My suggestion that dimer impact was important
in setting up the genetic code would suggest that
there must be a lot of UV.
But I also note that if the following is true, the
excessive UV may well have also made the RNA
and bases the centerpiece of life to begin with.
Thus everything seems to be beginning to fit with
UV the power source and selective force for much of it.

http://www.newscientist.com/news/news.jsp?id=ns99993778
The World's No.1 Science & Technology News Service 
 UV light may have sparked life on Earth 

Ultra-violet light, long thought to be an impediment to the early formation of
long organic molecules, may in fact hold the key to the origin of life,
according to a new study.
(snipped)

Life on Earth is thought to have evolved about 3.7 billion years ago, when
there was no protective ozone layer encasing the planet and UV radiation was
100 times more intense than today.

The nucleotides that make up RNA have three components - a sugar, a phosphate
and nitrogen-containing base. "And these bases have very peculiar properties of
being extremely efficient at quenching UV light," says Mulkidjanian, protecting
the sugar and phosphate components which form the spine of the chain.

The team fed data on the photochemistry of various organic molecules into a
computer model designed to simulate the effects of UV light on stability. "The
effect was very pronounced in RNA," he says. In the presence of strong UV
light, RNA was much more likely to form long chains than other molecules.

Comment?
Tom
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