wrote in message
news:c9lduo$2vuk$1{at}darwin.ediacara.org...
> Two separate ideas.
>
> IF I am correct and UV caused pyrimidine dimers
> was a selection force
> that  gave an advantage to purine codons which
> I associate with  mostly hydrophilic amino acids,
> as opposed to pyrimidine codons which I associate
> with mostly hydrophobic amino acids.
>
> THEN that would favor purine coded
> hydrophilic amino acid chemistry.
> Thus they would have the edge over
> pyrimidine coded  hydrophobic chemistry.
>
> But that also means that hydrophilic chemical
> reactions would probably precede hydrophobic ones.
>
> That would suggest that the naked gene hydrophilic
> chemical reactions would probably precede they
> membrane hydrophobic chemical reactions.

Membrane forming lipid molecules have both a hydrophobic
part and a hydrophilic part.  My OOL scenario assumes
that of the interesting chemical reactions happen in
the hydrophilic part - at least until photo-pigments
are invented.

> Thus this might set up a time line. First the
> chemistroy of hydrophilic amino acids would be
> favored,  with the chemistry of hydrophobic
> amino acids coming 2nd.
>
> ALSO,
>
> IF UV light is an important selection part of
> the origin, and its intensity must be at similar
> levels as that that struck earth,
> THEN Mars is too far out to get that same UV
> proponent of life. That would suggest life could
> not start there.

The surface of Mars today gets more UV than earth.
Back before earth got its ozone layer, the surface
of Mars still got more UV than does the average
surface location on earth - if you remember that
those locations are mostly under water.  And even
if you limit yourself to dry land, the UV photons
on Mars are just as strong as on earth - only they
are about 2.5 times less abundant.  That is still
a lot of selection against pyrimidines.
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