energy can lead to 'capture of energy", "genetic information, etc.
And why doing that would make them last longer.

For example, the capture of energy involves structures fine tuned to perform
their function coupled with a catalytic activity to facilitate the necessary
chemical reactions..... One of the requirements of any description of chemical
evolution is to suggest a plausible mechanism by which the evolving molecules
can fit into one or more of these roles.

Th
Chemicals do not look for roles to get to life. The 'plausible mechanism' is
rational chemical reaction to forced energy.

Three different scenarios for chemical evolution are discussed in the review;
co-evolution; self-replicating peptides and the RNA world.
Co-evolution makes no specific assumptions about a sequence of events. It
argues that a protein and nucleic acid based life emerged more or less fully
formed from a cocktail of pre-cursor molecules.It is the simplest of the models,
requiring perhaps the least detailed explanation but it is not a particularly
satisfying description.

TH
Protein and nucleic acid on a mission to pop up fully formed.
This is a fluke/magic event that will not, could not, and has never happened.

The self-replicating peptide scenario argues that short strands of protein,
peptides, were the first class of large biochemical to emerge from the soup. It
requires them to perform their modern functions of catalysis and structure
but also to store early genetic information... Peptide Nucleic Acid (PNA) it has
a similar structure to RNA but is not a nucleic acid. However, a fundamental
problem with this is that it would have to have been replaced at some later
period by RNA and then DNA....

TH
This is the 'aren't we clever scientists' version. PNA's are clever - like
trying
to solve a mystery plot. But the 'origin of life' if it is anything is NOT
clever. It's basic, methodical, and straightforward. Too often I, a non
scientist, see scientists
playing the 'oh I'm so clever to fathom this possibility' that they forget
that nature does not play that way. Usually anything this clever is NOT right.

The model that receives the most attention (and perhaps the most
straightforward) is that of the RNA world. In this short strings of nucleic acid, RNA, are
the first complex biochemical molecules to emerge from the soup. They have to
perform the three functions of structure, catalysis and genetic storage.
However, RNA does indeed perform all of these functions to some degree in modern
cells â?? perhaps the smoking gun of molecular evolution.

TH
Let's say it again. RNA is not on a mission to perform for you. RNA in
response to outside forces can do all these things that are claimed.
But for it to last or stabilize or develop into life,
RNA's development, and response to the environment must be one that is the
most
stable reaction to the environment, or it would not have lasted.

For many years RNA was seen as a bit player in cellular metabolism but the
last couple of decades have seen a significant re-think of its role. RNA can,
like DNA, store genetic information along its length and does so in a number of
modern viruses. It is also a key structural element of the ribosome, the
cellular component responsible for protein synthesis. But what has become clear
recently is that it can also be a catalyst for a number of chemical reactions.
RNAzymes, as they are called, are now known to have all sorts of different
activities including some of those essential for the reading and writing of genetic
information. However, an efficient prebiotic pathway for nucleotide synthesis
remains to be found.

TH
Whatever that prebiotic pathway is, we know it's the natural response
of chemicals to the energy forces in that environment. That is a massive
clue that should break the OOL impasse we now face.

..... It is impossible to say which of these scenarios most accurately
represents what really happened on an early Earth. The discussion is also far more
complex than this brief race through some of the ideas. Astrobiology, though,
might provide one of the few opportunities to help unravel some of the puzzle.
What might we find in the oceans of Europa or even the methane lakes of Titan?
Even if there is nothing that might be classified as life, could they contain
elements of the pre-biotic soup? If so, which model might they support?

TH
More and more I see life as a specific response to a liquid water
temperature range under very specific conditions. Again scientist like to
imagine
exotic origin pathways - that may be fun, but nature is, if anything down-to
earth
(pun) and straight forward, and again never clever.

This book covers every element of the evolution of life from the emergence of
simple organic molecules to theories on how the first cells might have got
together. How did groups of chemicals and their associated reactions become
compartmentalised into prototype cells? What was the involvement of inorganic
matrices and, the big one, how did complexity arise from simple origins? The
authors painstakingly pore over the limited evidence and maks intelligent, though
guarded, speculations as appropriate. Anyone who is not comfortable with
biochemistry might struggle at times but the summaries are less intense and will
allow virtually all readers to grasp the concepts and uncertainties. In
describing the problem of how life emerged the authors also illustrate why astrobiology
might provide one of the few experimental opportunities to test the
hypotheses.
By Toby Murcott

TH
I think they've missed the obvious. Until people see the origin
as not a start but a reaction to what went before, they will
be stuck and unable to move forward in prebiotic study.


Prebiotic chemistry- Biochemistry-Emergence of life, R.Pascal et al,
"From suns to life: a chronological approach to the history of life on Earth",
p153-203, Gargaud et al, Springer, 2006

Tom Hendricks </HTML>


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