I'm still thinking about the origin and some possibilities.
If we look at peptides we see a strand of amino acids.
With that in mind, this quote suggests some ideas

"The hydrophobic (amino acids) inside, hydrophilic
outside arrangement is an adaptation to an aqueous 
environment. IF this environment is markedly altered
as within biological membranes, the organization 
of the proteins may be reversed" Cell and Molecular
Biology, Karp.

So this suggests that any peptides so formed that
they had hydrophilic amino acids on the outside and
hydrophobic amino acids on the inside would be
favored in the watery environment on early earth.

Yet that suggests certain other things. It suggests that
in those cases there would be strands with MOSTLY
hydrophilic amino acids and FEW hydrophobic acids,
with the few phobic acids being towards the middle of
the chain)

And that suggests that strands with mostly hydrophilic
amino acids would have the advantage.
_____

At the same time purines (A,G) would have an advantage over
pyrimidines (C,U) in strands of nucleotides - due to dimer 
damage on any adjacent pyrimidines from UV.

NOW putting the two propositions together we have:
1. peptides with mostly hydrophilic amino acids
2. nucleotides with mostly purine bases.

I would suggest that perhaps the two were then
linked in the genetic code:
hydrophilic amino acids - purine bases

And because there was some but fewer hydrophobic amino
acids,  and at the same time some, but fewer pyrimidines
- that they too would be linked in the genetic code
in a sort of 2nd tier group or set.
hydrophobic amino acids - pyrimidine bases

Comment?

This argument is based on these premises, and
if they are false then the argument is  probably false
or much weakened.

1. Sun cycle (with UV) produces monomers, and some
polymerization.
2 at the start all strands of RNA are single stranded
and there is yet no need for exact W-C base pairing.
That'll come later
3. Mostly purine bases A and G (due to dimer damage
on adjacent pyrimidines C and U)
4 Mostly hydrophilic peptides, because they are favored
in a water environment
5. Dimer damage eliminates adjacent pyrimidines
6. dimer damage may be an advantage when base pairing
is not an advantage.
7. There is little C due to deanimation.
8 Disulfide bridges may be important in first protein
structures.
9 Codons are 3 bases long. One reason may be that
that is the first configuration that gives a center
and protected base.
10 Coding , due to limitations in either the codon
or the anticodon of any two pyrimidines adjacent,
would limit to 2 configurations:
pyrimidine/purine/pyrimidine  OR
purine/pyrimidine/purine.

Also  monomers that are near ATP may have an advantage
that those not near ATP would not have.
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